Add Precise Seam placement feature (#12974)

Co-authored-by: Ioannis Giannakas <59056762+igiannakas@users.noreply.github.com>
Co-authored-by: Rodrigo Faselli <162915171+RF47@users.noreply.github.com>
Co-authored-by: Ian Bassi <ian.bassi@outlook.com>

docs/HLSD/precise-seam.md

@@ -0,0 +1,239 @@
+# Precise Seam — High Level Design
+
+## Purpose and scope
+
+Precise Seam places the seam where a helper volume intersects the external
+wall. The user attaches a mesh to an object as a Precise Seam modifier, and on
+every layer the seam placer reads the modifier's slice to decide where the seam
+of each external perimeter may, must or must not go. The same mesh keeps
+working after the model changes, so the seam does not have to be repainted
+after every design revision, and a swept helper body can guide the seam along
+any path.
+
+The modifier is non-printing geometry. It does not take part in slicing, region
+assignment, filament selection or brim adhesion. It affects only seam
+placement, which runs during G-code export.
+
+## Volume types and priority
+
+Precise Seam adds six `ModelVolumeType` values after `SUPPORT_ENFORCER`. The
+strong types come first and the weak types follow. `is_precise_seam()`,
+`is_precise_seam_strong()` and `is_precise_seam_weak()` are range checks that
+depend on this order.
+
+| Type | Group | Effect on the perimeter |
+| --- | --- | --- |
+| `PRECISE_SEAM_CENTER` | strong | seam at the arc-length midpoint of the intersection |
+| `PRECISE_SEAM_LEFT` | strong | seam at the first point of the intersection |
+| `PRECISE_SEAM_RIGHT` | strong | seam at the last point of the intersection |
+| `PRECISE_SEAM_ENFORCED` | weak | intersection marked as enforced |
+| `PRECISE_SEAM_BLOCKED` | weak | intersection marked as blocked |
+| `PRECISE_SEAM_NEUTRAL` | weak | intersection reset to neutral |
+
+A strong modifier fixes one point. A weak modifier only changes the
+enforced/blocked type of seam candidates, and the configured seam position then
+chooses among them. First and last are taken along the perimeter made
+counter-clockwise seen from above. On an outer wall seen from outside, Left is
+the left end of the intersection. On the wall of a hole seen from inside the
+hole, the two ends are swapped.
+
+The order of volumes in the object is the priority order, highest first.
+`ModelObject::sort_volumes()` keeps every strong modifier before every weak one
+and preserves the user's order within each group. The object list lets the user
+drag a modifier only within its own group. A type change that crosses a group
+boundary moves the volume to the end of its new group, where it has the lowest
+priority. Strong modifiers are tried in this order, and the first one that
+yields a seam on a perimeter wins. Weak modifiers are applied from the lowest
+priority to the highest, so the highest one overwrites any overlapping zone.
+
+## Model storage and 3MF compatibility
+
+Projects must stay readable by earlier releases, and the modifier must not
+change a print there. Both 3MF writers therefore store a Precise Seam volume as
+an ordinary parameter modifier: `modifier_part` in the Bambu-format part
+subtype, and `ParameterModifier` together with the legacy `modifier` flag in
+the Prusa-format volume metadata. The seam mode is written separately under
+`precise_seam_type`, using the names from `ModelVolume::type_to_string()`
+(`precise_seam_center` and so on).
+
+On load, the mode applies after all other volume metadata, regardless of XML
+key order, and only when the base type is a modifier. Missing or unknown modes
+leave an ordinary modifier. Seam metadata on any other base type is ignored.
+Files that stored the seam mode directly as the volume type still load.
+
+A Precise Seam volume keeps any per-volume settings it had as a part or
+modifier, but they are inactive and the object list shows no settings item for
+it. The writers prefix these keys with `precise_seam_config:`, so an earlier
+reader drops them as unknown options. The volume therefore loads there as a
+modifier without settings and has no effect on the print. The current reader
+restores the keys only when the volume ends up as a Precise Seam type, so the
+settings return when the user changes the type back. Configuration values are
+XML-escaped in both writers, for every volume type.
+
+## Print invalidation
+
+`Print::apply()` compares the Precise Seam volumes of each object by type, ID
+and transformation. Adding, removing, moving, reordering or retyping one
+cancels background processing and invalidates only `psGCodeExport`; the sliced
+layers are kept. `model_volume_list_update_supports_and_seams()` then brings
+the support and Precise Seam volumes of the print's model copy in line with the
+new model in one pass. A volume may switch between the two families, since
+neither affects slicing. A conversion to or from a part or ordinary modifier
+changes the solid and modifier volume lists and reslices as before.
+
+## Modifier slices
+
+`SeamPlacer::init()` collects the Precise Seam volumes of each object once:
+strong ones in priority order and weak ones reversed. It slices each volume
+separately with `PrintObject::slice_single_volume()`, which shares
+`slice_modifier_volumes()` with support blockers and enforcers but does not
+merge volumes, so each keeps its own priority. The result is cached per volume
+and indexed by object layer; `Layer::id()` includes raft layers, which are
+subtracted. Seam candidates are then gathered in parallel over the layers and
+read the cache without locking.
+
+Objects without Precise Seam volumes follow the unchanged seam placement path.
+For objects that have them, perimeter extraction also removes consecutive
+duplicate points and the repeated closing point of each extrusion loop.
+Zero-length edges at path junctions would otherwise prevent point insertion
+there. Distinct visits to one point of a self-touching contour are kept.
+
+## Finding the wall segment
+
+The seam placer works on the external perimeter loops of each layer, both
+outer contours and holes, each made counter-clockwise. For every modifier
+polygon on the layer that overlaps the perimeter's bounding box, the region
+enclosed by the perimeter is clipped against the modifier polygon. The boundary
+of each intersection polygon alternates between runs that follow the perimeter
+and runs that follow the modifier outline. The wall segment is the longest
+continuous run of intersection vertices that lie on the perimeter, measured in
+vertices.
+
+The fast path first finds an intersection vertex that exactly matches a
+perimeter vertex. It then walks forward and backward, expecting the adjacent
+perimeter vertex and falling back to projection when Clipper has merged or
+split collinear edges. A vertex counts as on the perimeter when its projection
+is within about 1.6 nm, which covers Clipper's rounding. If no vertex matches
+exactly, or every vertex lies on the perimeter, the general path projects all
+vertices. When every vertex is on the perimeter, the edge midpoints are checked
+instead: a modifier chord can join two perimeter vertices directly, and the
+chords split the vertex ring into runs. If no edge leaves the perimeter, the
+perimeter lies entirely inside the modifier.
+
+`Polygon::point_projection()` optionally reports the edge that holds the
+projection, and every point of the segment keeps the index of its perimeter
+edge. New points are inserted on that edge. A point within 1 µm of an existing
+vertex snaps to that vertex instead.
+
+## Strong modifiers
+
+For a strong modifier, the target is the first point, the last point or the
+arc-length midpoint of the segment. The midpoint is projected back onto the
+original perimeter, because Clipper may have merged several perimeter edges
+into one segment edge. The target is inserted into the perimeter, and a helper
+point is inserted 1 µm before and after it. Strong modifiers are tried in
+priority order, the first valid intersection decides the seam, and weak
+modifiers are not processed for that perimeter.
+
+When candidates are built, the inserted point is the only enforced candidate
+and becomes the central enforcer; every other candidate is blocked. The seam
+position modes then pick that point: Aligned and Aligned Back prefer the central
+enforcer, while Back, Random and Nearest rank enforced candidates above blocked
+ones. Alignment and random placement can still move the final position along an
+edge. After alignment, `restore_precise_seam_positions()` writes the exact point
+and its index back into every perimeter that has a strong seam. Inner walls take
+their seam from the external seam as usual, including staggering.
+
+## Weak modifiers
+
+Weak modifiers produce one segment per intersection polygon, so one modifier can
+mark several zones on one perimeter. All segment boundaries are inserted into
+the perimeter in order of decreasing arc length. Each insertion then leaves the
+indices of the pending, shorter ones unchanged; a point on the closing edge is
+appended rather than inserted at index zero. A helper point is added 1 µm
+outside each boundary. Random placement picks a position along the edge that
+follows a candidate. These helpers keep that edge 1 µm long at each boundary, so
+a zone cannot extend or intrude further than that. Boundaries that coincide
+share their helper points.
+
+The zone types are then resolved in priority order, and the edges of enforced
+zones are subdivided into steps of at most
+`SeamPlacer::enforcer_oversampling_distance` (0.2 mm). The middle candidate of
+the longest enforced patch is therefore close to the geometric middle of the
+zone. That patch is measured in candidates, across the closing edge, regardless
+of where the contour starts; the same rule applies to painted seams.
+
+Candidates first receive their type from seam painting. The weak zones then
+overwrite it, lowest priority first. Blocked and Enforced zones therefore take
+precedence over painting, and Neutral clears painting inside its zone.
+
+## Unsupported geometry and warnings
+
+Some modifier shapes cannot be resolved to one seam or one zone per crossing.
+They are detected cheaply and reported rather than guessed:
+
+- A strong modifier that crosses a perimeter in more than one place uses only
+  its first valid segment. The other crossings are ignored.
+- A modifier that crosses the whole region enclosed by the perimeter is
+  detected when the modifier outline minus that region leaves more than one
+  piece, none of them a hole. Its intersection holds two wall runs, and only
+  one of them is used.
+- A modifier whose slice has a hole on a layer, found as a clockwise polygon in
+  the flattened slice, is skipped on that layer. The flattened slice no longer
+  records which hole belongs to which contour.
+- A perimeter that lies entirely inside a modifier is ignored by that modifier.
+
+The conditions are atomic flags shared by all layers and objects. After all
+objects are processed, `SeamPlacer::init()` issues at most one non-critical
+warning with the ID `SlicingPreciseSeamWarning`. The warning is a single line
+that lists every cause found, because the export warnings dialog shows only the
+first line of each warning. Repeated warning events replace this notification
+instead of appending text to it.
+
+## User interface
+
+- *Add Precise Seam* in the object menu creates a Center modifier from a
+  primitive or a loaded mesh. Text and SVG volumes cannot become Precise Seam
+  modifiers: the menu does not offer them, and `ObjectList::set_volume_type()`
+  refuses the change.
+- *Change Type* has a single *Precise Seam* entry. It converts other volumes to
+  Center and keeps the mode of volumes that are already Precise Seam. The
+  *Precise Seam Type* submenu appears only when every selected item is a
+  Precise Seam volume, including settings rows that resolve to one. It sets the
+  chosen mode on all selected volumes.
+- Each mode has its own icon in the object list and its own color in the 3D
+  view, at 60% opacity: warm oranges for the strong modes, and green, red and
+  gray for Enforced, Blocked and Neutral.
+- Object list drops map visible rows to volume indices while skipping hidden
+  cut connectors, and they refresh the row-to-volume map of the object.
+- Precise Seam volumes have no filament, block pasting into SLA, and are exposed
+  to Python plugins as `ModelVolumeType` values plus the `is_precise_seam*()`
+  methods.
+
+## Implementation and verification
+
+- [PreciseSeam.cpp](../../src/libslic3r/GCode/PreciseSeam.cpp) implements segment
+  detection, point insertion, weak-zone resolution and position restoration.
+  [SeamPlacer.cpp](../../src/libslic3r/GCode/SeamPlacer.cpp) integrates it into
+  candidate gathering and issues the warning.
+- [Model.hpp](../../src/libslic3r/Model.hpp) defines the types and their order,
+  [PrintApply.cpp](../../src/libslic3r/PrintApply.cpp) handles invalidation, and
+  [PrintObjectSlice.cpp](../../src/libslic3r/PrintObjectSlice.cpp) slices the
+  modifiers. [bbs_3mf.cpp](../../src/libslic3r/Format/bbs_3mf.cpp) and
+  [3mf.cpp](../../src/libslic3r/Format/3mf.cpp) store them.
+- [GUI_Factories.cpp](../../src/slic3r/GUI/GUI_Factories.cpp) and
+  [GUI_ObjectList.cpp](../../src/slic3r/GUI/GUI_ObjectList.cpp) provide the menus,
+  type changes and ordering.
+- [Precise Seam tests](../../tests/fff_print/test_precise_seam.cpp) cover the
+  strong positions, including a midpoint on an existing vertex or the closing
+  edge. They also cover shared and coincident weak boundaries, every warning,
+  and the priority order.
+- [Seam placer tests](../../tests/fff_print/test_seam_placer.cpp) cover
+  enforced-patch selection independent of the contour start, fully painted
+  contours, duplicate removal, and `Print::apply()` synchronization through
+  type changes and restored model snapshots.
+- [3MF tests](../../tests/libslic3r/test_precise_seam_3mf.cpp) cover the round
+  trip of every mode and of inactive settings, attribute escaping, and which
+  metadata combinations restore a seam mode.
+  [Plugin tests](../../tests/slic3rutils/test_precise_seam_plugin.cpp) cover the
+  Python bindings.

localization/i18n/list.txt

@@ -213,6 +213,7 @@ src/slic3r/Utils/Process.cpp
 src/libslic3r/GCode.cpp
 src/libslic3r/GCodeWriter.cpp
 src/libslic3r/GCode/ToolOrdering.cpp
+src/libslic3r/GCode/SeamPlacer.cpp
 src/libslic3r/ExtrusionEntity.cpp
 src/libslic3r/Flow.cpp
 src/libslic3r/Format/AMF.cpp

localization/i18n/ru/OrcaSlicer_ru.po

@@ -27357,3 +27357,22 @@ msgstr ""
 
 #~ msgid "Right click to reset value to system default."
 #~ msgstr "Правая кнопка мыши - сброс значения до системного по умолчанию."
+
+# AI Translated
+msgid "Precise Seam"
+msgstr "Точный шов"
+
+msgid "multiple intersections with a perimeter detected"
+msgstr "обнаружены множественные пересечения с периметром"
+
+msgid "modifier fully crosses the printable perimeter"
+msgstr "модификатор пересекает печатаемый периметр насквозь"
+
+msgid "modifier shape is not solid (has holes inside) and was ignored"
+msgstr "форма модификатора не сплошная (имеет отверстия) и была проигнорирована"
+
+msgid "perimeter is fully contained inside modifier and was ignored"
+msgstr "периметр полностью содержится внутри модификатора и был проигнорирован"
+
+msgid "Seam placement may differ from expected."
+msgstr "Размещение шва может отличаться от ожидаемого."

resources/images/menu_precise_seam_add.svg

@@ -0,0 +1,19 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Центральные пунктирные линии -->
+  <rect x="7" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Правая стрелка -->
+  <path d="M9.5,7.5L12.504,4.504L12.5,10.5L9.5,7.5Z" style="fill:#949494;stroke:#949494;stroke-width:1px;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:1.5"/>
+
+  <!-- Левая стрелка -->
+  <path d="M5.501928,7.5L2.497928,4.504L2.501928,10.5L5.501928,7.5Z" style="fill:#949494;stroke:#949494;stroke-width:1px;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:1.5"/>
+</svg>

resources/images/menu_precise_seam_blocked.svg

@@ -0,0 +1,28 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Левые пунктирные линии -->
+  <rect x="2" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Правые пунктирные линии -->
+  <rect x="12" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Круг (blocked seam marker) -->
+  <circle cx="7.5" cy="7.5" r="3.5" style="fill:#B16F6D"/>
+
+  <!-- Горизонтальная линия (минус) -->
+  <rect x="5" y="7" width="5" height="1" style="fill:#fff"/>
+</svg>

resources/images/menu_precise_seam_center.svg

@@ -0,0 +1,19 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Центральные пунктирные линии -->
+  <rect x="7" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Правая стрелка -->
+  <path d="M9.5,7.5L12.504,4.504L12.5,10.5L9.5,7.5Z" style="fill:#949494;stroke:#949494;stroke-width:1px;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:1.5"/>
+
+  <!-- Левая стрелка -->
+  <path d="M5.501928,7.5L2.497928,4.504L2.501928,10.5L5.501928,7.5Z" style="fill:#949494;stroke:#949494;stroke-width:1px;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:1.5"/>
+</svg>

resources/images/menu_precise_seam_enforced.svg

@@ -0,0 +1,29 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Левые пунктирные линии -->
+  <rect x="2" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Правые пунктирные линии -->
+  <rect x="12" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Круг (enforced seam marker) -->
+  <circle cx="7.5" cy="7.5" r="3.5" style="fill:#009688"/>
+
+  <!-- Крест внутри круга -->
+  <rect x="7" y="5" width="1" height="5" style="fill:#fff"/>
+  <rect x="5" y="7" width="5" height="1" style="fill:#fff"/>
+</svg>

resources/images/menu_precise_seam_left.svg

@@ -0,0 +1,16 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Центральные пунктирные линии -->
+  <rect x="7" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Правая стрелка -->
+  <path d="M9.5,7.5L12.504,4.504L12.5,10.5L9.5,7.5Z" style="fill:#949494;stroke:#949494;stroke-width:1px;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:1.5"/>
+</svg>

resources/images/menu_precise_seam_neutral.svg

@@ -0,0 +1,25 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Левые пунктирные линии -->
+  <rect x="2" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Правые пунктирные линии -->
+  <rect x="12" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Круг (neutral seam marker) -->
+  <circle cx="7.5" cy="7.5" r="3.5" style="fill:#949494"/>
+</svg>

resources/images/menu_precise_seam_right.svg

@@ -0,0 +1,16 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Центральные пунктирные линии -->
+  <rect x="7" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="7" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Левая стрелка -->
+  <path d="M5.501928,7.5L2.497928,4.504L2.501928,10.5L5.501928,7.5Z" style="fill:#949494;stroke:#949494;stroke-width:1px;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:1.5"/>
+</svg>

resources/images/menu_precise_seam_type.svg

@@ -0,0 +1,25 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Рамка -->
+  <path d="M0.5,13.5L0.5,1.5C0.5,0.951,0.951,0.5,1.5,0.5L13.5,0.5C14.049,0.5,14.5,0.951,14.5,1.5L14.5,13.5C14.5,14.049,14.049,14.5,13.5,14.5L1.5,14.5C0.951,14.5,0.5,14.049,0.5,13.5Z" style="fill:none;stroke:#949494;stroke-width:1px"/>
+
+  <!-- Левые пунктирные линии -->
+  <rect x="2" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="2" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Правые пунктирные линии -->
+  <rect x="12" y="1" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="3" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="5" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="7" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="9" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="11" width="1" height="1" style="fill:#009688"/>
+  <rect x="12" y="13" width="1" height="1" style="fill:#009688"/>
+
+  <!-- Галочка (checkmark) -->
+  <path d="M5.086213,7.556L6.953213,9.867L10.420213,4.444" style="fill:none;stroke:#949494;stroke-width:1.41px;stroke-linecap:round;stroke-linejoin:round"/>
+</svg>

src/libslic3r/Brim.cpp

@@ -131,6 +131,7 @@ double getadhesionCoeff(const PrintObject* printObject)
     }
     double adhesionCoeff = 1;
     for (const ModelVolume* modelVolume : objectVolumes) {
+        if (modelVolume->is_precise_seam()) continue; // non-printing helper geometry
         for (auto iter = extrudersFirstLayer.begin(); iter != extrudersFirstLayer.end(); iter++) {
             if (modelVolume->extruder_id() == *iter) {
                 if (Model::extruderParamsMap.find(modelVolume->extruder_id()) != Model::extruderParamsMap.end()) {

src/libslic3r/CMakeLists.txt

@@ -259,6 +259,8 @@ set(lisbslic3r_sources
     GCode/RetractWhenCrossingPerimeters.hpp
     GCode/SeamPlacer.cpp
     GCode/SeamPlacer.hpp
+    GCode/PreciseSeam.cpp
+    GCode/PreciseSeam.hpp
     #GCodeSender.cpp
     #GCodeSender.hpp
     GCode/SmallAreaInfillFlowCompensator.cpp

src/libslic3r/Format/3mf.cpp

@@ -125,6 +125,10 @@ static constexpr const char* VOLUME_TYPE = "volume";
 static constexpr const char* NAME_KEY = "name";
 static constexpr const char* MODIFIER_KEY = "modifier";
 static constexpr const char* VOLUME_TYPE_KEY = "volume_type";
+// Keep seam modes separate from the base type so older readers see a non-printing modifier.
+static constexpr const char* PRECISE_SEAM_TYPE_KEY = "precise_seam_type";
+// Preserve dormant settings without turning an older reader's modifier into an active override.
+static constexpr char PRECISE_SEAM_CONFIG_PREFIX[] = "precise_seam_config:";
 static constexpr const char* MATRIX_KEY = "matrix";
 static constexpr const char* SOURCE_FILE_KEY = "source_file";
 static constexpr const char* SOURCE_OBJECT_ID_KEY = "source_object_id";
@@ -368,7 +372,14 @@ ModelVolumeType type_from_string(const std::string &s)
     if (s == "ParameterModifier") return ModelVolumeType::PARAMETER_MODIFIER;
     if (s == "SupportEnforcer") return ModelVolumeType::SUPPORT_ENFORCER;
     if (s == "SupportBlocker") return ModelVolumeType::SUPPORT_BLOCKER;
-    // Default value if invalud type string received.
+    // Precise Seam types (snake_case strings from ModelVolume::type_to_string)
+    if (s == "precise_seam_center")   return ModelVolumeType::PRECISE_SEAM_CENTER;
+    if (s == "precise_seam_left")     return ModelVolumeType::PRECISE_SEAM_LEFT;
+    if (s == "precise_seam_right")    return ModelVolumeType::PRECISE_SEAM_RIGHT;
+    if (s == "precise_seam_enforced") return ModelVolumeType::PRECISE_SEAM_ENFORCED;
+    if (s == "precise_seam_blocked")  return ModelVolumeType::PRECISE_SEAM_BLOCKED;
+    if (s == "precise_seam_neutral")  return ModelVolumeType::PRECISE_SEAM_NEUTRAL;
+    // Default value if invalid type string received.
     return ModelVolumeType::MODEL_PART;
 }
 
@@ -2035,6 +2046,7 @@ ModelVolumeType type_from_string(const std::string &s)
         std::vector<std::string> valid_keys = {
             "name",
             "volume_type",
+            PRECISE_SEAM_TYPE_KEY,
             "matrix",
             "source_file",
             "source_object_id",
@@ -2047,7 +2059,7 @@ ModelVolumeType type_from_string(const std::string &s)
         };
 
         auto itor = std::find(valid_keys.begin(), valid_keys.end(), key);
-        if (itor == valid_keys.end()) {
+        if (itor == valid_keys.end() && !(type == VOLUME_TYPE && boost::starts_with(key, PRECISE_SEAM_CONFIG_PREFIX))) {
             // do nothing if not valid keys
             return true;
         }
@@ -2180,6 +2192,8 @@ ModelVolumeType type_from_string(const std::string &s)
             volume->mmu_segmentation_facets.shrink_to_fit();
             volume->fuzzy_skin_facets.shrink_to_fit();
 
+            // Apply the seam mode after all base-type metadata, regardless of XML key order.
+            ModelVolumeType precise_seam_type = ModelVolumeType::INVALID;
             // apply the remaining volume's metadata
             for (const Metadata& metadata : volume_data.metadata) {
                 if (metadata.key == NAME_KEY)
@@ -2188,6 +2202,10 @@ ModelVolumeType type_from_string(const std::string &s)
 					volume->set_type(ModelVolumeType::PARAMETER_MODIFIER);
                 else if (metadata.key == VOLUME_TYPE_KEY)
                     volume->set_type(type_from_string(metadata.value));
+                else if (metadata.key == PRECISE_SEAM_TYPE_KEY)
+                    precise_seam_type = ModelVolume::type_from_string(metadata.value);
+                else if (boost::starts_with(metadata.key, PRECISE_SEAM_CONFIG_PREFIX))
+                    continue; // Restore dormant settings only after the final volume type is known.
                 else if (metadata.key == SOURCE_FILE_KEY)
                     volume->source.input_file = metadata.value;
                 else if (metadata.key == SOURCE_OBJECT_ID_KEY)
@@ -2208,6 +2226,22 @@ ModelVolumeType type_from_string(const std::string &s)
                     volume->config.set_deserialize(metadata.key, metadata.value, config_substitutions);
             }
 
+            // Missing or unknown seam modes retain the ordinary modifier fallback.
+            // Ignore seam metadata on other base types; legacy inline seam types still load above.
+            if (volume->is_modifier() && is_precise_seam(precise_seam_type))
+                volume->set_type(precise_seam_type);
+
+            // Unknown seam modes must remain inert modifiers, even when dormant settings are present.
+            if (volume->is_precise_seam()) {
+                for (const Metadata& metadata : volume_data.metadata) {
+                    if (boost::starts_with(metadata.key, PRECISE_SEAM_CONFIG_PREFIX)) {
+                        const std::string key = metadata.key.substr(sizeof(PRECISE_SEAM_CONFIG_PREFIX) - 1);
+                        if (!key.empty())
+                            volume->config.set_deserialize(key, metadata.value, config_substitutions);
+                    }
+                }
+            }
+
             // this may happen for 3mf saved by 3rd part softwares
             if (volume->name.empty()) {
                 volume->name = object.name;
@@ -3123,10 +3157,17 @@ ModelVolumeType type_from_string(const std::string &s)
                                 stream << "   <" << METADATA_TAG << " " << TYPE_ATTR << "=\"" << VOLUME_TYPE << "\" " << KEY_ATTR << "=\"" << NAME_KEY << "\" " << VALUE_ATTR << "=\"" << xml_escape(volume->name) << "\"/>\n";
 
                             // stores volume's modifier field (legacy, to support old slicers)
-                            if (volume->is_modifier())
+                            // Readers with only the legacy flag still see helper geometry as a modifier.
+                            if (volume->is_modifier() || volume->is_precise_seam())
                                 stream << "   <" << METADATA_TAG << " " << TYPE_ATTR << "=\"" << VOLUME_TYPE << "\" " << KEY_ATTR << "=\"" << MODIFIER_KEY << "\" " << VALUE_ATTR << "=\"1\"/>\n";
-                            // stores volume's type (overrides the modifier field above)
+                            // This Prusa-format reader uses ParameterModifier, not Bambu's modifier_part.
+                            // The base type overrides the legacy flag, so it must also be backward-compatible.
+                            // Use the same spelling for ordinary modifiers, including a downgraded seam helper.
+                            const bool store_as_modifier = volume->is_modifier() || volume->is_precise_seam();
                             stream << "   <" << METADATA_TAG << " " << TYPE_ATTR << "=\"" << VOLUME_TYPE << "\" " << KEY_ATTR << "=\"" << VOLUME_TYPE_KEY << "\" " <<
+                                VALUE_ATTR << "=\"" << (store_as_modifier ? "ParameterModifier" : ModelVolume::type_to_string(volume->type())) << "\"/>\n";
+                            if (volume->is_precise_seam())
+                                stream << "   <" << METADATA_TAG << " " << TYPE_ATTR << "=\"" << VOLUME_TYPE << "\" " << KEY_ATTR << "=\"" << PRECISE_SEAM_TYPE_KEY << "\" " <<
                                     VALUE_ATTR << "=\"" << ModelVolume::type_to_string(volume->type()) << "\"/>\n";
 
                             // stores volume's local matrix
@@ -3162,7 +3203,12 @@ ModelVolumeType type_from_string(const std::string &s)
 
                             // stores volume's config data
                             for (const std::string& key : volume->config.keys()) {
-                                stream << "   <" << METADATA_TAG << " " << TYPE_ATTR << "=\"" << VOLUME_TYPE << "\" " << KEY_ATTR << "=\"" << key << "\" " << VALUE_ATTR << "=\"" << volume->config.opt_serialize(key) << "\"/>\n";
+                                // Seam settings are inactive but must survive changing the helper back into a part/modifier.
+                                const bool dormant = volume->is_precise_seam();
+                                const std::string stored_key = dormant ? PRECISE_SEAM_CONFIG_PREFIX + key : key;
+                                const std::string value = volume->config.opt_serialize(key);
+                                // Config serialization is C-style, not XML: escape active settings too, including tabs.
+                                stream << "   <" << METADATA_TAG << " " << TYPE_ATTR << "=\"" << VOLUME_TYPE << "\" " << KEY_ATTR << "=\"" << stored_key << "\" " << VALUE_ATTR << "=\"" << xml_escape_double_quotes_attribute_value(value) << "\"/>\n";
                             }
 
                             // stores mesh's statistics

src/libslic3r/Format/bbs_3mf.cpp

@@ -353,6 +353,10 @@ static constexpr const char* PART_TYPE = "part";
 static constexpr const char* NAME_KEY = "name";
 static constexpr const char* VOLUME_TYPE_KEY = "volume_type";
 static constexpr const char* PART_TYPE_KEY = "part_type";
+// Keep seam modes separate from the base type so older readers see a non-printing modifier.
+static constexpr const char* PRECISE_SEAM_TYPE_KEY = "precise_seam_type";
+// Preserve dormant settings without turning an older reader's modifier into an active override.
+static constexpr char PRECISE_SEAM_CONFIG_PREFIX[] = "precise_seam_config:";
 static constexpr const char* MATRIX_KEY = "matrix";
 static constexpr const char* SOURCE_FILE_KEY = "source_file";
 static constexpr const char* SOURCE_OBJECT_ID_KEY = "source_object_id";
@@ -5212,6 +5216,8 @@ void PlateData::parse_filament_info(GCodeProcessorResult *result)
             if (auto &tc = volume_data->text_configuration; tc.has_value())
                 volume->text_configuration = std::move(tc);
 
+            // Apply the seam mode after all base-type metadata, regardless of XML key order.
+            ModelVolumeType precise_seam_type = ModelVolumeType::INVALID;
             // apply the remaining volume's metadata
             for (const Metadata& metadata : volume_data->metadata) {
                 if (metadata.key == NAME_KEY)
@@ -5221,6 +5227,10 @@ void PlateData::parse_filament_info(GCodeProcessorResult *result)
 				//for old format
                 else if ((metadata.key == VOLUME_TYPE_KEY) || (metadata.key == PART_TYPE_KEY))
                     volume->set_type(ModelVolume::type_from_string(metadata.value));
+                else if (metadata.key == PRECISE_SEAM_TYPE_KEY)
+                    precise_seam_type = ModelVolume::type_from_string(metadata.value);
+                else if (boost::starts_with(metadata.key, PRECISE_SEAM_CONFIG_PREFIX))
+                    continue; // Restore dormant settings only after the final volume type is known.
                 else if (metadata.key == SOURCE_FILE_KEY)
                     volume->source.input_file = metadata.value;
                 else if (metadata.key == SOURCE_OBJECT_ID_KEY)
@@ -5243,6 +5253,22 @@ void PlateData::parse_filament_info(GCodeProcessorResult *result)
                     volume->config.set_deserialize(metadata.key, metadata.value, config_substitutions);
             }
 
+            // Missing or unknown seam modes retain the ordinary modifier fallback.
+            // Ignore seam metadata on other base types; legacy inline seam types still load above.
+            if (volume->is_modifier() && is_precise_seam(precise_seam_type))
+                volume->set_type(precise_seam_type);
+
+            // Unknown seam modes must remain inert modifiers, even when dormant settings are present.
+            if (volume->is_precise_seam()) {
+                for (const Metadata& metadata : volume_data->metadata) {
+                    if (boost::starts_with(metadata.key, PRECISE_SEAM_CONFIG_PREFIX)) {
+                        const std::string key = metadata.key.substr(sizeof(PRECISE_SEAM_CONFIG_PREFIX) - 1);
+                        if (!key.empty())
+                            volume->config.set_deserialize(key, metadata.value, config_substitutions);
+                    }
+                }
+            }
+
             // this may happen for 3mf saved by 3rd part softwares
             if (volume->name.empty()) {
                 volume->name = object.name;
@@ -8002,7 +8028,12 @@ void PlateData::parse_filament_info(GCodeProcessorResult *result)
                                 volume_id = m_volume_paths.find(volume)->second.second;
                             stream << ID_ATTR << "=\"" << volume_id << "\" ";
 
-                            stream << SUBTYPE_ATTR << "=\"" << ModelVolume::type_to_string(volume->type()) << "\">\n";
+                            // Older slicers must recognize the base type even when they ignore seam metadata.
+                            const ModelVolumeType stored_type = volume->is_precise_seam() ? ModelVolumeType::PARAMETER_MODIFIER : volume->type();
+                            stream << SUBTYPE_ATTR << "=\"" << ModelVolume::type_to_string(stored_type) << "\">\n";
+                            if (volume->is_precise_seam())
+                                stream << "      <" << METADATA_TAG << " " << KEY_ATTR << "=\"" << PRECISE_SEAM_TYPE_KEY << "\" " << VALUE_ATTR << "=\"" <<
+                                    ModelVolume::type_to_string(volume->type()) << "\"/>\n";
                             //stream << "    <" << PART_TAG << " " << ID_ATTR << "=\"" << it->second << "\" " << SUBTYPE_ATTR << "=\"" << ModelVolume::type_to_string(volume->type()) << "\">\n";
 
                             // stores volume's name
@@ -8052,7 +8083,12 @@ void PlateData::parse_filament_info(GCodeProcessorResult *result)
 
                             // stores volume's config data
                             for (const std::string& key : volume->config.keys()) {
-                                stream << "      <" << METADATA_TAG << " "<< KEY_ATTR << "=\"" << key << "\" " << VALUE_ATTR << "=\"" << volume->config.opt_serialize(key) << "\"/>\n";
+                                // Seam settings are inactive but must survive changing the helper back into a part/modifier.
+                                const bool dormant = volume->is_precise_seam();
+                                const std::string stored_key = dormant ? PRECISE_SEAM_CONFIG_PREFIX + key : key;
+                                const std::string value = volume->config.opt_serialize(key);
+                                // Config serialization is C-style, not XML: escape active settings too, including tabs.
+                                stream << "      <" << METADATA_TAG << " "<< KEY_ATTR << "=\"" << stored_key << "\" " << VALUE_ATTR << "=\"" << xml_escape_double_quotes_attribute_value(value) << "\"/>\n";
                             }
 
                             if (const std::optional<EmbossShape> &es = volume->emboss_shape; es.has_value()) {

src/libslic3r/GCode/PreciseSeam.hpp

@@ -0,0 +1,146 @@
+#ifndef slic3r_PreciseSeam_hpp_
+#define slic3r_PreciseSeam_hpp_
+
+#include <atomic>
+#include <optional>
+#include <vector>
+#include <unordered_map>
+#include "libslic3r/Polygon.hpp"
+#include "libslic3r/Polyline.hpp"
+#include "libslic3r/Model.hpp"
+#include "libslic3r/Layer.hpp"
+#include "libslic3r/Print.hpp"
+#include "libslic3r/ClipperUtils.hpp"
+#include "SeamPlacer.hpp"
+
+// CURRENT STATUS:
+// Strong modifiers (Center/Left/Right): only one intersection per perimeter is supported,
+// since there can be only one seam. Additional intersections are ignored.
+//
+// Weak modifiers (Enforced/Blocked/Neutral): multiple intersections are supported,
+// but none of them should pass through the model entirely. A through-body intersection
+// produces multiple segments, of which only one will be processed.
+//
+// In both cases, a pop-up warning is shown when unsupported intersections are detected.
+//
+// If any modifier has a multiply-connected cross-section (e.g. a hollow shape),
+// it is skipped and a corresponding notification is shown.
+//
+// Full containment of the perimeter within modifier is not handled.
+//
+// FUTURE DIRECTION:
+// A lightweight algorithm is needed to detect and handle through-body intersections
+// for Weak modifiers. The algorithm must not slow down the 99.9% common case.
+// Possible approach: if intersection passes the diff check (no through-body),
+// use the current fast algorithm. If diff check fails, fall back to a heavier
+// method: compute midpoints of intersection polygon edges, then check which
+// midpoints lie strictly inside the modifier (not on boundary) using
+// point_in_polygon. Those edges originate from the perimeter; the rest
+// originate from the modifier boundary. Collect perimeter edges into a polyline.
+// Additionally, multiply-connected cross-sections could be supported instead of
+// being skipped entirely (e.g. by decomposing them into simple polygons).
+// For Enforced and Neutral weak modifiers, full containment of the perimeter
+// within modifier could be handled (currently ignored).
+
+namespace Slic3r {
+namespace PreciseSeam {
+
+// Import EnforcedBlockedSeamPoint from SeamPlacerImpl namespace for convenience
+using SeamPlacerImpl::EnforcedBlockedSeamPoint;
+
+// Pre-sliced modifier cache: ModelVolume pointer → per-layer Polygons.
+// Built once in SeamPlacer::init(), then passed read-only into per-perimeter functions.
+using ModifierSlicesCache = std::unordered_map<const ModelVolume*, std::vector<Polygons>>;
+
+// Warning flags set during Precise Seam processing (thread-safe)
+struct PreciseSeamWarnings {
+    std::atomic<bool> multiple_intersections{false};  // modifier intersects perimeter in multiple separate places (strong only)
+    std::atomic<bool> through_body{false};            // modifier passes through the model body entirely
+    std::atomic<bool> multiply_connected{false};      // modifier has holes (multiply-connected cross-section)
+    std::atomic<bool> full_containment{false};        // modifier fully contains perimeter, no intersection edges
+};
+
+// Result of finding common segment between perimeter and intersection
+struct SegmentData {
+    Polyline segment;                         // Points from intersection_polygon forming the segment
+    std::vector<size_t> perimeter_edge_indices; // edge_index for each point in segment
+};
+
+// Result of weak modifier segment processing
+struct WeakModifierSegment {
+    EnforcedBlockedSeamPoint type;  // Enforced/Blocked/Neutral
+    Point left_point;               // Coordinates of left (first) point of segment
+    size_t left_idx;                // Perimeter vertex index for left_point
+    Point right_point;              // Coordinates of right (last) point of segment
+    size_t right_idx;               // Perimeter vertex index for right_point
+};
+
+// Initialize Precise Seam data by populating provided vectors and flag
+// Collects precise seam modifiers and fills output parameters
+// Call once during SeamPlacer::init() before gather_seam_candidates()
+// Parameters:
+//   strong_volumes_out - output vector for strong modifiers (CENTER/LEFT/RIGHT)
+//   weak_volumes_out   - output vector for weak modifiers (ENFORCED/BLOCKED/NEUTRAL)
+//   has_strong_out     - output flag indicating presence of strong modifiers
+//   model_object       - model object containing volumes
+void init_precise_seam_data(
+    std::vector<const ModelVolume*>& strong_volumes_out,
+    std::vector<const ModelVolume*>& weak_volumes_out,
+    bool& has_strong_out,
+    const ModelObject* model_object);
+
+// Insert strong seam point into perimeter polygon
+// Processes strong modifiers (CENTER/LEFT/RIGHT) and inserts seam point into polygon
+// Parameters:
+//   strong_volumes    - list of strong precise seam modifiers
+//   polygon           - perimeter polygon (will be modified if point inserted)
+//   layer             - current layer
+//   slices_cache      - pre-sliced modifier polygons (built once in SeamPlacer::init)
+// Returns:
+//   Coordinates of inserted point (internal units) or std::nullopt if nothing inserted
+std::optional<Point> insert_strong_seam_point(
+    const std::vector<const ModelVolume*> &strong_volumes,
+    Polygon &polygon,
+    const Layer *layer,
+    const ModifierSlicesCache &slices_cache,
+    PreciseSeamWarnings* warnings = nullptr);
+
+// Collect all weak modifier segments for a perimeter polygon
+// Processes weak modifiers (ENFORCED/BLOCKED/NEUTRAL) and collects segment boundaries
+// Also inserts boundary points into the perimeter polygon (sorted by descending arc length)
+// Refines enforced edges by subdividing them into segments ≤ enforcer_oversampling_distance
+// Parameters:
+//   weak_volumes      - list of weak precise seam modifiers
+//   polygon           - perimeter polygon (will be modified with inserted points and refined edges)
+//   layer             - current layer
+//   slices_cache      - pre-sliced modifier polygons (built once in SeamPlacer::init)
+// Returns:
+//   Ordered vector of segments with updated coordinates (same order as weak_volumes list)
+std::vector<WeakModifierSegment> collect_weak_modifier_segments(
+    const std::vector<const ModelVolume*> &weak_volumes,
+    Polygon &polygon,
+    const Layer *layer,
+    const ModifierSlicesCache &slices_cache,
+    PreciseSeamWarnings* warnings = nullptr);
+
+// Apply weak modifier types to perimeter points based on segment boundaries
+// Finds boundary points in refined polygon and sets types for points within segments
+// Parameters:
+//   weak_segments       - segments with boundary coordinates and types
+//   result              - layer seams data to modify
+//   perimeter           - perimeter info (start/end indices)
+//   some_point_enforced - flag to update if Enforced points are set
+void apply_weak_modifiers_to_perimeter(
+    const std::vector<WeakModifierSegment> &weak_segments,
+    PrintObjectSeamData::LayerSeams &result,
+    const SeamPlacerImpl::Perimeter &perimeter,
+    bool &some_point_enforced);
+
+// Restore precise seam positions that may have been modified by alignment
+// Iterates through all perimeters and restores precise_seam_point positions
+void restore_precise_seam_positions(std::vector<PrintObjectSeamData::LayerSeams> &layers);
+
+} // namespace PreciseSeam
+} // namespace Slic3r
+
+#endif // slic3r_PreciseSeam_hpp_

src/libslic3r/GCode/SeamPlacer.cpp

@@ -9,6 +9,7 @@
 #include <random>
 #include <algorithm>
 #include <queue>
+#include <unordered_map>
 
 #include "libslic3r/AABBTreeLines.hpp"
 #include "libslic3r/KDTreeIndirect.hpp"
@@ -19,10 +20,12 @@
 #include "libslic3r/Layer.hpp"
 
 #include "libslic3r/Geometry/Curves.hpp"
+#include "libslic3r/I18N.hpp"
 #include "libslic3r/ShortEdgeCollapse.hpp"
 #include "libslic3r/TriangleSetSampling.hpp"
 
 #include "libslic3r/Utils.hpp"
+#include "PreciseSeam.hpp"
 
 //#define DEBUG_FILES
 
@@ -303,6 +306,16 @@ struct GlobalModelInfo {
   AABBTreeIndirect::Tree<3, float> enforcers_tree;
   AABBTreeIndirect::Tree<3, float> blockers_tree;
 
+  // Precise Seam modifiers: strong modifiers (CENTER/LEFT/RIGHT) determine exact seam placement
+  std::vector<const ModelVolume*> precise_seam_strong_volumes;
+  // Precise Seam modifiers: weak modifiers (ENFORCED/BLOCKED/NEUTRAL) provide hints for seam placement
+  std::vector<const ModelVolume*> precise_seam_weak_volumes;
+
+  // Pre-sliced modifier polygons, keyed by ModelVolume pointer.
+  // Populated once in SeamPlacer::init() to avoid re-slicing on every perimeter.
+  // Each value is a per-layer vector of Polygons for that modifier volume.
+  std::unordered_map<const ModelVolume*, std::vector<Polygons>> precise_seam_slices;
+
   bool is_enforced(const Vec3f &position, float radius) const {
     if (enforcers.empty()) {
       return false;
@@ -403,7 +416,8 @@ struct GlobalModelInfo {
 ;
 
 //Extract perimeter polygons of the given layer
-Polygons extract_perimeter_polygons(const Layer *layer, std::vector<const LayerRegion*> &corresponding_regions_out) {
+Polygons extract_perimeter_polygons(const Layer *layer, std::vector<const LayerRegion*> &corresponding_regions_out,
+                                   bool has_precise_seam_modifiers) {
   Polygons polygons;
   for (const LayerRegion *layer_region : layer->regions()) {
     for (const ExtrusionEntity *ex_entity : layer_region->perimeters.entities) {
@@ -440,6 +454,18 @@ Polygons extract_perimeter_polygons(const Layer *layer, std::vector<const LayerR
     }
   }
 
+  if (has_precise_seam_modifiers) {
+    // Extrusion loops repeat their start point; Polygon closes the contour implicitly.
+    // Normalize here for Precise Seam without changing ordinary seam candidates.
+    for (Polygon &polygon : polygons) {
+      // Adjacent extrusion paths share endpoints; zero-length edges would prevent refinement at their junctions.
+      // Remove only consecutive duplicates, preserving distinct visits to a self-touching contour point.
+      polygon.points.erase(std::unique(polygon.points.begin(), polygon.points.end()), polygon.points.end());
+      while (polygon.size() > 1 && polygon.points.front() == polygon.points.back())
+        polygon.points.pop_back();
+    }
+  }
+
   if (polygons.empty()) { // If there are no perimeter polygons for whatever reason (disabled perimeters .. ) insert dummy point
     // it is easier than checking everywhere if the layer is not emtpy, no seam will be placed to this layer anyway
     polygons.emplace_back(Points{ { 0, 0 } });
@@ -449,17 +475,42 @@ Polygons extract_perimeter_polygons(const Layer *layer, std::vector<const LayerR
   return polygons;
 }
 
-// Insert SeamCandidates created from perimeter polygons in to the result vector.
+// Build SeamCandidates for each vertex of the perimeter polygon and attach them to a shared Perimeter.
-// Compute its type (Enfrocer,Blocker), angle, and position
+// For each vertex: computes position, angle, and type (Enforcer / Blocker / Neutral).
-//each SeamCandidate also contains pointer to shared Perimeter structure representing the polygon
+// When Precise Seam modifiers are present: inserts strong seam point,
-// if Custom Seam modifiers are present, oversamples the polygon if necessary to better fit user intentions
+// oversamples enforcer edges, applies weak modifiers, marks one enforced point as central for alignment.
 void process_perimeter_polygon(const Polygon &orig_polygon, float z_coord, const LayerRegion *region,
-                               const GlobalModelInfo &global_model_info, PrintObjectSeamData::LayerSeams &result) {
+                               const GlobalModelInfo &global_model_info, PrintObjectSeamData::LayerSeams &result,
+                               PreciseSeam::PreciseSeamWarnings* warnings = nullptr) {
   if (orig_polygon.size() == 0) {
     return;
   }
   Polygon polygon = orig_polygon;
   bool was_clockwise = polygon.make_counter_clockwise();
+
+  // Process Precise Seam modifiers to find seam placement
+  const Layer* layer = region ? region->layer() : nullptr;
+
+  // Use pre-computed Precise Seam volumes from global_model_info (computed once in init)
+  const auto& strong_volumes = global_model_info.precise_seam_strong_volumes;
+  const auto& weak_volumes = global_model_info.precise_seam_weak_volumes;
+
+  // Use pre-sliced cache from global_model_info instead of re-slicing on every call
+  auto seam_point = PreciseSeam::insert_strong_seam_point(strong_volumes, polygon, layer, global_model_info.precise_seam_slices, warnings);
+
+  // Store the inserted point position for marking as central_enforcer later
+  std::optional<Vec3f> inserted_seam_position;
+  if (seam_point.has_value()) {
+    Vec2f unscaled_p = unscale(seam_point.value()).cast<float>();
+    inserted_seam_position = Vec3f(unscaled_p.x(), unscaled_p.y(), z_coord);
+  }
+
+  // Process weak modifiers (ENFORCED/BLOCKED/NEUTRAL) only if no strong modifier was inserted
+  std::vector<PreciseSeam::WeakModifierSegment> weak_segments;
+  if (!inserted_seam_position.has_value()) {
+    weak_segments = PreciseSeam::collect_weak_modifier_segments(weak_volumes, polygon, layer, global_model_info.precise_seam_slices, warnings);
+  }
+
   float angle_arm_len = region != nullptr ? region->flow(FlowRole::frExternalPerimeter).nozzle_diameter() : 0.5f;
 
   std::vector<float> lengths { };
@@ -528,10 +579,15 @@ void process_perimeter_polygon(const Polygon &orig_polygon, float z_coord, const
 
   perimeter.end_index = result.points.size();
 
+  // Apply weak modifiers if no strong modifier was inserted
+  if (!inserted_seam_position.has_value() && !weak_segments.empty()) {
+    PreciseSeam::apply_weak_modifiers_to_perimeter(
+        weak_segments, result, perimeter, some_point_enforced);
+  }
+
   if (some_point_enforced) {
-    // We will patches of enforced points (patch: continuous section of enforced points), choose
+    // Choose the continuous enforced patch with the most candidates, then select its middle
-    // the longest patch, and select the middle point or sharp point (depending on the angle)
+    // candidate or a sharp corner. Patch length here is a point count, not geometric distance.
-    // this point will have high priority on this perimeter
     size_t perimeter_size = perimeter.end_index - perimeter.start_index;
     const auto next_index = [&](size_t idx) {
       return perimeter.start_index + Slic3r::next_idx_modulo(idx - perimeter.start_index, perimeter_size);
@@ -562,7 +618,9 @@ void process_perimeter_polygon(const Polygon &orig_polygon, float z_coord, const
       std::pair<size_t, size_t> longest_patch { 0, 0 };
       auto patch_len = [perimeter_size](const std::pair<size_t, size_t> &start_end) {
         if (start_end.second < start_end.first) {
-          return start_end.first + (perimeter_size - start_end.second);
+          // Count [start, end) across the closing edge, independently of the contour's start.
+          // Subtract indices first: they are offsets in the layer, not local perimeter indices.
+          return perimeter_size - (start_end.first - start_end.second);
         } else {
           return start_end.second - start_end.first;
         }
@@ -595,30 +653,24 @@ void process_perimeter_polygon(const Polygon &orig_polygon, float z_coord, const
     }
   }
 
-}
+  // Apply precise seam point if it was inserted
-
+  if (inserted_seam_position.has_value()) {
-// Get index of previous and next perimeter point of the layer. Because SeamCandidates of all polygons of the given layer
+    // Set single point as Enforced, block all others
-// are sequentially stored in the vector, each perimeter contains info about start and end index. These vales are used to
+    for (size_t i = perimeter.start_index; i < perimeter.end_index; ++i) {
-// deduce index of previous and next neigbour in the corresponding perimeter.
+      if (result.points[i].position == inserted_seam_position.value()) {
-std::pair<size_t, size_t> find_previous_and_next_perimeter_point(const std::vector<SeamCandidate> &perimeter_points,
+        // Mark as the single enforced point with highest priority
-                                                                 size_t point_index) {
+        result.points[i].type = EnforcedBlockedSeamPoint::Enforced;
-  const SeamCandidate &current = perimeter_points[point_index];
+        result.points[i].central_enforcer = true;
-  int prev = point_index - 1; //for majority of points, it is true that neighbours lie behind and in front of them in the vector
+        perimeter.precise_seam_point = inserted_seam_position;
-  int next = point_index + 1;
+        perimeter.precise_seam_index = i;
-
+      } else {
-  if (point_index == current.perimeter.start_index) {
+        // Block all other points
-    // if point_index is equal to start, it means that the previous neighbour is at the end
+        result.points[i].type = EnforcedBlockedSeamPoint::Blocked;
-    prev = current.perimeter.end_index;
+        result.points[i].central_enforcer = false;
+      }
+    }
   }
 
-  if (point_index == current.perimeter.end_index - 1) {
-    // if point_index is equal to end, than next neighbour is at the start
-    next = current.perimeter.start_index;
-  }
-
-  assert(prev >= 0);
-  assert(next >= 0);
-  return {size_t(prev),size_t(next)};
 }
 
 // Computes all global model info - transforms object, performs raycasting
@@ -636,7 +688,9 @@ void compute_global_occlusion(GlobalModelInfo &result, const PrintObject *po,
         || model_volume->type() == ModelVolumeType::NEGATIVE_VOLUME) {
       auto model_transformation = model_volume->get_matrix();
       indexed_triangle_set model_its = model_volume->mesh().its;
-      // ORCA: Mirrored transforms flip winding, keep normals outward
+      // ORCA fix (not related to Precise Seam, discovered during its development):
+      // Mirror transforms have negative determinant which flips triangle winding.
+      // fix_left_handed=true swaps indices to keep normals pointing outward.
       its_transform(model_its, model_transformation, true);
       if (model_volume->type() == ModelVolumeType::MODEL_PART) {
         its_merge(triangle_set, model_its);
@@ -657,7 +711,10 @@ void compute_global_occlusion(GlobalModelInfo &result, const PrintObject *po,
 
   size_t negative_volumes_start_index = triangle_set.indices.size();
   its_merge(triangle_set, negative_volumes_set);
-  // ORCA: Mirroring flips normals, keep them outward for visibility sampling
+  // ORCA fix (not related to Precise Seam, discovered during its development):
+  // Object-level transform may include mirroring (negative determinant),
+  // which inverts triangle winding. fix_left_handed=true corrects this
+  // so visibility ray sampling sees outward-facing normals.
   its_transform(triangle_set, obj_transform, true);
   BOOST_LOG_TRIVIAL(debug)
       << "SeamPlacer: decimate: end";
@@ -719,12 +776,12 @@ void gather_enforcers_blockers(GlobalModelInfo &result, const PrintObject *po) {
       auto model_transformation = obj_transform * mv->get_matrix();
 
       indexed_triangle_set enforcers = mv->seam_facets.get_facets(*mv, EnforcerBlockerType::ENFORCER);
-      // ORCA: Keep normals outward when mirroring seam enforcers
+      // ORCA fix (not related to Precise Seam): fix winding for mirrored transforms
       its_transform(enforcers, model_transformation, true);
       its_merge(result.enforcers, enforcers);
 
       indexed_triangle_set blockers = mv->seam_facets.get_facets(*mv, EnforcerBlockerType::BLOCKER);
-      // ORCA: Keep normals outward when mirroring seam blockers
+      // ORCA fix (not related to Precise Seam): fix winding for mirrored transforms
       its_transform(blockers, model_transformation, true);
       its_merge(result.blockers, blockers);
     }
@@ -1011,13 +1068,14 @@ void pick_random_seam_point(const std::vector<SeamCandidate> &perimeter_points,
 // Parallel process and extract each perimeter polygon of the given print object.
 // Gather SeamCandidates of each layer into vector and build KDtree over them
 // Store results in the SeamPlacer variables m_seam_per_object
-void SeamPlacer::gather_seam_candidates(const PrintObject *po, const SeamPlacerImpl::GlobalModelInfo &global_model_info) {
+void SeamPlacer::gather_seam_candidates(const PrintObject *po, const SeamPlacerImpl::GlobalModelInfo &global_model_info,
+                                        PreciseSeam::PreciseSeamWarnings* warnings) {
   using namespace SeamPlacerImpl;
   PrintObjectSeamData &seam_data = m_seam_per_object.emplace(po, PrintObjectSeamData { }).first->second;
   seam_data.layers.resize(po->layer_count());
 
   tbb::parallel_for(tbb::blocked_range<size_t>(0, po->layers().size()),
-                    [po, &global_model_info, &seam_data]
+                    [po, &global_model_info, &seam_data, warnings]
                     (tbb::blocked_range<size_t> r) {
                       for (size_t layer_idx = r.begin(); layer_idx < r.end(); ++layer_idx) {
                         PrintObjectSeamData::LayerSeams &layer_seams = seam_data.layers[layer_idx];
@@ -1025,10 +1083,13 @@ void SeamPlacer::gather_seam_candidates(const PrintObject *po, const SeamPlacerI
                         auto unscaled_z = layer->slice_z;
                         std::vector<const LayerRegion*> regions;
                         //NOTE corresponding region ptr may be null, if the layer has zero perimeters
-                        Polygons polygons = extract_perimeter_polygons(layer, regions);
+                        const bool has_precise_seam_modifiers = !global_model_info.precise_seam_strong_volumes.empty() ||
+                                                               !global_model_info.precise_seam_weak_volumes.empty();
+                        Polygons polygons = extract_perimeter_polygons(layer, regions, has_precise_seam_modifiers);
                         for (size_t poly_index = 0; poly_index < polygons.size(); ++poly_index) {
                           process_perimeter_polygon(polygons[poly_index], unscaled_z,
-                                                    regions[poly_index], global_model_info, layer_seams);
+                                                    regions[poly_index], global_model_info, layer_seams,
+                                                    warnings);
                         }
                         auto functor = SeamCandidateCoordinateFunctor { layer_seams.points };
                         seam_data.layers[layer_idx].points_tree =
@@ -1424,10 +1485,13 @@ void SeamPlacer::align_seam_points(const PrintObject *po, const SeamPlacerImpl::
 
 }
 
-void SeamPlacer::init(const Print &print, std::function<void(void)> throw_if_canceled_func) {
+void SeamPlacer::init(Print &print, std::function<void(void)> throw_if_canceled_func) {
   using namespace SeamPlacerImpl;
   m_seam_per_object.clear();
 
+  // Warning flags for Precise Seam processing — shared across all objects
+  PreciseSeam::PreciseSeamWarnings precise_seam_warnings;
+
   for (const PrintObject *po : print.objects()) {
     throw_if_canceled_func();
     SeamPosition configured_seam_preference = po->config().seam_position.value;
@@ -1436,14 +1500,29 @@ void SeamPlacer::init(const Print &print, std::function<void(void)> throw_if_can
     {
       GlobalModelInfo global_model_info { };
       gather_enforcers_blockers(global_model_info, po);
+      PreciseSeam::init_precise_seam_data(
+          global_model_info.precise_seam_strong_volumes,
+          global_model_info.precise_seam_weak_volumes,
+          m_seam_per_object[po].has_precise_seam_strong_volumes,
+          po->model_object());
+
+      // Pre-slice all precise seam modifier volumes once per object.
+      // Without this cache, slice_single_volume() would be called for every
+      // modifier × every perimeter × every layer — thousands of redundant slicing operations.
+      for (const ModelVolume* vol : global_model_info.precise_seam_strong_volumes)
+          global_model_info.precise_seam_slices[vol] = po->slice_single_volume(vol);
+      for (const ModelVolume* vol : global_model_info.precise_seam_weak_volumes)
+          global_model_info.precise_seam_slices[vol] = po->slice_single_volume(vol);
+
       throw_if_canceled_func();
       if (configured_seam_preference == spAligned || configured_seam_preference == spNearest || configured_seam_preference == spAlignedBack) {
         compute_global_occlusion(global_model_info, po, throw_if_canceled_func, configured_seam_preference);
       }
       throw_if_canceled_func();
+
       BOOST_LOG_TRIVIAL(debug)
           << "SeamPlacer: gather_seam_candidates: start";
-      gather_seam_candidates(po, global_model_info);
+      gather_seam_candidates(po, global_model_info, &precise_seam_warnings);
       BOOST_LOG_TRIVIAL(debug)
           << "SeamPlacer: gather_seam_candidates: end";
       throw_if_canceled_func();
@@ -1491,10 +1570,49 @@ void SeamPlacer::init(const Print &print, std::function<void(void)> throw_if_can
           << "SeamPlacer: align_seam_points : end";
     }
 
+    // Restore precise seam positions that were potentially modified
+    if (m_seam_per_object[po].has_precise_seam_strong_volumes) {
+      PreciseSeam::restore_precise_seam_positions(m_seam_per_object[po].layers);
+    }
+
 #ifdef DEBUG_FILES
     debug_export_points(m_seam_per_object[po].layers, po->bounding_box(), comparator);
 #endif
   }
+
+  // Show Precise Seam warnings (once for all objects).
+  // Only ONE active_step_add_warning() call — multiple calls generate multiple UI events,
+  // each re-pushing ALL current warnings via Plater handler, causing NotificationManager::append()
+  // to duplicate text within each popup.
+  {
+      const bool mi = precise_seam_warnings.multiple_intersections.load(std::memory_order_relaxed);
+      const bool tb = precise_seam_warnings.through_body.load(std::memory_order_relaxed);
+      const bool mc = precise_seam_warnings.multiply_connected.load(std::memory_order_relaxed);
+      const bool fc = precise_seam_warnings.full_containment.load(std::memory_order_relaxed);
+      std::vector<std::string> parts;
+      if (mi)
+          parts.push_back(_u8L("multiple intersections with a perimeter detected"));
+      if (tb)
+          parts.push_back(_u8L("modifier fully crosses the printable perimeter"));
+      if (mc)
+          parts.push_back(_u8L("modifier shape is not solid (has holes inside) and was ignored"));
+      if (fc)
+          parts.push_back(_u8L("perimeter is fully contained inside modifier and was ignored"));
+      if (!parts.empty()) {
+          // One line: the export warnings dialog shows only the first line of each warning.
+          std::string warning_text = _u8L("Precise Seam") + ": ";
+          for (size_t i = 0; i < parts.size(); ++i) {
+              if (i > 0) warning_text += "; ";
+              warning_text += parts[i];
+          }
+          warning_text += ". ";
+          warning_text += _u8L("Seam placement may differ from expected.");
+          print.active_step_add_warning(
+              PrintStateBase::WarningLevel::NON_CRITICAL,
+              warning_text,
+              PrintStateBase::SlicingPreciseSeamWarning);
+      }
+  }
 }
 
 void SeamPlacer::place_seam(const Layer *layer, ExtrusionLoop &loop,

src/libslic3r/GCode/SeamPlacer.hpp

@@ -18,6 +18,7 @@
 namespace Slic3r {
 
 class PrintObject;
+namespace PreciseSeam { struct PreciseSeamWarnings; }
 class ExtrusionLoop;
 class Print;
 class Layer;
@@ -41,7 +42,7 @@ enum class EnforcedBlockedSeamPoint {
 // struct representing single perimeter loop
 struct Perimeter {
   size_t start_index{};
-  size_t end_index{}; //inclusive!
+  size_t end_index{}; //exclusive (one-past-the-end)
   size_t seam_index{};
   float flow_width{};
 
@@ -50,6 +51,10 @@ struct Perimeter {
   // Random position also uses this flexibility to set final seam point position
   bool finalized = false;
   Vec3f final_seam_position = Vec3f::Zero();
+
+  // Stores precise seam coordinates found by Precise Seam modifiers
+  std::optional<Vec3f> precise_seam_point;
+  size_t precise_seam_index{};
 };
 
 //Struct over which all processing of perimeters is done. For each perimeter point, its respective candidate is created,
@@ -102,6 +107,9 @@ struct PrintObjectSeamData
   // Map of PrintObjects (PO) -> vector of layers of PO -> unique_ptr to KD
   // tree of all points of the given layer
 
+  // Indicates presence of strong Precise Seam modifiers (CENTER/LEFT/RIGHT) for this object
+  bool has_precise_seam_strong_volumes = false;
+
   void clear()
   {
     layers.clear();
@@ -141,11 +149,12 @@ public:
   //The following data structures hold all perimeter points for all PrintObject.
   std::unordered_map<const PrintObject*, PrintObjectSeamData> m_seam_per_object;
 
-  void init(const Print &print, std::function<void(void)> throw_if_canceled_func);
+  void init(Print &print, std::function<void(void)> throw_if_canceled_func);
 
   void place_seam(const Layer *layer, ExtrusionLoop &loop, const Point &last_pos, float& overhang) const;
 private:
-  void gather_seam_candidates(const PrintObject *po, const SeamPlacerImpl::GlobalModelInfo &global_model_info);
+  void gather_seam_candidates(const PrintObject *po, const SeamPlacerImpl::GlobalModelInfo &global_model_info,
+                              PreciseSeam::PreciseSeamWarnings* warnings = nullptr);
   void calculate_candidates_visibility(const PrintObject *po,
                                        const SeamPlacerImpl::GlobalModelInfo &global_model_info);
   void calculate_overhangs_and_layer_embedding(const PrintObject *po);

src/libslic3r/Model.cpp

@@ -23,6 +23,7 @@
 
 #include "libslic3r/Geometry/ConvexHull.hpp"
 
+#include <algorithm>
 #include <float.h>
 
 #include <boost/algorithm/string/predicate.hpp>
@@ -1229,6 +1230,7 @@ ModelObject& ModelObject::assign_copy(const ModelObject &rhs)
         this->volumes.emplace_back(new ModelVolume(*model_volume));
         this->volumes.back()->set_model_object(this);
     }
+
     this->clear_instances();
 	this->instances.reserve(rhs.instances.size());
     for (const ModelInstance *model_instance : rhs.instances) {
@@ -1267,6 +1269,7 @@ ModelObject& ModelObject::assign_copy(ModelObject &&rhs)
 	rhs.volumes.clear();
     for (ModelVolume *model_volume : this->volumes)
         model_volume->set_model_object(this);
+
     this->clear_instances();
 	this->instances = std::move(rhs.instances);
 	rhs.instances.clear();
@@ -1390,7 +1393,9 @@ ModelVolume* ModelObject::add_volume_with_shared_mesh(const ModelVolume &other,
 void ModelObject::delete_volume(size_t idx)
 {
     ModelVolumePtrs::iterator i = this->volumes.begin() + idx;
-    delete *i;
+    ModelVolume* volume_to_delete = *i;
+
+    delete volume_to_delete;
     this->volumes.erase(i);
 
     if (this->volumes.size() == 1)
@@ -1449,6 +1454,20 @@ void ModelObject::sort_volumes(bool full_sort)
     // sort volumes inside the object to order "Model Part, Negative Volume, Modifier, Support Blocker and Support Enforcer. "
     if (full_sort)
         std::stable_sort(volumes.begin(), volumes.end(), [](ModelVolume* vl, ModelVolume* vr) {
+            // Special handling for Precise Seam modifiers: group-based sorting with user order preservation
+            if (vl->is_precise_seam() && vr->is_precise_seam()) {
+                // Strong (center/left/right) always before weak (enforced/blocked/neutral)
+                bool vl_strong = vl->is_precise_seam_strong();
+                bool vr_strong = vr->is_precise_seam_strong();
+                if (vl_strong != vr_strong)
+                    return vl_strong; // strong < weak → strong group appears first
+
+                // Within same group (both strong or both weak): preserve current order
+                // stable_sort will maintain relative positions when comparator returns false
+                return false;
+            }
+
+            // For non-Precise-Seam or mixed types: use standard enum-based ordering
             return vl->type() < vr->type();
         });
     // sort have to controll "place" of the support blockers/enforcers. But one of the model parts have to be on the first place.
@@ -1456,10 +1475,19 @@ void ModelObject::sort_volumes(bool full_sort)
         std::stable_sort(volumes.begin(), volumes.end(), [](ModelVolume* vl, ModelVolume* vr) {
             ModelVolumeType vl_type = vl->type() > ModelVolumeType::PARAMETER_MODIFIER ? vl->type() : ModelVolumeType::PARAMETER_MODIFIER;
             ModelVolumeType vr_type = vr->type() > ModelVolumeType::PARAMETER_MODIFIER ? vr->type() : ModelVolumeType::PARAMETER_MODIFIER;
+
+            // Apply same Precise Seam grouping logic for partial sort
+            if (vl->is_precise_seam() && vr->is_precise_seam()) {
+                bool vl_strong = vl->is_precise_seam_strong();
+                bool vr_strong = vr->is_precise_seam_strong();
+                if (vl_strong != vr_strong)
+                    return vl_strong;
+                return false; // preserve order within same group
+            }
+
             return vl_type < vr_type;
         });
 }
-
 ModelInstance* ModelObject::add_instance()
 {
     ModelInstance* i = new ModelInstance(this);
@@ -2615,7 +2643,8 @@ std::vector<int> ModelVolume::get_extruders() const
     if (m_type == ModelVolumeType::INVALID
         || m_type == ModelVolumeType::NEGATIVE_VOLUME
         || m_type == ModelVolumeType::SUPPORT_BLOCKER
-        || m_type == ModelVolumeType::SUPPORT_ENFORCER)
+        || m_type == ModelVolumeType::SUPPORT_ENFORCER
+        || this->is_precise_seam()) // Precise Seam is non-printing helper geometry
         return std::vector<int>();
 
     if (mmu_segmentation_facets.timestamp() != mmuseg_ts) {
@@ -2819,6 +2848,19 @@ ModelVolumeType ModelVolume::type_from_string(const std::string &s)
 		return ModelVolumeType::SUPPORT_ENFORCER;
     if (s == "support_blocker")
 		return ModelVolumeType::SUPPORT_BLOCKER;
+    // Precise Seam types
+    if (s == "precise_seam_center")
+		return ModelVolumeType::PRECISE_SEAM_CENTER;
+    if (s == "precise_seam_left")
+		return ModelVolumeType::PRECISE_SEAM_LEFT;
+    if (s == "precise_seam_right")
+		return ModelVolumeType::PRECISE_SEAM_RIGHT;
+    if (s == "precise_seam_enforced")
+		return ModelVolumeType::PRECISE_SEAM_ENFORCED;
+    if (s == "precise_seam_blocked")
+		return ModelVolumeType::PRECISE_SEAM_BLOCKED;
+    if (s == "precise_seam_neutral")
+		return ModelVolumeType::PRECISE_SEAM_NEUTRAL;
     //assert(s == "0");
     // Default value if invalud type string received.
 	return ModelVolumeType::MODEL_PART;
@@ -2833,6 +2875,12 @@ std::string ModelVolume::type_to_string(const ModelVolumeType t)
 	case ModelVolumeType::PARAMETER_MODIFIER: return "modifier_part";
 	case ModelVolumeType::SUPPORT_ENFORCER:   return "support_enforcer";
 	case ModelVolumeType::SUPPORT_BLOCKER:    return "support_blocker";
+	case ModelVolumeType::PRECISE_SEAM_CENTER:   return "precise_seam_center";
+	case ModelVolumeType::PRECISE_SEAM_LEFT:     return "precise_seam_left";
+	case ModelVolumeType::PRECISE_SEAM_RIGHT:    return "precise_seam_right";
+	case ModelVolumeType::PRECISE_SEAM_ENFORCED: return "precise_seam_enforced";
+	case ModelVolumeType::PRECISE_SEAM_BLOCKED:  return "precise_seam_blocked";
+	case ModelVolumeType::PRECISE_SEAM_NEUTRAL:  return "precise_seam_neutral";
     default:
         assert(false);
         return "normal_part";
@@ -3785,6 +3833,7 @@ bool model_volume_list_changed(const ModelObject &model_object_old, const ModelO
     });
 }
 
+
 template< typename TypeFilterFn, typename CompareFn>
 bool model_property_changed(const ModelObject &model_object_old, const ModelObject &model_object_new, TypeFilterFn type_filter, CompareFn compare)
 {

src/libslic3r/Model.hpp

@@ -347,8 +347,22 @@ enum class ModelVolumeType : int {
     PARAMETER_MODIFIER,
     SUPPORT_BLOCKER,
     SUPPORT_ENFORCER,
+    // Precise seam modifiers (6 subtypes for seam placement control).
+    // Order is critical: strong types first, then weak. Range checks in is_precise_seam*() depend on it.
+    PRECISE_SEAM_CENTER,
+    PRECISE_SEAM_LEFT,
+    PRECISE_SEAM_RIGHT,
+    PRECISE_SEAM_ENFORCED,
+    PRECISE_SEAM_BLOCKED,
+    PRECISE_SEAM_NEUTRAL,
 };
 
+// Free functions for checking ModelVolumeType without a ModelVolume object.
+// Keep in sync with ModelVolume::is_precise_seam*() methods below.
+inline bool is_precise_seam(ModelVolumeType t)       { return t >= ModelVolumeType::PRECISE_SEAM_CENTER && t <= ModelVolumeType::PRECISE_SEAM_NEUTRAL; }
+inline bool is_precise_seam_strong(ModelVolumeType t) { return t >= ModelVolumeType::PRECISE_SEAM_CENTER && t <= ModelVolumeType::PRECISE_SEAM_RIGHT; }
+inline bool is_precise_seam_weak(ModelVolumeType t)   { return t >= ModelVolumeType::PRECISE_SEAM_ENFORCED && t <= ModelVolumeType::PRECISE_SEAM_NEUTRAL; }
+
 // A printable object, possibly having multiple print volumes (each with its own set of parameters and materials),
 // and possibly having multiple modifier volumes, each modifier volume with its set of parameters and materials.
 // Each ModelObject may be instantiated mutliple times, each instance having different placement on the print bed,
@@ -917,6 +931,14 @@ public:
 	bool                is_support_enforcer()   const { return m_type == ModelVolumeType::SUPPORT_ENFORCER; }
 	bool                is_support_blocker()    const { return m_type == ModelVolumeType::SUPPORT_BLOCKER; }
 	bool                is_support_modifier()   const { return m_type == ModelVolumeType::SUPPORT_BLOCKER || m_type == ModelVolumeType::SUPPORT_ENFORCER; }
+	// Check if this volume is any of the precise seam modifier subtypes
+	bool                is_precise_seam()       const { return m_type >= ModelVolumeType::PRECISE_SEAM_CENTER && m_type <= ModelVolumeType::PRECISE_SEAM_NEUTRAL; }
+	// Helper to check if volume is a "strong" Precise Seam type (center, left, right)
+	// Strong modifiers have priority and always appear above weak modifiers in UI
+	bool                is_precise_seam_strong() const { return m_type >= ModelVolumeType::PRECISE_SEAM_CENTER && m_type <= ModelVolumeType::PRECISE_SEAM_RIGHT; }
+	// Helper to check if volume is a "weak" Precise Seam type (enforced, blocked, neutral)
+	// Weak modifiers always appear below strong modifiers in UI
+	bool                is_precise_seam_weak()   const { return m_type >= ModelVolumeType::PRECISE_SEAM_ENFORCED && m_type <= ModelVolumeType::PRECISE_SEAM_NEUTRAL; }
     bool                is_text()               const { return text_configuration.has_value(); }
     bool                is_svg() const { return emboss_shape.has_value()  && !text_configuration.has_value(); }
     bool                is_the_only_one_part() const; // behave like an object
@@ -1041,7 +1063,7 @@ protected:
     friend class SLAPrint;
     friend class Model;
 	friend class ModelObject;
-    friend void model_volume_list_update_supports(ModelObject& model_object_dst, const ModelObject& model_object_new);
+    friend void model_volume_list_update_supports_and_seams(ModelObject& model_object_dst, const ModelObject& model_object_new);
 
 	// Copies IDs of both the ModelVolume and its config.
 	explicit ModelVolume(const ModelVolume &rhs) = default;

src/libslic3r/Polygon.cpp

@@ -318,10 +318,13 @@ Points Polygon::concave_points(double angle_threshold) const
 }
 
 // Projection of a point onto the polygon.
-Point Polygon::point_projection(const Point &point) const
+Point Polygon::point_projection(const Point &point, size_t *edge_index) const
 {
     Point proj = point;
     double dmin = std::numeric_limits<double>::max();
+    // Preserve the existing projection and tie order while optionally tracking its edge.
+    if (edge_index)
+        *edge_index = std::numeric_limits<size_t>::max();
     if (! this->points.empty()) {
         for (size_t i = 0; i < this->points.size(); ++ i) {
             const Point &pt0 = this->points[i];
@@ -330,11 +333,15 @@ Point Polygon::point_projection(const Point &point) const
             if (d < dmin) {
                 dmin = d;
                 proj = pt0;
+                if (edge_index)
+                    *edge_index = i;
             }
             d = (point - pt1).cast<double>().norm();
             if (d < dmin) {
                 dmin = d;
                 proj = pt1;
+                if (edge_index)
+                    *edge_index = (i + 1) % this->points.size();
             }
             Vec2d v1(coordf_t(pt1(0) - pt0(0)), coordf_t(pt1(1) - pt0(1)));
             coordf_t div = v1.squaredNorm();
@@ -347,6 +354,8 @@ Point Polygon::point_projection(const Point &point) const
                     if (d < dmin) {
                         dmin = d;
                         proj = foot;
+                        if (edge_index)
+                            *edge_index = i;
                     }
                 }
             }

src/libslic3r/Polygon.hpp

@@ -85,7 +85,9 @@ public:
     Points convex_points(double angle_threshold = 0.) const;
     Points concave_points(double angle_threshold = 0.) const;
     // Projection of a point onto the polygon.
-    Point point_projection(const Point &point) const;
+    // Optional index: start of the closest edge, or the vertex itself for an endpoint.
+    // Empty polygons return the query point and std::numeric_limits<size_t>::max() as the index.
+    Point point_projection(const Point &point, size_t *edge_index = nullptr) const;
     std::vector<float> parameter_by_length() const;
     
     //BBS

src/libslic3r/Print.hpp

@@ -470,6 +470,10 @@ public:
     std::vector<Polygons>       slice_support_volumes(const ModelVolumeType model_volume_type) const;
     std::vector<Polygons>       slice_support_blockers() const { return this->slice_support_volumes(ModelVolumeType::SUPPORT_BLOCKER); }
     std::vector<Polygons>       slice_support_enforcers() const { return this->slice_support_volumes(ModelVolumeType::SUPPORT_ENFORCER); }
+    // Shared slicing path; multiple volumes are united per layer.
+    std::vector<Polygons>       slice_modifier_volumes(const std::vector<const ModelVolume*> &volumes) const;
+    // Keep Precise Seam volumes separate so their individual priority is preserved.
+    std::vector<Polygons>       slice_single_volume(const ModelVolume* volume) const { return this->slice_modifier_volumes({volume}); }
 
     // Helpers to project custom facets on slices
     void project_and_append_custom_facets(bool seam, EnforcerBlockerType type, std::vector<Polygons>& expolys, std::vector<std::pair<Vec3f,Vec3f>>* vertical_points=nullptr) const;

src/libslic3r/PrintApply.cpp

@@ -8,12 +8,12 @@
 
 namespace Slic3r {
 
-// Add or remove support modifier ModelVolumes from model_object_dst to match the ModelVolumes of model_object_new
+// Add or remove support and Precise Seam modifier ModelVolumes from model_object_dst to match the ModelVolumes of model_object_new
 // in the exact order and with the same IDs.
-// It is expected, that the model_object_dst already contains the non-support volumes of model_object_new in the correct order.
+// Other volume types must already match model_object_new in the correct order.
 // Friend to ModelVolume to allow copying.
 // static is not accepted by gcc if declared as a friend of ModelObject.
-/* static */ void model_volume_list_update_supports(ModelObject &model_object_dst, const ModelObject &model_object_new)
+/* static */ void model_volume_list_update_supports_and_seams(ModelObject &model_object_dst, const ModelObject &model_object_new)
 {
     typedef std::pair<const ModelVolume*, bool> ModelVolumeWithStatus;
     std::vector<ModelVolumeWithStatus> old_volumes;
@@ -33,18 +33,22 @@ namespace Slic3r {
             assert(! it->second); // not consumed yet
             it->second = true;
             ModelVolume *model_volume_dst = const_cast<ModelVolume*>(it->first);
-            // For support modifiers, the type may have been switched from blocker to enforcer and vice versa.
+            // Type may switch within support family, within precise_seam family, or between them.
-            assert((model_volume_dst->is_support_modifier() && model_volume_src->is_support_modifier()) || model_volume_dst->type() == model_volume_src->type());
+            assert((model_volume_dst->is_support_modifier() && model_volume_src->is_support_modifier()) ||
+                   (model_volume_dst->is_precise_seam()     && model_volume_src->is_precise_seam())     ||
+                   (model_volume_dst->is_support_modifier() && model_volume_src->is_precise_seam())     ||
+                   (model_volume_dst->is_precise_seam()     && model_volume_src->is_support_modifier()) ||
+                   model_volume_dst->type() == model_volume_src->type());
             model_object_dst.volumes.emplace_back(model_volume_dst);
-            if (model_volume_dst->is_support_modifier()) {
+            if (model_volume_dst->is_support_modifier() || model_volume_dst->is_precise_seam()) {
-                // For support modifiers, the type may have been switched from blocker to enforcer and vice versa.
+                // Type may have been switched within or between support/precise_seam families.
                 model_volume_dst->set_type(model_volume_src->type());
                 model_volume_dst->set_transformation(model_volume_src->get_transformation());
             }
             assert(model_volume_dst->get_matrix().isApprox(model_volume_src->get_matrix()));
         } else {
             // The volume was not found in the old list. Create a new copy.
-            assert(model_volume_src->is_support_modifier());
+            assert(model_volume_src->is_support_modifier() || model_volume_src->is_precise_seam());
             model_object_dst.volumes.emplace_back(new ModelVolume(*model_volume_src));
             model_object_dst.volumes.back()->set_model_object(&model_object_dst);
         }
@@ -55,17 +59,20 @@ namespace Slic3r {
             delete mv_with_status.first;
 }
 
-static inline void model_volume_list_copy_configs(ModelObject &model_object_dst, const ModelObject &model_object_src, const ModelVolumeType type)
+// Copy configs of ModelVolumes matching type_filter predicate from src to dst.
+// Mirrors the template pattern of model_volume_list_changed() in Model.cpp.
+template<typename TypeFilterFn>
+static inline void model_volume_list_copy_configs(ModelObject &model_object_dst, const ModelObject &model_object_src, TypeFilterFn type_filter)
 {
     size_t i_src, i_dst;
     for (i_src = 0, i_dst = 0; i_src < model_object_src.volumes.size() && i_dst < model_object_dst.volumes.size();) {
         const ModelVolume &mv_src = *model_object_src.volumes[i_src];
         ModelVolume       &mv_dst = *model_object_dst.volumes[i_dst];
-        if (mv_src.type() != type) {
+        if (! type_filter(mv_src.type())) {
             ++ i_src;
             continue;
         }
-        if (mv_dst.type() != type) {
+        if (! type_filter(mv_dst.type())) {
             ++ i_dst;
             continue;
         }
@@ -88,6 +95,20 @@ static inline void model_volume_list_copy_configs(ModelObject &model_object_dst,
     }
 }
 
+// Convenience overload: single volume type.
+static inline void model_volume_list_copy_configs(ModelObject &model_object_dst, const ModelObject &model_object_src, const ModelVolumeType type)
+{
+    model_volume_list_copy_configs(model_object_dst, model_object_src, [type](const ModelVolumeType t) { return t == type; });
+}
+
+// Convenience overload: multiple volume types at once (e.g. all precise seam types).
+static inline void model_volume_list_copy_configs(ModelObject &model_object_dst, const ModelObject &model_object_src, const std::initializer_list<ModelVolumeType> &types)
+{
+    model_volume_list_copy_configs(model_object_dst, model_object_src, [&types](const ModelVolumeType t) {
+        return std::find(types.begin(), types.end(), t) != types.end();
+    });
+}
+
 static inline void layer_height_ranges_copy_configs(t_layer_config_ranges &lr_dst, const t_layer_config_ranges &lr_src)
 {
     assert(lr_dst.size() == lr_src.size());
@@ -1618,6 +1639,11 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
 
     // 3) Synchronize ModelObjects & PrintObjects.
     const std::initializer_list<ModelVolumeType> solid_or_modifier_types { ModelVolumeType::MODEL_PART, ModelVolumeType::NEGATIVE_VOLUME, ModelVolumeType::PARAMETER_MODIFIER };
+    const std::initializer_list<ModelVolumeType> precise_seam_types {
+        ModelVolumeType::PRECISE_SEAM_CENTER, ModelVolumeType::PRECISE_SEAM_LEFT,
+        ModelVolumeType::PRECISE_SEAM_RIGHT,  ModelVolumeType::PRECISE_SEAM_ENFORCED,
+        ModelVolumeType::PRECISE_SEAM_BLOCKED, ModelVolumeType::PRECISE_SEAM_NEUTRAL
+    };
     for (size_t idx_model_object = 0; idx_model_object < model.objects.size(); ++ idx_model_object) {
         ModelObject       &model_object        = *m_model.objects[idx_model_object];
         ModelObjectStatus &model_object_status = const_cast<ModelObjectStatus&>(model_object_status_db.reuse(model_object));
@@ -1635,6 +1661,7 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
                                           model_fuzzy_skin_data_changed(model_object, model_object_new);
         bool supports_differ            = model_volume_list_changed(model_object, model_object_new, ModelVolumeType::SUPPORT_BLOCKER) ||
                                           model_volume_list_changed(model_object, model_object_new, ModelVolumeType::SUPPORT_ENFORCER);
+        bool precise_seam_differ        = model_volume_list_changed(model_object, model_object_new, precise_seam_types);
         bool layer_height_ranges_differ = ! layer_height_ranges_equal(model_object.layer_config_ranges, model_object_new.layer_config_ranges, model_object_new.layer_height_profile.empty());
         bool model_origin_translation_differ = model_object.origin_translation != model_object_new.origin_translation;
         bool brim_points_differ = model_brim_points_data_changed(model_object, model_object_new);
@@ -1677,13 +1704,20 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
                 // Invalidate just the supports step.
                 for (const PrintObjectStatus &print_object_status : print_objects_range)
                     update_apply_status(print_object_status.print_object->invalidate_step(posSupportMaterial));
-                if (supports_differ) {
-                    // Copy just the support volumes.
-                    model_volume_list_update_supports(model_object, model_object_new);
             }
+            if (precise_seam_differ) {
+                // First stop background processing before shuffling or deleting the ModelVolumes in the ModelObject's list.
+                this->call_cancel_callback();
+                update_apply_status(false);
+                // Invalidate seam placement (affects G-code export).
+                update_apply_status(this->invalidate_step(psGCodeExport));
             } else if (model_custom_seam_data_changed(model_object, model_object_new)) {
                 update_apply_status(this->invalidate_step(psGCodeExport));
             }
+            // Synchronize both families once, after cancellation and all affected-step invalidations.
+            // This also handles type changes between supports and Precise Seam before copying configs below.
+            if (supports_differ || precise_seam_differ)
+                model_volume_list_update_supports_and_seams(model_object, model_object_new);
             if (brim_points_differ) {
                 model_object.brim_points = model_object_new.brim_points;
                 update_apply_status(this->invalidate_all_steps());
@@ -1708,6 +1742,8 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
             //FIXME What to do with m_material_id?
 			model_volume_list_copy_configs(model_object /* dst */, model_object_new /* src */, ModelVolumeType::MODEL_PART);
 			model_volume_list_copy_configs(model_object /* dst */, model_object_new /* src */, ModelVolumeType::PARAMETER_MODIFIER);
+			// Synchronize Precise Seam modifier volumes
+			model_volume_list_copy_configs(model_object /* dst */, model_object_new /* src */, precise_seam_types);
             layer_height_ranges_copy_configs(model_object.layer_config_ranges /* dst */, model_object_new.layer_config_ranges /* src */);
             // Copy the ModelObject name, input_file and instances. The instances will be compared against PrintObject instances in the next step.
             model_object.name       = model_object_new.name;

src/libslic3r/PrintBase.hpp

@@ -62,7 +62,8 @@ public:
         SlicingReplaceInitEmptyLayers,
         SlicingNeedSupportOn,
         SlicingEmptyGcodeLayers,
-        SlicingGcodeOverlap
+        SlicingGcodeOverlap,
+        SlicingPreciseSeamWarning
     };
 
     typedef size_t TimeStamp;

src/libslic3r/PrintObjectSlice.cpp

@@ -1551,12 +1551,19 @@ ExPolygons PrintObject::_shrink_contour_holes(double contour_delta, double hole_
 
 std::vector<Polygons> PrintObject::slice_support_volumes(const ModelVolumeType model_volume_type) const
 {
-    auto it_volume     = this->model_object()->volumes.begin();
+    // Supports merge every matching volume; Precise Seam calls the shared slicer one volume at a time.
-    auto it_volume_end = this->model_object()->volumes.end();
+    std::vector<const ModelVolume*> volumes;
-    for (; it_volume != it_volume_end && (*it_volume)->type() != model_volume_type; ++ it_volume) ;
+    for (const ModelVolume *volume : this->model_object()->volumes)
+        if (volume->type() == model_volume_type)
+            volumes.push_back(volume);
+    return this->slice_modifier_volumes(volumes);
+}
+
+std::vector<Polygons> PrintObject::slice_modifier_volumes(const std::vector<const ModelVolume*> &volumes) const
+{
     std::vector<Polygons> slices;
-    if (it_volume != it_volume_end) {
+    if (!volumes.empty()) {
-        // Found at least a single support volume of model_volume_type.
+        // Share layer heights, transforms and cancellation handling across the selected volumes.
         std::vector<float> zs = zs_from_layers(this->layers());
         std::vector<char>  merge_layers;
         bool               merge = false;
@@ -1564,9 +1571,8 @@ std::vector<Polygons> PrintObject::slice_support_volumes(const ModelVolumeType m
         auto               throw_on_cancel_callback = std::function<void()>([print](){ print->throw_if_canceled(); });
         MeshSlicingParamsEx params;
         params.trafo = this->trafo_centered();
-        for (; it_volume != it_volume_end; ++ it_volume)
+        for (const ModelVolume *volume : volumes) {
-            if ((*it_volume)->type() == model_volume_type) {
+            std::vector<ExPolygons> slices2 = slice_volume(*volume, zs, params, throw_on_cancel_callback);
-                std::vector<ExPolygons> slices2 = slice_volume(*(*it_volume), zs, params, throw_on_cancel_callback);
             if (slices.empty()) {
                 slices.reserve(slices2.size());
                 for (ExPolygons &src : slices2)

src/slic3r/GUI/3DScene.cpp

@@ -162,6 +162,14 @@ ColorRGBA GLVolume::SUPPORT_BLOCKER_COL  = {1.0f, 0.3f, 0.3f, 0.4f};
 
 ColorRGBA GLVolume::MODEL_HIDDEN_COL  = {0.f, 0.f, 0.f, 0.3f};
 
+// Precise Seam modifier colors
+ColorRGBA GLVolume::PRECISE_SEAM_CENTER_COL   = {1.0f,   0.627f, 0.082f, 0.6f};  // FFA015 - orange
+ColorRGBA GLVolume::PRECISE_SEAM_LEFT_COL     = {1.0f,   0.753f, 0.0f,   0.6f};  // FFC000 - golden
+ColorRGBA GLVolume::PRECISE_SEAM_RIGHT_COL    = {1.0f,   0.514f, 0.0f,   0.6f};  // FF8300 - dark orange
+ColorRGBA GLVolume::PRECISE_SEAM_ENFORCED_COL = {0.412f, 0.820f, 0.412f, 0.6f};  // 69D169 - green
+ColorRGBA GLVolume::PRECISE_SEAM_NEUTRAL_COL  = {0.655f, 0.655f, 0.655f, 0.6f};  // A7A7A7 - gray
+ColorRGBA GLVolume::PRECISE_SEAM_BLOCKED_COL  = {0.820f, 0.412f, 0.412f, 0.6f};  // D16969 - red
+
 std::array<ColorRGBA, 5> GLVolume::MODEL_COLOR = { {
     { 1.0f, 1.0f, 0.0f, 1.f },
     { 1.0f, 0.5f, 0.5f, 1.f },
@@ -363,6 +371,28 @@ ColorRGBA color_from_model_volume(const ModelVolume& model_volume)
     ColorRGBA color;
     if (model_volume.is_negative_volume())
         return GLVolume::MODEL_NEGTIVE_COL;
+    else if (model_volume.is_precise_seam()) {
+        // Return color based on Precise Seam subtype.
+        // Exhaustive switch (no default) so -Wswitch flags any future PRECISE_SEAM_* additions.
+        switch (model_volume.type()) {
+            case ModelVolumeType::PRECISE_SEAM_CENTER:   return GLVolume::PRECISE_SEAM_CENTER_COL;
+            case ModelVolumeType::PRECISE_SEAM_LEFT:     return GLVolume::PRECISE_SEAM_LEFT_COL;
+            case ModelVolumeType::PRECISE_SEAM_RIGHT:    return GLVolume::PRECISE_SEAM_RIGHT_COL;
+            case ModelVolumeType::PRECISE_SEAM_ENFORCED: return GLVolume::PRECISE_SEAM_ENFORCED_COL;
+            case ModelVolumeType::PRECISE_SEAM_NEUTRAL:  return GLVolume::PRECISE_SEAM_NEUTRAL_COL;
+            case ModelVolumeType::PRECISE_SEAM_BLOCKED:  return GLVolume::PRECISE_SEAM_BLOCKED_COL;
+            // Non-seam types are unreachable due to the outer is_precise_seam() guard;
+            // listed explicitly so this switch stays exhaustive over ModelVolumeType.
+            case ModelVolumeType::INVALID:
+            case ModelVolumeType::MODEL_PART:
+            case ModelVolumeType::NEGATIVE_VOLUME:
+            case ModelVolumeType::PARAMETER_MODIFIER:
+            case ModelVolumeType::SUPPORT_BLOCKER:
+            case ModelVolumeType::SUPPORT_ENFORCER:
+                break;
+        }
+        return GLVolume::MODEL_MIDIFIER_COL; // unreachable fallback
+    }
     else if (model_volume.is_modifier())
 #if ENABLE_MODIFIERS_ALWAYS_TRANSPARENT
         return GLVolume::MODEL_MIDIFIER_COL;

src/slic3r/GUI/3DScene.hpp

@@ -93,6 +93,13 @@ public:
     static ColorRGBA SUPPORT_ENFORCER_COL;
     static ColorRGBA SUPPORT_BLOCKER_COL;
     static ColorRGBA MODEL_HIDDEN_COL;
+    // Precise Seam modifier colors
+    static ColorRGBA PRECISE_SEAM_CENTER_COL;
+    static ColorRGBA PRECISE_SEAM_LEFT_COL;
+    static ColorRGBA PRECISE_SEAM_RIGHT_COL;
+    static ColorRGBA PRECISE_SEAM_ENFORCED_COL;
+    static ColorRGBA PRECISE_SEAM_NEUTRAL_COL;
+    static ColorRGBA PRECISE_SEAM_BLOCKED_COL;
 
     static void update_render_colors();
     static void load_render_colors();

src/slic3r/GUI/GUI_Factories.cpp

@@ -336,13 +336,20 @@ wxBitmap SettingsFactory::get_category_bitmap(const std::string& category_name,
 //-------------------------------------
 
 // Note: id accords to type of the sub-object (adding volume), so sequence of the menu items is important
-static const constexpr std::array<std::pair<const char *, const char *>, 5> ADD_VOLUME_MENU_ITEMS = {{
+static const constexpr std::array<std::pair<const char *, const char *>, 11> ADD_VOLUME_MENU_ITEMS = {{
     //       menu_item Name              menu_item bitmap name
         {L("Add Part"),              "menu_add_part" },           // ~ModelVolumeType::MODEL_PART
         {L("Add Negative Part"),     "menu_add_negative" },       // ~ModelVolumeType::NEGATIVE_VOLUME
         {L("Add Modifier"),          "menu_add_modifier"},         // ~ModelVolumeType::PARAMETER_MODIFIER
         {L("Add Support Blocker"),   "menu_support_blocker"},     // ~ModelVolumeType::SUPPORT_BLOCKER
         {L("Add Support Enforcer"),  "menu_support_enforcer"},     // ~ModelVolumeType::SUPPORT_ENFORCER
+        // Precise Seam modifiers (all 6 subtypes - only first one shown in menu, others used for tree icons)
+        {L("Add Precise Seam"),      "menu_precise_seam_center"},     // ~ModelVolumeType::PRECISE_SEAM_CENTER
+        {L("Add Precise Seam"),      "menu_precise_seam_left"},       // ~ModelVolumeType::PRECISE_SEAM_LEFT
+        {L("Add Precise Seam"),      "menu_precise_seam_right"},      // ~ModelVolumeType::PRECISE_SEAM_RIGHT
+        {L("Add Precise Seam"),      "menu_precise_seam_enforced"},   // ~ModelVolumeType::PRECISE_SEAM_ENFORCED
+        {L("Add Precise Seam"),      "menu_precise_seam_blocked"},    // ~ModelVolumeType::PRECISE_SEAM_BLOCKED
+        {L("Add Precise Seam"),      "menu_precise_seam_neutral"},    // ~ModelVolumeType::PRECISE_SEAM_NEUTRAL
 }};
 
 // Note: id accords to type of the sub-object (adding volume), so sequence of the menu items is important
@@ -740,10 +747,20 @@ void MenuFactory::append_menu_items_add_volume(wxMenu* menu)
 
     for (size_t type = 0; type < ADD_VOLUME_MENU_ITEMS.size(); type++)
     {
+        // Skip Precise Seam subtypes except the first one (CENTER) - they are only used for tree icons
+        if (type >= int(ModelVolumeType::PRECISE_SEAM_LEFT) &&
+            type <= int(ModelVolumeType::PRECISE_SEAM_NEUTRAL))
+            continue;
+
         auto& item = ADD_VOLUME_MENU_ITEMS[type];
 
+        // Use special icon for "Add Precise Seam" menu (different from tree icon)
+        std::string icon_name = item.second;
+        if (type == int(ModelVolumeType::PRECISE_SEAM_CENTER))
+            icon_name = "menu_precise_seam_add";
+
         wxMenu* sub_menu = append_submenu_add_generic(menu, ModelVolumeType(type));
-        append_submenu(menu, sub_menu, wxID_ANY, _(item.first), "", item.second,
+        append_submenu(menu, sub_menu, wxID_ANY, _(item.first), "", icon_name,
             []() { return obj_list()->is_instance_or_object_selected(); }, m_parent);
     }
 
@@ -847,7 +864,12 @@ wxMenuItem* MenuFactory::append_menu_item_change_type(wxMenu* menu)
         { ModelVolumeType::NEGATIVE_VOLUME,     _L("Negative Part") },
         { ModelVolumeType::PARAMETER_MODIFIER,  _L("Modifier") },
         { ModelVolumeType::SUPPORT_BLOCKER,     _L("Support Blocker") },
-        { ModelVolumeType::SUPPORT_ENFORCER,   _L("Support Enforcer") }
+        { ModelVolumeType::SUPPORT_ENFORCER,    _L("Support Enforcer") },
+        // Single "Precise Seam" entry that maps to PRECISE_SEAM_CENTER as the default subtype.
+        // set_volume_type() preserves the existing subtype (LEFT/RIGHT/etc.) for volumes that
+        // are already Precise Seam; subtype picking is done via the separate
+        // "Precise Seam Type" submenu (see append_menu_item_precise_seam_submenu).
+        { ModelVolumeType::PRECISE_SEAM_CENTER, _L("Precise Seam") }
     };
 
     for (const auto& info : types) {
@@ -863,7 +885,13 @@ wxMenuItem* MenuFactory::append_menu_item_change_type(wxMenu* menu)
             obj_list()->GetSelections(sels);
             for (auto item : sels) {
                 ModelVolumeType vol_type = model->GetVolumeType(item);
-                if (vol_type == type) {
+                // The "Precise Seam" entry represents all six PS subtypes, so any subtype
+                // among selected volumes counts as a match (keeps the checkbox ticked when
+                // user has PS_LEFT/RIGHT/etc. selected, not only plain CENTER).
+                const bool match = (type == ModelVolumeType::PRECISE_SEAM_CENTER)
+                    ? is_precise_seam(vol_type)
+                    : (vol_type == type);
+                if (match) {
                     has_type = true;
                     break;
                 }
@@ -871,6 +899,7 @@ wxMenuItem* MenuFactory::append_menu_item_change_type(wxMenu* menu)
             evt.Check(has_type);
 
             // ORCA Fix crash caused by SVG/TEXT volumes cant be Support Enforcer/Blocker type
+            // The same applies to Precise Seam subtypes.
             for (auto item : sels) {
                 if (model->GetItemType(item) == itVolume){
                     auto vol_idx = model->GetVolumeIdByItem(item);
@@ -881,7 +910,8 @@ wxMenuItem* MenuFactory::append_menu_item_change_type(wxMenu* menu)
                     auto vol = (*objs)[obj_idx]->volumes[vol_idx];
 
                     // disable Support Enforcer/Blocker if selection contains svg or text
-                    if (vol != nullptr && (vol->is_svg() || vol->is_text()) && (type == ModelVolumeType::SUPPORT_BLOCKER || type == ModelVolumeType::SUPPORT_ENFORCER)){
+                    // (same applies to Precise Seam)
+                    if (vol != nullptr && (vol->is_svg() || vol->is_text()) && (type == ModelVolumeType::SUPPORT_BLOCKER || type == ModelVolumeType::SUPPORT_ENFORCER || is_precise_seam(type))){
                         evt.Enable(false);
                         break;
                     }
@@ -1309,6 +1339,131 @@ void MenuFactory::append_menu_items_mirror(wxMenu* menu)
         []() { return plater()->can_mirror(); }, m_parent);
 }
 
+void MenuFactory::append_menu_item_precise_seam_submenu(wxMenu* menu)
+{
+    wxString submenu_name = _L("Precise Seam Type");
+
+    // Remove existing submenu if present (menu is rebuilt on every right-click)
+    const int menu_item_id = menu->FindItem(submenu_name);
+    if (menu_item_id != wxNOT_FOUND)
+        menu->Destroy(menu_item_id);
+
+    // --- Precondition: ALL selected volumes must be Precise Seam ---
+    // Mixed selections (PS + non-PS) are ambiguous for subtype switching: applying a subtype
+    // would implicitly convert the non-PS volumes to PS, which is not what the user expects
+    // from a subtype picker. For mixed selections the user should first use
+    // "Change type → Precise Seam" to unify them, then come back to this submenu.
+    wxDataViewItemArray sels;
+    obj_list()->GetSelections(sels);
+    if (sels.IsEmpty())
+        return;
+
+    // A right-click on a volume in the 3D view routes through Plater::show_context_menu
+    // → ObjectList::update_selections(), which intentionally preserves a pre-existing
+    // settings-row selection alongside the clicked volume. As a result GetSelections()
+    // may return settings-row items that are children of a volume. Treat each settings
+    // row as selecting its parent volume (same pattern used by set_volume_type()
+    // internally). Without this resolution, GetVolumeType(settings_row) returns INVALID
+    // and the submenu would hide even when a valid PS volume is in the selection.
+    //
+    // Precomputing the resolved PS types once here also lets the checkmark loop below
+    // display checks correctly for selections that contain settings rows.
+    std::vector<ModelVolumeType> selected_ps_types;
+    selected_ps_types.reserve(sels.size());
+
+    auto* model = obj_list()->GetModel();
+    for (const auto& sel_item : sels) {
+        wxDataViewItem vol_item  = sel_item;
+        const ItemType type_mask = model->GetItemType(sel_item);
+        if (!(type_mask & itVolume)) {
+            // Only settings rows whose parent is a volume map to "selecting that volume".
+            // Any other non-volume item (object, instance, etc.) means the selection is
+            // not purely PS-volume-based — hide the submenu.
+            if ((type_mask & itSettings) && (model->GetItemType(model->GetParent(sel_item)) & itVolume))
+                vol_item = model->GetParent(sel_item);
+            else
+                return;
+        }
+        const ModelVolumeType vol_type = model->GetVolumeType(vol_item);
+        if (!is_precise_seam(vol_type))
+            return;
+        selected_ps_types.push_back(vol_type);
+    }
+
+    // Match the label used by append_menu_item_change_type, including its translation.
+    int insert_pos     = wxNOT_FOUND;
+    int change_type_id = menu->FindItem(_L("Change Type"));
+    if (change_type_id != wxNOT_FOUND) {
+        for (size_t i = 0; i < menu->GetMenuItemCount(); i++) {
+            wxMenuItem* item = menu->FindItemByPosition(i);
+            if (item && item->GetId() == change_type_id) {
+                insert_pos = i + 1;  // insert directly after the Change Type entry
+                break;
+            }
+        }
+    }
+
+    // --- Build the subtype submenu ---
+    wxMenu* ps_menu = new wxMenu();
+
+    // Array of all 6 Precise Seam subtypes with labels.
+    // "Seam ..." prefix disambiguates from other i18n contexts (extruder Left/Right, "Center on bed", etc.)
+    static const std::array<std::pair<const char*, ModelVolumeType>, 6> PS_TYPES = {{
+        {L("Seam Center"),   ModelVolumeType::PRECISE_SEAM_CENTER},
+        {L("Seam Left"),     ModelVolumeType::PRECISE_SEAM_LEFT},
+        {L("Seam Right"),    ModelVolumeType::PRECISE_SEAM_RIGHT},
+        {L("Seam Enforced"), ModelVolumeType::PRECISE_SEAM_ENFORCED},
+        {L("Seam Blocked"),  ModelVolumeType::PRECISE_SEAM_BLOCKED},
+        {L("Seam Neutral"),  ModelVolumeType::PRECISE_SEAM_NEUTRAL},
+    }};
+
+    for (const auto& ps_type : PS_TYPES) {
+        wxString label = _(ps_type.first);
+
+        // Check-items (not radio): radio groups in wxWidgets auto-select the first item when
+        // no item is explicitly checked, which misleads the user into seeing "Seam Center"
+        // as the active subtype for mixed-subtype selections. With check-items we can leave
+        // every item unchecked in that case, and the menu matches the pattern of the
+        // neighbouring "Change type" submenu (see append_menu_item_change_type).
+        //
+        // Handler: delegate to set_volume_type() with preserve_ps_subtype=false. The user
+        // explicitly picked a subtype here, so "Seam Center" must set every selected PS
+        // volume to CENTER verbatim — preservation (the default for "Change type") would
+        // keep existing subtypes and make CENTER unreachable on mixed-subtype selections.
+        // set_volume_type() already handles multi-select iteration, the last-solid-part
+        // guard, and the PS group-change reordering via move_volume_to_end().
+        wxMenuItem* item = append_menu_check_item(ps_menu, wxID_ANY, label, "",
+            [ps_type](wxCommandEvent&) {
+                obj_list()->set_volume_type(ps_type.second, /*preserve_ps_subtype=*/false);
+            },
+            ps_menu);
+
+        // Tick every subtype that is present in the current selection. Same pattern as the
+        // neighbouring "Change type" submenu (where selecting [Part + Modifier] ticks both
+        // "Part" and "Modifier" boxes). For homogeneous selections exactly one checkbox is
+        // ticked; for mixed subtype selections (e.g. PS_LEFT + PS_RIGHT) both "Seam Left"
+        // and "Seam Right" appear checked, so the user sees at a glance which subtypes are
+        // currently in the selection.
+        //
+        // Uses the precomputed selected_ps_types (which resolved settings rows to their
+        // parent volumes); iterating sels directly would miss subtypes for selections that
+        // arrive via settings rows from right-click-in-3D-view.
+        const bool is_present = std::find(selected_ps_types.begin(), selected_ps_types.end(),
+                                          ps_type.second) != selected_ps_types.end();
+        if (is_present && item)
+            item->Check(true);
+    }
+
+    // Add submenu to parent menu
+    append_submenu(menu, ps_menu, wxID_ANY,
+                   submenu_name,
+                   _L("Choose precise seam subtype"),
+                   "menu_precise_seam_type",
+                   []() { return true; },
+                   m_parent,
+                   insert_pos);
+}
+
 void MenuFactory::append_menu_item_edit_text(wxMenu *menu)
 {
     wxString name        = _L("Edit text");
@@ -1578,6 +1733,7 @@ void MenuFactory::create_part_menu()
 
     menu->AppendSeparator();
     append_menu_item_change_type(menu);
+    append_menu_item_precise_seam_submenu(menu);
     append_menu_items_mirror(&m_part_menu);
     append_menu_item(&m_part_menu, wxID_ANY, _L("Split"), _L("Split the selected object into multiple parts"),
         [](wxCommandEvent&) { plater()->split_volume(); }, "split_parts", nullptr,
@@ -1875,6 +2031,7 @@ wxMenu* MenuFactory::part_menu()
 {
     append_menu_items_convert_unit(&m_part_menu);
     append_menu_item_change_filament(&m_part_menu);
+    append_menu_item_precise_seam_submenu(&m_part_menu);
     append_menu_item_per_object_settings(&m_part_menu);
     return &m_part_menu;
 }
@@ -1989,6 +2146,10 @@ wxMenu* MenuFactory::multi_selection_menu()
         append_menu_item_per_object_process(menu);
         menu->AppendSeparator();
         append_menu_item_change_type(menu);
+        // Subtype picker for Precise Seam — shown only when all selected volumes are PS.
+        // Must be paired with Change Type here (as in single-volume part_menu), otherwise
+        // the user cannot switch PS subtypes (LEFT/RIGHT/etc.) on multi-selection.
+        append_menu_item_precise_seam_submenu(menu);
         append_menu_item_change_filament(menu);
     }
     return menu;

src/slic3r/GUI/GUI_Factories.hpp

@@ -174,6 +174,7 @@ private:
     void        append_menu_item_merge_to_single_object(wxMenu* menu);
     void        append_menu_item_merge_parts_to_single_part(wxMenu *menu);
     void        append_menu_items_mirror(wxMenu *menu);
+    void        append_menu_item_precise_seam_submenu(wxMenu* menu);
     void        append_menu_item_invalidate_cut_info(wxMenu *menu);
     void        append_menu_item_edit_text(wxMenu *menu);
     void        append_menu_item_edit_svg(wxMenu *menu);

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -1944,6 +1944,12 @@ bool ObjectList::can_drop(const wxDataViewItem& item, int& src_obj_id, int& src_
 
         if (dragged_item_v_type == item_v_type && dragged_item_v_type != ModelVolumeType::MODEL_PART)
             return true;
+
+        // Use tree item types: hidden cut connectors make tree indices differ from volumes indices.
+        if (is_precise_seam(dragged_item_v_type) && is_precise_seam(item_v_type))
+            // Allow reordering only within the strong or weak modifier group.
+            return is_precise_seam_strong(dragged_item_v_type) == is_precise_seam_strong(item_v_type);
+
         if ((dragged_item_v_type != item_v_type) ||   // we can't reorder volumes outside of types
             item_v_type >= ModelVolumeType::SUPPORT_BLOCKER)        // support blockers/enforcers can't change its place
             return false;
@@ -2033,10 +2039,43 @@ void ObjectList::OnDrop(wxDataViewEvent &event)
         int to_volume_id   = m_objects_model->GetVolumeIdByItem(item);
         int delta          = to_volume_id < from_volume_id ? -1 : 1;
 
-        auto &volumes = (*m_objects)[m_dragged_data.obj_idx()]->volumes;
+        const int obj_idx = m_dragged_data.obj_idx();
+        // Object-indexed UI maps may be stale after another object is removed or reordered.
+        if (obj_idx < 0 || size_t(obj_idx) >= m_objects->size()) {
+            event.Veto();
+            m_dragged_data.clear();
+            return;
+        }
+        const ModelObject *object = (*m_objects)[obj_idx];
+        auto &volumes = (*m_objects)[obj_idx]->volumes;
+        std::vector<size_t> visible_volume_indices;
+        visible_volume_indices.reserve(volumes.size());
+        for (size_t idx = 0; idx < volumes.size(); ++idx)
+            // Match add_volumes_to_object_in_list: only connectors of cut objects are hidden.
+            if (!(object->is_cut() && volumes[idx]->is_cut_connector()))
+                visible_volume_indices.push_back(idx);
 
-        int cnt = 0;
+        // Validate the entire move before any swap; these checks must also protect Release builds.
-        for (int id = from_volume_id; cnt < abs(from_volume_id - to_volume_id); id += delta, cnt++) std::swap(volumes[id], volumes[id + delta]);
+        if (from_volume_id < 0 || to_volume_id < 0 ||
+            size_t(from_volume_id) >= visible_volume_indices.size() || size_t(to_volume_id) >= visible_volume_indices.size()) {
+            event.Veto();
+            m_dragged_data.clear();
+            return;
+        }
+
+        // Move through visible slots only, keeping hidden cut connectors at their original indices.
+        // The local mapping stays valid because no hidden volume changes slots during the move.
+        for (int id = from_volume_id; id != to_volume_id; id += delta) {
+            const size_t current_idx = visible_volume_indices[id];
+            const size_t next_idx = visible_volume_indices[id + delta];
+            std::swap(volumes[current_idx], volumes[next_idx]);
+        }
+
+        // Later selection/filament handlers use this cache; repair it for the moved object too.
+        auto &ui_to_model = m_objects_model->get_ui_and_3d_volume_map()[obj_idx];
+        ui_to_model.clear();
+        for (size_t idx = 0; idx < visible_volume_indices.size(); ++idx)
+            ui_to_model[int(idx)] = int(visible_volume_indices[idx]);
 
         select_item(m_objects_model->ReorganizeChildren(from_volume_id, to_volume_id, m_objects_model->GetParent(item)));
 
@@ -3855,7 +3894,8 @@ wxDataViewItem ObjectList::add_settings_item(wxDataViewItem parent_item, const D
     const bool is_layer_settings = m_objects_model->GetItemType(parent_item) == itLayer;
     if (!is_object_settings) {
         ModelVolumeType volume_type = m_objects_model->GetVolumeType(parent_item);
-        if (volume_type == ModelVolumeType::NEGATIVE_VOLUME || volume_type == ModelVolumeType::SUPPORT_BLOCKER || volume_type == ModelVolumeType::SUPPORT_ENFORCER)
+        // Precise Seam is non-printing helper geometry — no per-volume settings
+        if (volume_type == ModelVolumeType::NEGATIVE_VOLUME || volume_type == ModelVolumeType::SUPPORT_BLOCKER || volume_type == ModelVolumeType::SUPPORT_ENFORCER || is_precise_seam(volume_type))
             return ret;
     }
 
@@ -5712,7 +5752,6 @@ void ObjectList::change_part_type()
   return;
 }
 #endif
-
 ModelVolumeType ObjectList::get_selected_volume_type()
 {
     ModelVolume* volume = get_selected_model_volume();
@@ -5721,13 +5760,49 @@ ModelVolumeType ObjectList::get_selected_volume_type()
     return ModelVolumeType::INVALID;
 }
 
-void ObjectList::set_volume_type(ModelVolumeType new_type)
+// ---------------- Helpers for Precise Seam group-aware type changes ----------------
+// Used by set_volume_type() and the "Precise Seam Type" subtype submenu handler.
+
+// Detects whether a type change crosses a Precise Seam "group boundary" that requires
+// manual repositioning inside ModelObject::volumes[]:
+//   - between any non-PS type and any PS subtype, or
+//   - between strong PS (CENTER/LEFT/RIGHT) and weak PS (ENFORCED/BLOCKED/NEUTRAL).
+// sort_volumes() is stable and groups strong PS before weak PS. When a volume crosses
+// a group, moving it to the end of volumes[] lets the subsequent stable sort place it
+// at the end of its new group. Without this, a strong→weak transition would leave the
+// volume stuck in the strong segment and break the modifier-application order.
+static bool precise_seam_group_changed(ModelVolumeType old_type, ModelVolumeType new_type)
+{
+    const bool old_is_ps = is_precise_seam(old_type);
+    const bool new_is_ps = is_precise_seam(new_type);
+    if (old_is_ps != new_is_ps)
+        return true;                 // non-PS ↔ PS transition
+    if (!old_is_ps)
+        return false;                // both non-PS — regular enum ordering is enough
+    return is_precise_seam_strong(old_type) != is_precise_seam_strong(new_type);
+}
+
+// Moves a volume to the end of ModelObject::volumes[]. Combined with stable sort_volumes(),
+// this places it at the end of its new group while preserving relative order of others.
+static void move_volume_to_end(ModelObject* obj, ModelVolume* volume)
+{
+    if (obj == nullptr || volume == nullptr)
+        return;
+    auto it = std::find(obj->volumes.begin(), obj->volumes.end(), volume);
+    if (it != obj->volumes.end()) {
+        obj->volumes.erase(it);
+        obj->volumes.push_back(volume);
+    }
+}
+
+void ObjectList::set_volume_type(ModelVolumeType new_type, bool preserve_ps_subtype)
 {
     struct VolumeSelection {
         int          object_idx;
         ModelVolume* volume;
     };
 
+    // --- Collect selected volumes from the object tree, falling back to the 3D canvas ---
     std::vector<VolumeSelection> volumes;
     auto add_volume = [&volumes](int obj_idx, ModelVolume* volume) {
         if (volume == nullptr)
@@ -5792,25 +5867,49 @@ void ObjectList::set_volume_type(ModelVolumeType new_type)
     // and historically crashed (originally fixed in the now-disabled change_part_type() by hiding
     // Support entries in the old choice dialog; the UI-side guard for the current submenu lives
     // in MenuFactory::append_menu_item_change_type, see #13120).
+    // The same rationale applies to any Precise Seam subtype.
     // This block must never be reachable under a healthy UI; if it ever logs, the UI guard has
     // been bypassed (new entry point, refactor, plugin, etc.) and should be investigated.
-    if (new_type == ModelVolumeType::SUPPORT_BLOCKER || new_type == ModelVolumeType::SUPPORT_ENFORCER) {
+    if (new_type == ModelVolumeType::SUPPORT_BLOCKER || new_type == ModelVolumeType::SUPPORT_ENFORCER
+        || is_precise_seam(new_type)) {
         const bool has_text_or_svg = std::any_of(volumes.begin(), volumes.end(),
             [](const VolumeSelection& sel) { return sel.volume->is_svg() || sel.volume->is_text(); });
         if (has_text_or_svg) {
             BOOST_LOG_TRIVIAL(error) << __FUNCTION__
-                << ": blocked attempt to set SUPPORT_BLOCKER/ENFORCER on SVG/text volume; "
+                << ": blocked attempt to set SUPPORT_BLOCKER/ENFORCER or Precise Seam on SVG/text volume; "
                 << "UI guard should have prevented this -- possible regression in the Change Type menu";
             return;
         }
     }
 
+    // --- Subtype preservation for the generic "Precise Seam" entry ---
+    // The Change Type submenu uses PRECISE_SEAM_CENTER as the single "Precise Seam" entry
+    // and calls with preserve_ps_subtype=true: volumes that are already Precise Seam keep
+    // their subtype (LEFT/RIGHT/etc.); only non-PS volumes get converted to the default
+    // CENTER subtype.
+    //
+    // The "Precise Seam Type" subtype submenu calls with preserve_ps_subtype=false: the
+    // user explicitly picked CENTER and all selected PS volumes must be set to CENTER
+    // verbatim (otherwise CENTER would become unreachable for mixed-subtype selections,
+    // since preservation would keep every volume at its current subtype and any_diff would
+    // be false — the reason for this two-parameter split).
+    auto effective_new_type = [new_type, preserve_ps_subtype](const ModelVolume* v) -> ModelVolumeType {
+        if (preserve_ps_subtype && new_type == ModelVolumeType::PRECISE_SEAM_CENTER && v->is_precise_seam())
+            return v->type();
+        return new_type;
+    };
+
+    // --- Any-change check using the effective target type ---
     const bool any_diff = std::any_of(volumes.begin(), volumes.end(),
-        [new_type](const VolumeSelection& sel) { return sel.volume->type() != new_type; });
+        [&effective_new_type](const VolumeSelection& sel) {
+            return sel.volume->type() != effective_new_type(sel.volume);
+        });
 
     if (!any_diff)
         return;
 
+    // --- Last-solid-part guard (pre-existing behavior) ---
+    // When converting away from MODEL_PART, ensure at least one solid part remains per object.
     if (new_type != ModelVolumeType::MODEL_PART) {
         std::map<int, int> total_part_cnt;
         std::map<int, int> selected_part_cnt;
@@ -5838,14 +5937,36 @@ void ObjectList::set_volume_type(ModelVolumeType new_type)
 
     take_snapshot(_u8L("Change part type"));
 
+    // --- Apply type changes with Precise Seam group-aware repositioning ---
+    // Note: `changed_volumes` / `touched_objects` track every processed volume, including
+    // subtype-preservation no-ops. Tracking no-ops is intentional — it ensures such
+    // volumes remain selected after the post-apply rebuild. Without that, a cross-object
+    // mixed selection like [Part, PS_LEFT] clicking "Change type → Precise Seam" would
+    // silently deselect the preserved PS_LEFT: only the Part's object would enter
+    // touched_objects, and the final SetSelections(new_selection) would drop PS_LEFT.
     std::set<const ModelVolume*> changed_volumes;
     std::set<int>                touched_objects;
     for (const auto& sel : volumes) {
-        sel.volume->set_type(new_type);
+        const ModelVolumeType target   = effective_new_type(sel.volume);
+        const ModelVolumeType old_type = sel.volume->type();
+
+        // Record the volume for reselection before the no-op short-circuit (see comment above).
         changed_volumes.insert(sel.volume);
         touched_objects.insert(sel.object_idx);
+
+        if (old_type == target)
+            continue; // subtype-preservation no-op — nothing to write/move
+
+        sel.volume->set_type(target);
+
+        // If the change crosses a PS group boundary, move the volume to the end of volumes[]
+        // so the stable sort in reorder_volumes_and_get_selection() places it at the end of
+        // its new group (see precise_seam_group_changed() for details).
+        if (precise_seam_group_changed(old_type, target))
+            move_volume_to_end((*m_objects)[sel.object_idx], sel.volume);
     }
 
+    // --- Rebuild selection to follow the changed volumes ---
     wxDataViewItemArray new_selection;
     for (int obj_idx : touched_objects) {
         wxDataViewItemArray sel_items = reorder_volumes_and_get_selection(obj_idx, [&changed_volumes](const ModelVolume* volume) {

src/slic3r/GUI/GUI_ObjectList.hpp

@@ -88,6 +88,7 @@ struct MeshErrorsInfo
 class ObjectList : public wxDataViewCtrl
 {
 public:
+
     enum SELECTION_MODE
     {
         smUndef     = 0,
@@ -430,7 +431,12 @@ public:
 #if 0 // ORCA: disabled alongside definition in GUI_ObjectList.cpp (see #if 0 block there)
     void change_part_type();
 #endif
-	void set_volume_type(ModelVolumeType new_type);
+	// preserve_ps_subtype = true: when new_type is PRECISE_SEAM_CENTER, volumes that are
+	//   already Precise Seam keep their existing subtype (LEFT/RIGHT/etc.). Used by the
+	//   generic "Change type → Precise Seam" entry where CENTER is a default fallback.
+	// preserve_ps_subtype = false: no preservation — target type is applied verbatim. Used
+	//   by the "Precise Seam Type" subtype picker where the user explicitly wants CENTER.
+	void set_volume_type(ModelVolumeType new_type, bool preserve_ps_subtype = true);
     ModelVolumeType get_selected_volume_type();
 
     void last_volume_is_deleted(const int obj_idx);

src/slic3r/GUI/GUI_ObjectTable.cpp

@@ -348,7 +348,8 @@ void GridCellFilamentsRenderer::Draw(wxGrid &grid, wxGridCellAttr &attr, wxDC &d
         if ((grid_row->model_volume_type != ModelVolumeType::NEGATIVE_VOLUME) && \
             (grid_row->model_volume_type != ModelVolumeType::SUPPORT_BLOCKER) && \
             (grid_row->model_volume_type != ModelVolumeType::SUPPORT_ENFORCER) && \
-            (grid_row->model_volume_type != ModelVolumeType::PARAMETER_MODIFIER)) {
+            (grid_row->model_volume_type != ModelVolumeType::PARAMETER_MODIFIER) && \
+            !is_precise_seam(grid_row->model_volume_type)) { // Precise Seam is non-printing helper geometry
             dc.DrawBitmap(*bitmap, wxPoint(rect.x + offset_x, rect.y + offset_y));
         }
         else if (grid_row->model_volume_type == ModelVolumeType::PARAMETER_MODIFIER){
@@ -3011,7 +3012,8 @@ void ObjectTablePanel::load_data()
                         if (col == ObjectGridTable::col_filaments) {
                             if ((grid_row->model_volume_type != ModelVolumeType::NEGATIVE_VOLUME) && \
                                 (grid_row->model_volume_type != ModelVolumeType::SUPPORT_BLOCKER) && \
-                                (grid_row->model_volume_type != ModelVolumeType::SUPPORT_ENFORCER)) {
+                                (grid_row->model_volume_type != ModelVolumeType::SUPPORT_ENFORCER) && \
+                                !is_precise_seam(grid_row->model_volume_type)) { // Precise Seam is non-printing helper geometry
                                 GridCellFilamentsEditor* filament_editor = new GridCellFilamentsEditor(grid_col->choices, false, &m_color_bitmaps);
                                 m_object_grid->SetCellEditor(row, col, filament_editor);
                                 m_object_grid->SetCellRenderer(row, col, new GridCellFilamentsRenderer());

src/slic3r/GUI/Gizmos/GLGizmoEmboss.cpp

@@ -2000,7 +2000,7 @@ void GLGizmoEmboss::draw_model_type()
         if ((is_volume_move_inside || is_volume_move_outside))
             process();
 
-        // inspiration in ObjectList::change_part_type()
+        // inspiration in ObjectList::set_volume_type()
         // how to view correct side panel with objects
         ObjectList *obj_list = app.obj_list();
         wxDataViewItemArray sel = obj_list->reorder_volumes_and_get_selection(

src/slic3r/GUI/Gizmos/GLGizmoSVG.cpp

@@ -2052,7 +2052,7 @@ void GLGizmoSVG::draw_model_type()
         if ((is_volume_move_inside || is_volume_move_outside))
             process();
 
-        // inspiration in ObjectList::change_part_type()
+        // inspiration in ObjectList::set_volume_type()
         // how to view correct side panel with objects
         ObjectList *obj_list = app.obj_list();
         wxDataViewItemArray sel = obj_list->reorder_volumes_and_get_selection(

src/slic3r/GUI/NotificationManager.cpp

@@ -11,6 +11,7 @@
 #include "MainFrame.hpp"
 #include "Tab.hpp"
 #include "libslic3r/Config.hpp"
+#include "libslic3r/PrintBase.hpp"
 #include "format.hpp"
 
 #include <boost/algorithm/string.hpp>
@@ -3084,7 +3085,9 @@ bool NotificationManager::push_notification_data(std::unique_ptr<NotificationMan
 	bool retval = false;
 	if (this->activate_existing(notification.get())) {
 		if (m_initialized && m_imgui_ready) {
-			if (notification->get_type() == NotificationType::SlicingWarning) {
+			// Precise Seam already aggregates all causes; replace it on repeated warning events.
+			if (notification->get_type() == NotificationType::SlicingWarning &&
+                notification->get_data().sub_msg_id != PrintStateBase::SlicingPreciseSeamWarning) {
 				m_pop_notifications.back()->append(notification->get_data().ori_text);
 			} else {
                 m_pop_notifications.back()->update(notification->get_data());

src/slic3r/GUI/ObjectDataViewModel.cpp

@@ -546,7 +546,8 @@ void ObjectDataViewModel::UpdateBitmapForNode(ObjectDataViewModelNode *node)
 {
     bool is_volume_node = node->GetType() & itVolume;
     int  vol_type       = static_cast<int>(node->GetVolumeType());
-    is_volume_node &= (vol_type >= int(ModelVolumeType::MODEL_PART) && vol_type <= int(ModelVolumeType::SUPPORT_ENFORCER));
+    // Extended range to include Precise Seam modifier types
+    is_volume_node &= (vol_type >= int(ModelVolumeType::MODEL_PART) && vol_type <= int(ModelVolumeType::PRECISE_SEAM_NEUTRAL));
 
     if (!node->has_warning_icon() && !node->has_lock()) {
         node->SetBitmap(is_volume_node ? (

src/slic3r/GUI/Selection.cpp

@@ -67,7 +67,7 @@ bool Selection::Clipboard::is_sla_compliant() const
             return false;
 
         for (const ModelVolume* v : o->volumes) {
-            if (v->is_modifier())
+            if (v->is_modifier() || v->is_precise_seam()) // Precise Seam not supported in SLA
                 return false;
         }
     }

src/slic3r/plugin/host/PluginHostModel.cpp

@@ -24,7 +24,14 @@ void host_bindings::register_model(py::module_& host)
         .value("NegativeVolume", ModelVolumeType::NEGATIVE_VOLUME)
         .value("ParameterModifier", ModelVolumeType::PARAMETER_MODIFIER)
         .value("SupportBlocker", ModelVolumeType::SUPPORT_BLOCKER)
-        .value("SupportEnforcer", ModelVolumeType::SUPPORT_ENFORCER);
+        .value("SupportEnforcer", ModelVolumeType::SUPPORT_ENFORCER)
+        // Expose every seam mode returned by ModelVolume::type() to plugins.
+        .value("PreciseSeamCenter", ModelVolumeType::PRECISE_SEAM_CENTER)
+        .value("PreciseSeamLeft", ModelVolumeType::PRECISE_SEAM_LEFT)
+        .value("PreciseSeamRight", ModelVolumeType::PRECISE_SEAM_RIGHT)
+        .value("PreciseSeamEnforced", ModelVolumeType::PRECISE_SEAM_ENFORCED)
+        .value("PreciseSeamBlocked", ModelVolumeType::PRECISE_SEAM_BLOCKED)
+        .value("PreciseSeamNeutral", ModelVolumeType::PRECISE_SEAM_NEUTRAL);
 
     py::class_<ModelVolume, std::unique_ptr<ModelVolume, py::nodelete>>(host, "ModelVolume")
         .def("id", [](const ModelVolume& volume) { return volume.id().id; })
@@ -36,6 +43,10 @@ void host_bindings::register_model(py::module_& host)
         .def("is_support_enforcer", &ModelVolume::is_support_enforcer)
         .def("is_support_blocker", &ModelVolume::is_support_blocker)
         .def("is_support_modifier", &ModelVolume::is_support_modifier)
+        // Let plugins identify seam helper geometry without relying on enum ordering.
+        .def("is_precise_seam", &ModelVolume::is_precise_seam)
+        .def("is_precise_seam_strong", &ModelVolume::is_precise_seam_strong)
+        .def("is_precise_seam_weak", &ModelVolume::is_precise_seam_weak)
         // Extruder ID is 1-based for FFF, -1 for SLA or support volumes.
         .def("extruder_id", &ModelVolume::extruder_id)
         .def("offset", [](const ModelVolume& volume) { return vec3_to_tuple(volume.get_offset()); })

tests/fff_print/CMakeLists.txt

@@ -14,9 +14,12 @@ add_executable(${_TEST_NAME}_tests
 	test_model.cpp
 	test_multifilament.cpp
 	test_perimeters.cpp
+	# Seam geometry and pipeline regressions share the existing Print fixtures.
+	test_precise_seam.cpp
 	test_print.cpp
 	test_printobject.cpp
 	test_mixed_filament.cpp
+	test_seam_placer.cpp
 	test_skirt_brim.cpp
 	test_slicing_pipeline_hook.cpp
 	test_support_material.cpp

tests/fff_print/test_precise_seam.cpp

@@ -0,0 +1,279 @@
+#include <catch2/catch_all.hpp>
+
+#include "test_helpers.hpp"
+#include "libslic3r/GCode/PreciseSeam.hpp"
+
+#include <algorithm>
+
+using namespace Slic3r;
+
+namespace {
+Point mm(double x, double y) { return Point(scale_(x), scale_(y)); }
+
+Polygon rectangle(double x0, double y0, double x1, double y1)
+{
+    // Counterclockwise contours match the modifier-slice cache contract.
+    return Polygon(Points{mm(x0, y0), mm(x1, y0), mm(x1, y1), mm(x0, y1)});
+}
+
+struct SeamFixture {
+    Model model;
+    Print print;
+    Model modifiers;
+    Layer *layer = nullptr;
+    PreciseSeam::ModifierSlicesCache cache;
+
+    SeamFixture()
+    {
+        // Only the layer/PrintObject context is needed; clipping uses explicit cached slices below.
+        Test::init_print({Test::cube(20)}, print, model, {{"raft_layers", "0"}});
+        REQUIRE(print.objects().size() == 1);
+        PrintObject *object = print.get_object(0);
+        layer = object->add_layer(int(object->slicing_parameters().raft_layers()), 0.2, 0.2, 0.1);
+        modifiers.add_object();
+    }
+
+    const ModelVolume *add(ModelVolumeType type, Polygons slices)
+    {
+        // These volumes own cache keys; mesh slicing is deliberately outside this geometry fixture.
+        ModelVolume *volume = modifiers.objects.front()->add_volume(Test::cube(1));
+        volume->set_type(type);
+        cache.emplace(volume, std::vector<Polygons>{std::move(slices)});
+        return volume;
+    }
+};
+
+void check_square_boundary(const Polygon &polygon)
+{
+    // Wrong insertion edges can retrace a side without changing area: check length as well.
+    CHECK_THAT(unscale<double>(polygon.length()), Catch::Matchers::WithinAbs(80.0, 0.00001));
+    for (const Point &p : polygon.points) {
+        CAPTURE(p.x(), p.y());
+        CHECK(p.x() >= scale_(0));
+        CHECK(p.x() <= scale_(20));
+        CHECK(p.y() >= scale_(0));
+        CHECK(p.y() <= scale_(20));
+        const bool on_boundary = p.x() == 0 || p.x() == scale_(20) || p.y() == 0 || p.y() == scale_(20);
+        CHECK(on_boundary);
+    }
+}
+
+void require_vertex(const Polygon &polygon, const Point &point)
+{
+    // Integer coordinates make the micron transition helpers exact on these axis-aligned edges.
+    CAPTURE(point.x(), point.y());
+    REQUIRE(std::find(polygon.points.begin(), polygon.points.end(), point) != polygon.points.end());
+}
+} // namespace
+
+TEST_CASE("Strong seam modes select the requested location on a clipped side", "[PreciseSeam]")
+{
+    const auto mode = GENERATE(ModelVolumeType::PRECISE_SEAM_LEFT, ModelVolumeType::PRECISE_SEAM_CENTER,
+                              ModelVolumeType::PRECISE_SEAM_RIGHT);
+    SeamFixture fixture;
+    // Clipper may collapse all three original collinear edges into one.
+    Polygon perimeter(Points{mm(0, 0), mm(2, 0), mm(4, 0), mm(8, 0), mm(20, 0), mm(20, 20), mm(0, 20)});
+    const ModelVolume *modifier = fixture.add(mode, {rectangle(1, -2, 13, 2)});
+    PreciseSeam::PreciseSeamWarnings warnings;
+    const auto seam = PreciseSeam::insert_strong_seam_point({modifier}, perimeter, fixture.layer, fixture.cache, &warnings);
+    REQUIRE(seam.has_value());
+    const double expected_x = mode == ModelVolumeType::PRECISE_SEAM_LEFT ? 1.0 :
+                              mode == ModelVolumeType::PRECISE_SEAM_RIGHT ? 13.0 : 7.0;
+    CHECK(*seam == mm(expected_x, 0));
+    require_vertex(perimeter, mm(expected_x - 0.001, 0));
+    require_vertex(perimeter, mm(expected_x + 0.001, 0));
+    check_square_boundary(perimeter);
+    CHECK_FALSE(warnings.through_body.load());
+    CHECK_FALSE(warnings.full_containment.load());
+    CHECK_FALSE(warnings.multiple_intersections.load());
+}
+
+TEST_CASE("Center seams preserve the edge order at vertices and across the contour origin", "[PreciseSeam]")
+{
+    const bool wrap = GENERATE(false, true);
+    SeamFixture fixture;
+    Polygon perimeter(Points{mm(0, 0), mm(4, 0), mm(20, 0), mm(20, 20), mm(0, 20)});
+    // Symmetric cuts put the arc midpoint exactly on an existing vertex, including vertex zero.
+    const Polygon cut = wrap ? rectangle(-2, -2, 4, 4) : rectangle(1, -2, 7, 2);
+    const ModelVolume *modifier = fixture.add(ModelVolumeType::PRECISE_SEAM_CENTER, {cut});
+    const auto seam = PreciseSeam::insert_strong_seam_point({modifier}, perimeter, fixture.layer, fixture.cache);
+    REQUIRE(seam.has_value());
+    CHECK(*seam == (wrap ? mm(0, 0) : mm(4, 0)));
+    require_vertex(perimeter, wrap ? mm(0, 0.001) : mm(3.999, 0));
+    require_vertex(perimeter, wrap ? mm(0.001, 0) : mm(4.001, 0));
+    check_square_boundary(perimeter);
+}
+
+TEST_CASE("Center seams land on the closing edge", "[PreciseSeam]")
+{
+    SeamFixture fixture;
+    Polygon perimeter = rectangle(0, 0, 20, 20);
+    const auto *modifier = fixture.add(ModelVolumeType::PRECISE_SEAM_CENTER, {rectangle(-2, 3, 2, 9)});
+    const auto seam = PreciseSeam::insert_strong_seam_point({modifier}, perimeter, fixture.layer, fixture.cache);
+    REQUIRE(seam.has_value());
+    CHECK(*seam == mm(0, 6));
+    require_vertex(perimeter, mm(0, 5.999));
+    require_vertex(perimeter, mm(0, 6.001));
+    check_square_boundary(perimeter);
+}
+
+TEST_CASE("Coincident weak boundaries do not prevent later boundary refinement", "[PreciseSeam][Regression]")
+{
+    SeamFixture fixture;
+    Polygon perimeter = rectangle(0, 0, 20, 20);
+    // Duplicate boundaries used to stall the reverse cursor before reaching the separate segment.
+    const auto *a = fixture.add(ModelVolumeType::PRECISE_SEAM_BLOCKED, {rectangle(2, -2, 6, 2)});
+    const auto *b = fixture.add(ModelVolumeType::PRECISE_SEAM_NEUTRAL, {rectangle(2, -2, 6, 2)});
+    const auto *c = fixture.add(ModelVolumeType::PRECISE_SEAM_BLOCKED, {rectangle(10, -2, 14, 2)});
+    const auto segments = PreciseSeam::collect_weak_modifier_segments({c, b, a}, perimeter, fixture.layer, fixture.cache);
+    REQUIRE(segments.size() == 3);
+    for (double x : {1.999, 6.001, 9.999, 14.001})
+        require_vertex(perimeter, mm(x, 0));
+    check_square_boundary(perimeter);
+}
+
+TEST_CASE("Weak boundaries sharing a vertex refine both sides", "[PreciseSeam]")
+{
+    SeamFixture fixture;
+    Polygon perimeter = rectangle(0, 0, 20, 20);
+    const auto *a = fixture.add(ModelVolumeType::PRECISE_SEAM_BLOCKED, {rectangle(2, -2, 6, 2)});
+    const auto *b = fixture.add(ModelVolumeType::PRECISE_SEAM_NEUTRAL, {rectangle(6, -2, 10, 2)});
+    const auto segments = PreciseSeam::collect_weak_modifier_segments({a, b}, perimeter, fixture.layer, fixture.cache);
+    REQUIRE(segments.size() == 2);
+    require_vertex(perimeter, mm(5.999, 0));
+    require_vertex(perimeter, mm(6.001, 0));
+    check_square_boundary(perimeter);
+}
+
+TEST_CASE("Unsupported modifier sections are skipped with the appropriate warning", "[PreciseSeam]")
+{
+    const int scenario = GENERATE(0, 1, 2, 3);
+    SeamFixture fixture;
+    Polygon perimeter = rectangle(0, 0, 20, 20);
+    const Points original = perimeter.points;
+    Polygons slices;
+    if (scenario == 0) slices = {rectangle(30, 30, 40, 40)}; // Disjoint bounds.
+    if (scenario == 1) slices = {rectangle(2, 2, 4, 4)};     // Wholly inside; no common boundary.
+    if (scenario == 2) slices = {rectangle(-2, -2, 22, 22)}; // Contains the entire perimeter.
+    if (scenario == 3) {
+        Polygon hole = rectangle(2, 2, 4, 4);
+        hole.reverse();
+        slices = {rectangle(-2, -2, 22, 22), hole};
+    }
+    const auto *modifier = fixture.add(ModelVolumeType::PRECISE_SEAM_CENTER, std::move(slices));
+    PreciseSeam::PreciseSeamWarnings warnings;
+    CHECK_FALSE(PreciseSeam::insert_strong_seam_point({modifier}, perimeter, fixture.layer, fixture.cache, &warnings).has_value());
+    CHECK(perimeter.points == original);
+    CHECK(warnings.full_containment.load() == (scenario == 2));
+    CHECK(warnings.multiply_connected.load() == (scenario == 3));
+    CHECK_FALSE(warnings.multiple_intersections.load());
+    CHECK_FALSE(warnings.through_body.load());
+}
+
+TEST_CASE("Strong modifiers warn when another slice polygon also intersects the perimeter", "[PreciseSeam]")
+{
+    SeamFixture fixture;
+    Polygon perimeter = rectangle(0, 0, 20, 20);
+    // One helper has two disconnected sections; only its first section supplies the seam.
+    const auto *modifier = fixture.add(ModelVolumeType::PRECISE_SEAM_CENTER,
+                                       {rectangle(2, -2, 6, 2), rectangle(12, -2, 16, 2)});
+    PreciseSeam::PreciseSeamWarnings warnings;
+    const auto seam = PreciseSeam::insert_strong_seam_point({modifier}, perimeter, fixture.layer, fixture.cache, &warnings);
+    REQUIRE(seam.has_value());
+    CHECK(*seam == mm(4, 0));
+    CHECK(warnings.multiple_intersections.load());
+    CHECK_FALSE(warnings.through_body.load());
+    CHECK_FALSE(warnings.full_containment.load());
+    CHECK_FALSE(warnings.multiply_connected.load());
+    check_square_boundary(perimeter);
+}
+
+TEST_CASE("Modifiers crossing the entire body raise a through body warning", "[PreciseSeam]")
+{
+    const bool strong = GENERATE(false, true);
+    CAPTURE(strong);
+    SeamFixture fixture;
+    Polygon perimeter = rectangle(0, 0, 20, 20);
+    // The strip exits on opposite sides, leaving two exterior pieces. None of the clipped
+    // vertices matches a square corner, so this also exercises the general segment-extraction path.
+    const auto type = strong ? ModelVolumeType::PRECISE_SEAM_CENTER : ModelVolumeType::PRECISE_SEAM_BLOCKED;
+    const auto *modifier = fixture.add(type, {rectangle(8, -2, 12, 22)});
+    PreciseSeam::PreciseSeamWarnings warnings;
+    if (strong) {
+        const auto seam = PreciseSeam::insert_strong_seam_point({modifier}, perimeter, fixture.layer, fixture.cache, &warnings);
+        REQUIRE(seam.has_value());
+        CHECK(seam->x() == mm(10, 0).x());
+        // Either boundary segment may be encountered first by the clipping traversal.
+        const bool on_crossed_side = seam->y() == 0 || seam->y() == mm(0, 20).y();
+        CHECK(on_crossed_side);
+    } else {
+        const auto segments = PreciseSeam::collect_weak_modifier_segments({modifier}, perimeter, fixture.layer, fixture.cache, &warnings);
+        REQUIRE_FALSE(segments.empty());
+    }
+    CHECK(warnings.through_body.load());
+    CHECK_FALSE(warnings.multiple_intersections.load()); // The clipped strip is one polygon.
+    CHECK_FALSE(warnings.full_containment.load());
+    CHECK_FALSE(warnings.multiply_connected.load());
+    check_square_boundary(perimeter);
+}
+
+TEST_CASE("Modifier hierarchy keeps strong order and applies the highest weak priority last", "[PreciseSeam]")
+{
+    const auto high_type = GENERATE(ModelVolumeType::PRECISE_SEAM_BLOCKED, ModelVolumeType::PRECISE_SEAM_NEUTRAL,
+                                   ModelVolumeType::PRECISE_SEAM_ENFORCED);
+    const auto expected_type = high_type == ModelVolumeType::PRECISE_SEAM_BLOCKED ? SeamPlacerImpl::EnforcedBlockedSeamPoint::Blocked :
+                               high_type == ModelVolumeType::PRECISE_SEAM_NEUTRAL ? SeamPlacerImpl::EnforcedBlockedSeamPoint::Neutral :
+                                                                                  SeamPlacerImpl::EnforcedBlockedSeamPoint::Enforced;
+    SeamFixture fixture;
+    const auto *strong_a = fixture.add(ModelVolumeType::PRECISE_SEAM_CENTER, {rectangle(1, -2, 3, 2)});
+    const auto *strong_b = fixture.add(ModelVolumeType::PRECISE_SEAM_CENTER, {rectangle(11, -2, 13, 2)});
+    const auto *high = fixture.add(high_type, {rectangle(2, -2, 6, 2)});
+    const auto low_type = high_type == ModelVolumeType::PRECISE_SEAM_ENFORCED ? ModelVolumeType::PRECISE_SEAM_BLOCKED :
+                                                                                           ModelVolumeType::PRECISE_SEAM_ENFORCED;
+    const auto *low = fixture.add(low_type, {rectangle(2, -2, 6, 2)});
+    std::vector<const ModelVolume*> strong, weak;
+    bool has_strong = false;
+    PreciseSeam::init_precise_seam_data(strong, weak, has_strong, fixture.modifiers.objects.front());
+    REQUIRE(has_strong);
+    CHECK(strong == std::vector<const ModelVolume*>{strong_a, strong_b});
+    CHECK(weak == std::vector<const ModelVolume*>{low, high});
+    Polygon perimeter = rectangle(0, 0, 20, 20);
+    const auto seam = PreciseSeam::insert_strong_seam_point(strong, perimeter, fixture.layer, fixture.cache);
+    REQUIRE(seam.has_value());
+    CHECK(*seam == mm(2, 0));
+
+    perimeter = rectangle(0, 0, 20, 20);
+    const auto segments = PreciseSeam::collect_weak_modifier_segments(weak, perimeter, fixture.layer, fixture.cache);
+    REQUIRE(segments.size() == 2);
+    PrintObjectSeamData::LayerSeams result;
+    result.perimeters.emplace_back();
+    auto &loop = result.perimeters.back();
+    // Include a preceding candidate to exercise nonzero global layer indices.
+    result.points.emplace_back(Vec3f(-1, -1, 0), loop, 0, SeamPlacerImpl::EnforcedBlockedSeamPoint::Neutral);
+    loop.start_index = 1;
+    for (const Point &p : perimeter.points) {
+        // Match production's double-to-float conversion: weak boundary lookup uses exact equality.
+        const Vec2f position = unscale(p).cast<float>();
+        result.points.emplace_back(Vec3f(position.x(), position.y(), 0), loop, 0,
+                                   SeamPlacerImpl::EnforcedBlockedSeamPoint::Neutral);
+    }
+    loop.end_index = result.points.size();
+    bool enforced = false;
+    PreciseSeam::apply_weak_modifiers_to_perimeter(segments, result, loop, enforced);
+    size_t patch_count = 0;
+    for (size_t i = loop.start_index; i < loop.end_index; ++i) {
+        const auto &candidate = result.points[i];
+        // Axis-aligned input and interpolation keep y exactly zero; this classifies, rather than measures, the patch.
+        const bool in_patch = candidate.position.y() == 0 && candidate.position.x() >= 2 && candidate.position.x() <= 6;
+        CHECK(candidate.type == (in_patch ? expected_type :
+                                          SeamPlacerImpl::EnforcedBlockedSeamPoint::Neutral));
+        if (in_patch) ++patch_count;
+    }
+    CHECK(patch_count >= 2);
+    if (high_type == ModelVolumeType::PRECISE_SEAM_ENFORCED) {
+        // Four millimetres of enforcement must be subdivided, not just marked at its endpoints.
+        CHECK(enforced);
+        CHECK(patch_count >= size_t(4.0f / SeamPlacer::enforcer_oversampling_distance));
+    }
+    CHECK(result.points.front().type == SeamPlacerImpl::EnforcedBlockedSeamPoint::Neutral);
+}

tests/fff_print/test_seam_placer.cpp

@@ -0,0 +1,281 @@
+#include <catch2/catch_all.hpp>
+
+#include "test_helpers.hpp"
+#include "libslic3r/GCode/SeamPlacer.hpp"
+#include "libslic3r/Layer.hpp"
+#include "libslic3r/TriangleSelector.hpp"
+
+#include <algorithm>
+#include <cmath>
+
+using namespace Slic3r;
+
+namespace {
+struct PipelineFixture {
+    Model model;
+    Print print;
+    DynamicPrintConfig config = DynamicPrintConfig::full_print_config();
+
+    explicit PipelineFixture(bool trapezoid = false)
+    {
+        auto mesh = its_make_cube(20, 20, 0.4);
+        if (trapezoid) {
+            // Opposite painted sides have deliberately different lengths: 20 mm and 6 mm.
+            for (auto &vertex : mesh.vertices)
+                if (vertex.y() == 20.0f) vertex.x() = 7.0f + 0.3f * vertex.x();
+        }
+        config.set_deserialize_strict("seam_position", "back"); // Rear needs no visibility ray tracing.
+        config.set_deserialize_strict("layer_height", "0.2");
+        config.set_deserialize_strict("initial_layer_print_height", "0.2");
+        config.set_deserialize_strict("outer_wall_line_width", "0.4");
+        config.set_deserialize_strict("initial_layer_line_width", "0.4");
+        config.set_deserialize_strict("wall_loops", "1");
+        config.set_deserialize_strict("raft_layers", "0");
+        config.set_deserialize_strict("gcode_comments", "1"); // Match init_print so later apply calls change only the model.
+        Test::init_print({TriangleMesh(std::move(mesh))}, print, model, config);
+    }
+
+    void paint(bool all_faces)
+    {
+        auto &volume = *model.objects.front()->volumes.front();
+        const auto &mesh = volume.mesh();
+        const auto bounds = mesh.bounding_box();
+        TriangleSelector selector(mesh);
+        size_t painted = 0;
+        for (size_t i = 0; i < mesh.its.indices.size(); ++i) {
+            const auto &face = mesh.its.indices[i];
+            bool lower = true, upper = true;
+            for (int j = 0; j < 3; ++j) {
+                const auto &v = mesh.its.vertices[face[j]];
+                lower = lower && std::abs(double(v.y()) - bounds.min.y()) < 1e-6;
+                upper = upper && std::abs(double(v.y()) - bounds.max.y()) < 1e-6;
+            }
+            if (all_faces || lower || upper) {
+                selector.set_facet(int(i), EnforcerBlockerType::ENFORCER);
+                ++painted;
+            }
+        }
+        REQUIRE(painted > 0);
+        volume.seam_facets.set(selector);
+        print.apply(model, config);
+    }
+
+    PrintObject &prepare()
+    {
+        REQUIRE(print.objects().size() == 1);
+        auto &object = *print.get_object(0);
+        object.slice(); // Real layers/regions are sufficient: each test supplies its own perimeter loops.
+        REQUIRE_FALSE(object.layers().empty());
+        REQUIRE_FALSE(object.layers().front()->regions().empty());
+        return object;
+    }
+
+    Points points_in_layer(const PrintObject &object, const std::vector<Vec2d> &xy) const
+    {
+        const auto &volume = *object.model_object()->volumes.front();
+        const auto minimum = volume.mesh().bounding_box().min;
+        const Transform3d transform = object.trafo_centered() * volume.get_matrix();
+        Points points;
+        for (const auto &point : xy) {
+            // add_volume centers the mesh; restore its local offset before applying the slicing transform.
+            const Vec3d local = minimum + Vec3d(point.x(), point.y(), 0.2);
+            const Vec3d placed = transform * local;
+            points.emplace_back(scale_(placed.x()), scale_(placed.y()));
+        }
+        return points;
+    }
+};
+
+void append_loop(LayerRegion &region, Points points, bool separate_paths = false)
+{
+    // Inject deterministic external loops while keeping the real layer, region and paint-query machinery.
+    REQUIRE(points.size() >= 3);
+    points.push_back(points.front());
+    ExtrusionPaths paths;
+    if (separate_paths) {
+        for (size_t i = 1; i < points.size(); ++i) {
+            ExtrusionPath path(erExternalPerimeter, 0.08, 0.4f, 0.2f);
+            path.polyline = Polyline3(Polyline(Points{points[i - 1], points[i]}));
+            paths.push_back(std::move(path));
+        }
+    } else {
+        ExtrusionPath path(erExternalPerimeter, 0.08, 0.4f, 0.2f);
+        path.polyline = Polyline3(Polyline(std::move(points)));
+        paths.push_back(std::move(path));
+    }
+    region.perimeters.append(ExtrusionLoop(std::move(paths)));
+}
+
+LayerRegion &clear_first_layer(PrintObject &object)
+{
+    Layer &layer = *object.layers().front();
+    for (LayerRegion *region : layer.regions()) region->perimeters.clear();
+    return *layer.get_region(0);
+}
+} // namespace
+
+TEST_CASE("Painted seams prefer the longer candidate patch regardless of contour origin", "[SeamPlacer][Regression]")
+{
+    const bool clockwise = GENERATE(false, true);
+    const bool wrapped = GENERATE(false, true);
+    CAPTURE(clockwise, wrapped);
+    PipelineFixture fixture(true);
+    fixture.paint(false);
+    PrintObject &object = fixture.prepare();
+    REQUIRE(object.model_object()->volumes.size() == 1);
+    CHECK_FALSE(object.model_object()->volumes.front()->is_precise_seam());
+
+    auto &region = clear_first_layer(object);
+    // A neutral loop ensures that patch indices are offsets in the layer, not zero-based local indices.
+    append_loop(region, fixture.points_in_layer(object, {{9, 8}, {11, 8}, {10, 10}}));
+    Points outline = fixture.points_in_layer(object, {{10, 20}, {7, 20}, {3.5, 10}, {0, 0}, {10, 0}, {20, 0}, {13, 20}});
+    if (clockwise) std::reverse(outline.begin() + 1, outline.end()); // Keep the same starting vertex.
+    if (!wrapped) {
+        const Point neutral = fixture.points_in_layer(object, {{3.5, 10}}).front();
+        const auto start = std::find(outline.begin(), outline.end(), neutral);
+        REQUIRE(start != outline.end());
+        std::rotate(outline.begin(), start, outline.end());
+    }
+    append_loop(region, std::move(outline));
+    SeamPlacer placer;
+    placer.init(fixture.print, [] {});
+    const auto &data = placer.m_seam_per_object.at(&object).layers.front();
+    REQUIRE(data.perimeters.size() == 2);
+    const auto &perimeter = data.perimeters[1];
+    REQUIRE(perimeter.start_index > 0);
+    REQUIRE(perimeter.end_index > perimeter.start_index);
+    using Type = SeamPlacerImpl::EnforcedBlockedSeamPoint;
+    CHECK((data.points[perimeter.start_index].type == Type::Enforced) == wrapped);
+    if (wrapped) CHECK(data.points[perimeter.end_index - 1].type == Type::Enforced);
+
+    const auto extremes = fixture.points_in_layer(object, {{10, 0}, {10, 20}});
+    const double bottom_y = unscale<double>(extremes[0].y()), top_y = unscale<double>(extremes[1].y());
+    size_t bottom_count = 0, top_count = 0, centers = 0;
+    for (size_t i = perimeter.start_index; i < perimeter.end_index; ++i) {
+        const auto &candidate = data.points[i];
+        if (candidate.type == Type::Enforced) {
+            // Include the small paint-radius fringe at the ends of each face.
+            const bool bottom = std::abs(candidate.position.y() - bottom_y) < 0.5;
+            const bool top = std::abs(candidate.position.y() - top_y) < 0.5;
+            const bool on_painted_face = bottom || top;
+            CAPTURE(candidate.position.x(), candidate.position.y());
+            CHECK(on_painted_face);
+            bottom_count += bottom;
+            top_count += top;
+        }
+        if (candidate.central_enforcer) {
+            ++centers;
+            CHECK(candidate.type == Type::Enforced);
+            CHECK_THAT(double(candidate.position.y()), Catch::Matchers::WithinAbs(bottom_y, 0.5));
+        }
+    }
+    REQUIRE(top_count > 0);
+    REQUIRE(bottom_count > top_count);
+    CHECK(centers == 1); // The old wrapped-length formula instead selected the short top patch.
+}
+
+TEST_CASE("Entirely painted contours keep valid enforced seam candidates", "[SeamPlacer]")
+{
+    PipelineFixture fixture;
+    fixture.paint(true);
+    PrintObject &object = fixture.prepare();
+    auto &region = clear_first_layer(object);
+    append_loop(region, fixture.points_in_layer(object, {{0, 0}, {20, 0}, {20, 20}, {0, 20}}));
+    SeamPlacer placer;
+    placer.init(fixture.print, [] {});
+    const auto &data = placer.m_seam_per_object.at(&object).layers.front();
+    REQUIRE(data.perimeters.size() == 1);
+    const auto &perimeter = data.perimeters.front();
+    CHECK(perimeter.seam_index >= perimeter.start_index);
+    CHECK(perimeter.seam_index < perimeter.end_index);
+    for (const auto &candidate : data.points) {
+        CHECK(candidate.type == SeamPlacerImpl::EnforcedBlockedSeamPoint::Enforced);
+        CHECK_FALSE(candidate.central_enforcer); // There is no bounded patch to mark as central.
+    }
+}
+
+TEST_CASE("Precise Seam removes path junction duplicates but preserves separate visits", "[SeamPlacer][PreciseSeam]")
+{
+    const bool enable_ps = GENERATE(false, true);
+    const bool self_touch = GENERATE(false, true);
+    PipelineFixture fixture;
+    if (enable_ps) {
+        auto *helper = fixture.model.objects.front()->add_volume(make_cube(1, 1, 1));
+        helper->set_type(ModelVolumeType::PRECISE_SEAM_NEUTRAL);
+        helper->set_offset(Vec3d(100, 100, 0)); // Enable normalization without intersecting the synthetic loop.
+        fixture.print.apply(fixture.model, fixture.config);
+    }
+    PrintObject &object = fixture.prepare();
+    auto &region = clear_first_layer(object);
+    const std::vector<Vec2d> vertices = self_touch ? std::vector<Vec2d>{{2, 2}, {10, 2}, {18, 10}, {10, 2}, {2, 18}} :
+                                                   std::vector<Vec2d>{{2, 2}, {18, 2}, {18, 18}, {2, 18}};
+    const Points outline = fixture.points_in_layer(object, vertices);
+    append_loop(region, outline, true);
+    SeamPlacer placer;
+    placer.init(fixture.print, [] {});
+    const auto &data = placer.m_seam_per_object.at(&object).layers.front();
+    REQUIRE(data.perimeters.size() == 1);
+    // Each separate path contributes both endpoints in ordinary mode; PS removes only adjacent copies.
+    REQUIRE(data.points.size() == (enable_ps ? outline.size() : 2 * outline.size()));
+    for (size_t i = 0; i < outline.size(); ++i) {
+        const Vec2f target = unscale(outline[i]).cast<float>();
+        const size_t input_count = std::count(outline.begin(), outline.end(), outline[i]);
+        // Both paths convert the same integer coordinates to float; exact identity detects duplicate copies.
+        const size_t actual_count = std::count_if(data.points.begin(), data.points.end(), [&](const auto &candidate) {
+            return candidate.position.template head<2>() == target;
+        });
+        CHECK(actual_count == (enable_ps ? input_count : 2 * input_count));
+    }
+    if (enable_ps) {
+        for (size_t i = 0; i < data.points.size(); ++i) {
+            CHECK(std::isfinite(data.points[i].local_ccw_angle));
+            CHECK(data.points[i].position != data.points[(i + 1) % data.points.size()].position);
+        }
+    }
+}
+
+TEST_CASE("Print apply synchronizes support and seam helpers through type changes and restored models", "[SeamPlacer][PreciseSeam][Print]")
+{
+    const int changed = GENERATE(0, 1, 2); // Support only, seam only, or both including cross-family switches.
+    PipelineFixture fixture;
+    auto *model_object = fixture.model.objects.front();
+    auto *support = model_object->add_volume(make_cube(1, 1, 1));
+    support->set_type(ModelVolumeType::SUPPORT_BLOCKER);
+    auto *seam = model_object->add_volume(make_cube(1, 1, 1));
+    seam->set_type(ModelVolumeType::PRECISE_SEAM_CENTER);
+    fixture.print.apply(fixture.model, fixture.config);
+    REQUIRE(fixture.print.objects().size() == 1);
+    const PrintObject *original_print_object = fixture.print.objects().front();
+    const ModelVolume *original_part = original_print_object->model_object()->volumes.front();
+    const Model before(fixture.model); // A restored model snapshot preserves IDs, as the apply path requires.
+    if (changed == 0 || changed == 2) {
+        support->set_type(changed == 2 ? ModelVolumeType::PRECISE_SEAM_LEFT : ModelVolumeType::SUPPORT_ENFORCER);
+        support->set_offset(Vec3d(3, 4, 0));
+    }
+    if (changed == 1 || changed == 2) {
+        seam->set_type(changed == 2 ? ModelVolumeType::SUPPORT_BLOCKER : ModelVolumeType::PRECISE_SEAM_RIGHT);
+        seam->set_offset(Vec3d(-3, 2, 0));
+    }
+    if (changed == 2) std::swap(model_object->volumes[1], model_object->volumes[2]);
+    const auto check_applied = [&](const Model &expected) {
+        REQUIRE(fixture.print.objects().size() == 1);
+        // Helper-only changes should preserve the print object and its unaffected printable volume.
+        CHECK(fixture.print.objects().front() == original_print_object);
+        const auto &actual = fixture.print.objects().front()->model_object()->volumes;
+        const auto &wanted = expected.objects.front()->volumes;
+        REQUIRE(actual.size() == wanted.size());
+        CHECK(actual.front() == original_part);
+        for (size_t i = 0; i < wanted.size(); ++i) {
+            CAPTURE(changed, i);
+            CHECK(actual[i]->id() == wanted[i]->id());
+            CHECK(actual[i]->type() == wanted[i]->type());
+            CHECK(actual[i]->get_matrix().isApprox(wanted[i]->get_matrix(), 1e-9));
+        }
+    };
+    fixture.print.apply(fixture.model, fixture.config);
+    check_applied(fixture.model);
+    fixture.print.apply(before, fixture.config);
+    check_applied(before);
+    fixture.print.apply(fixture.model, fixture.config);
+    check_applied(fixture.model);
+}

tests/libslic3r/CMakeLists.txt

@@ -3,6 +3,8 @@ get_filename_component(_TEST_NAME ${CMAKE_CURRENT_LIST_DIR} NAME)
 add_executable(${_TEST_NAME}_tests
     ${_TEST_NAME}_tests.cpp
     test_3mf.cpp
+    # Round-trip seam metadata and active/dormant volume settings in both formats.
+    test_precise_seam_3mf.cpp
     test_aabbindirect.cpp
     test_appconfig.cpp
     test_arachne_walls.cpp

tests/libslic3r/test_precise_seam_3mf.cpp

@@ -0,0 +1,372 @@
+#include <catch2/catch_all.hpp>
+
+#include "test_utils.hpp"
+#include "libslic3r/Model.hpp"
+#include "libslic3r/PrintConfig.hpp"
+#include "libslic3r/Preset.hpp"
+#include "libslic3r/Semver.hpp"
+#include "libslic3r/Format/3mf.hpp"
+#include "libslic3r/Format/bbs_3mf.hpp"
+#include "libslic3r/miniz_extension.hpp"
+#include "libslic3r/Zipper.hpp"
+
+#include <algorithm>
+#include <array>
+#include <string>
+#include <vector>
+
+using namespace Slic3r;
+
+namespace {
+enum class Backend { Bbs, Prusa };
+constexpr std::array<ModelVolumeType, 6> seam_types = {
+    ModelVolumeType::PRECISE_SEAM_CENTER, ModelVolumeType::PRECISE_SEAM_LEFT,
+    ModelVolumeType::PRECISE_SEAM_RIGHT, ModelVolumeType::PRECISE_SEAM_ENFORCED,
+    ModelVolumeType::PRECISE_SEAM_BLOCKED, ModelVolumeType::PRECISE_SEAM_NEUTRAL
+};
+// Literal entities and backslashes distinguish XML escaping from config serialization.
+const std::string notes = "quoted \"value\" & <tag>\tcolumn\nnext line &amp; path\\file";
+
+struct Scene {
+    // Keep the backup directory alive longer than its model; no project files are touched.
+    ScopedTemporaryDir backup{"orca_seam_3mf"};
+    Model model;
+    DynamicPrintConfig config = DynamicPrintConfig::full_print_config();
+    bool share_mesh = false;
+
+    Scene()
+    {
+        model.set_backup_path(backup.string());
+        // Import normalizes extruder indices against this list; explicitly provide filament 2.
+        config.set_key_value("filament_settings_id", new ConfigOptionStrings(std::vector<std::string>{"A", "B"}));
+    }
+
+    void populate(bool all_modes, bool shared_mesh = false)
+    {
+        share_mesh = shared_mesh;
+        auto *object = model.add_object();
+        object->name = "seam round trip";
+        auto *part = object->add_volume(make_cube(20, 20, 2));
+        part->name = "printable";
+        ModelVolume *first_helper = nullptr;
+        for (size_t i = 0; i < (all_modes ? seam_types.size() : size_t(1)); ++i) {
+            auto *volume = shared_mesh && first_helper ? object->add_volume_with_shared_mesh(*first_helper) :
+                                                        object->add_volume(make_cube(2, 3, 4));
+            if (!first_helper) first_helper = volume;
+            volume->name = "helper_" + std::to_string(i);
+            volume->set_type(seam_types[i]);
+            Geometry::Transformation transform;
+            transform.set_offset(Vec3d(3.0 * double(i), -2.0, 1.0));
+            transform.set_rotation(Vec3d(0.0, 0.0, 0.1 * double(i + 1)));
+            transform.set_scaling_factor(Vec3d(1.0, 1.2, 0.8));
+            volume->set_transformation(transform);
+            // Simulate settings retained by conversion from a part/modifier into a seam helper.
+            volume->config.set_key_value("extruder", new ConfigOptionInt(2));
+            volume->config.set_key_value("sparse_infill_density", new ConfigOptionPercent(100.0 - 5.0 * double(i)));
+            volume->config.set_key_value("notes", new ConfigOptionString(notes + std::to_string(i)));
+        }
+        object->add_instance();
+    }
+};
+
+void save(const std::string &path, Backend backend, Scene &scene)
+{
+    if (backend == Backend::Prusa) {
+        REQUIRE(store_3mf(path.c_str(), &scene.model, &scene.config, false));
+    } else {
+        StoreParams params;
+        params.path = path.c_str();
+        params.model = &scene.model;
+        params.config = &scene.config;
+        params.strategy = SaveStrategy::Zip64 | SaveStrategy::Silence;
+        if (scene.share_mesh) params.strategy = params.strategy | SaveStrategy::ShareMesh;
+        REQUIRE(store_bbs_3mf(params));
+    }
+}
+
+void load(const std::string &path, Backend backend, Scene &scene)
+{
+    ConfigSubstitutionContext substitutions{ForwardCompatibilitySubstitutionRule::Enable};
+    if (backend == Backend::Prusa) {
+        REQUIRE(load_3mf(path.c_str(), scene.config, substitutions, &scene.model, false));
+    } else {
+        struct ImportedResources {
+            PlateDataPtrs plates;
+            std::vector<Preset*> presets;
+            // Catch failures must not leak importer-owned auxiliary objects.
+            ~ImportedResources() { release_PlateData_list(plates); for (auto *preset : presets) delete preset; }
+        } resources;
+        bool bbs = false, orca = false;
+        Semver version;
+        REQUIRE(load_bbs_3mf(path.c_str(), &scene.config, &substitutions, &scene.model, &resources.plates,
+                            &resources.presets, &bbs, &orca, &version, nullptr,
+                            LoadStrategy::LoadModel | LoadStrategy::LoadConfig));
+    }
+}
+
+using Archive = std::vector<std::pair<std::string, std::string>>;
+Archive read_archive(const std::string &path)
+{
+    struct Reader {
+        mz_zip_archive zip{};
+        ~Reader() { if (zip.m_pState) close_zip_reader(&zip); }
+    } reader;
+    REQUIRE(open_zip_reader(&reader.zip, path));
+    Archive entries;
+    for (mz_uint i = 0; i < mz_zip_reader_get_num_files(&reader.zip); ++i) {
+        mz_zip_archive_file_stat stat;
+        REQUIRE(mz_zip_reader_file_stat(&reader.zip, i, &stat));
+        if (stat.m_is_directory) continue;
+        std::string name(stat.m_filename);
+        std::replace(name.begin(), name.end(), '\\', '/');
+        std::string data(size_t(stat.m_uncomp_size), '\0');
+        if (!data.empty()) REQUIRE(mz_zip_reader_extract_to_mem(&reader.zip, i, data.data(), data.size(), 0));
+        entries.emplace_back(std::move(name), std::move(data));
+    }
+    return entries;
+}
+
+std::string &model_xml(Archive &archive, Backend backend)
+{
+    const std::string name = backend == Backend::Bbs ? "Metadata/model_settings.config" : "Metadata/Slic3r_PE_model.config";
+    const auto found = std::find_if(archive.begin(), archive.end(), [&](const auto &entry) { return entry.first == name; });
+    REQUIRE(found != archive.end());
+    return found->second;
+}
+
+void write_archive(const std::string &path, const Archive &archive)
+{
+    // Repack a separate temporary archive, preserving every entry except explicitly edited metadata.
+    Zipper zipper(path);
+    for (const auto &entry : archive) zipper.add_entry(entry.first, entry.second.data(), entry.second.size());
+    zipper.finalize();
+}
+
+void replace_once(std::string &text, const std::string &from, const std::string &to)
+{
+    const auto pos = text.find(from);
+    REQUIRE(pos != std::string::npos);
+    REQUIRE(text.find(from, pos + from.size()) == std::string::npos);
+    text.replace(pos, from.size(), to);
+}
+
+std::string remove_mode_metadata(std::string &xml)
+{
+    // Mutations must locate exactly one real metadata element; silent no-op rewrites would give false confidence.
+    const auto key = xml.find("key=\"precise_seam_type\"");
+    REQUIRE(key != std::string::npos);
+    REQUIRE(xml.find("key=\"precise_seam_type\"", key + 1) == std::string::npos);
+    const auto begin = xml.rfind("<metadata ", key);
+    const auto end = xml.find("/>", key);
+    REQUIRE(begin != std::string::npos);
+    REQUIRE(end != std::string::npos);
+    const std::string result = xml.substr(begin, end + 2 - begin);
+    xml.erase(begin, result.size());
+    return result;
+}
+
+void check_config(const ModelVolume &volume, size_t index = 0)
+{
+    REQUIRE(volume.config.has("extruder"));
+    CHECK(volume.config.opt_int("extruder") == 2);
+    REQUIRE(volume.config.has("sparse_infill_density"));
+    CHECK_THAT(volume.config.opt_float("sparse_infill_density"), Catch::Matchers::WithinAbs(100.0 - 5.0 * double(index), 1e-9));
+    REQUIRE(volume.config.has("notes"));
+    CHECK(volume.config.get().opt_string("notes") == notes + std::to_string(index));
+}
+
+void check_modifier_config(const ModelVolume &volume, Backend backend, size_t index = 0)
+{
+    if (backend == Backend::Bbs) {
+        check_config(volume, index);
+    } else {
+        // The Prusa importer whitelists extruder, but drops ordinary notes and infill settings.
+        REQUIRE(volume.config.has("extruder"));
+        CHECK(volume.config.opt_int("extruder") == 2);
+        CHECK_FALSE(volume.config.has("sparse_infill_density"));
+        CHECK_FALSE(volume.config.has("notes"));
+    }
+}
+
+void check_geometry(const ModelVolume &before, const ModelVolume &after)
+{
+    // Importers may recenter a mesh and compensate in its transform; compare transformed vertices.
+    REQUIRE(after.mesh().its.vertices.size() == before.mesh().its.vertices.size());
+    CHECK(after.mesh().its.indices.size() == before.mesh().its.indices.size());
+    std::vector<Vec3d> expected;
+    for (const auto &v : before.mesh().its.vertices) expected.push_back(before.get_matrix() * v.cast<double>());
+    for (const auto &v : after.mesh().its.vertices) {
+        const Vec3d actual = after.get_matrix() * v.cast<double>();
+        const auto match = std::find_if(expected.begin(), expected.end(), [&](const Vec3d &p) { return (p - actual).norm() < 1e-4; });
+        REQUIRE(match != expected.end());
+        expected.erase(match); // Match multiplicities, not merely membership in the vertex set.
+    }
+    CHECK(expected.empty());
+}
+} // namespace
+
+TEST_CASE("All Precise Seam types and dormant settings survive a 3MF round trip", "[PreciseSeam3mf]")
+{
+    const auto backend = GENERATE(Backend::Bbs, Backend::Prusa);
+    const bool shared = GENERATE(false, true);
+    CAPTURE(int(backend), shared);
+    Scene source;
+    source.populate(true, shared);
+    ScopedTemporaryFile file(".3mf");
+    save(file.string(), backend, source);
+    Archive archive = read_archive(file.string());
+    const auto &xml = model_xml(archive, backend);
+    const std::string open = backend == Backend::Bbs ? "<part " : "<volume ";
+    const std::string close = backend == Backend::Bbs ? "</part>" : "</volume>";
+    size_t pos = 0, helpers = 0;
+    while ((pos = xml.find(open, pos)) != std::string::npos) {
+        const auto end = xml.find(close, pos);
+        REQUIRE(end != std::string::npos);
+        const auto block = xml.substr(pos, end - pos);
+        if (block.find("key=\"precise_seam_type\"") != std::string::npos) {
+            ++helpers;
+            CHECK(block.find(backend == Backend::Bbs ? "subtype=\"modifier_part\"" : "value=\"ParameterModifier\"") != std::string::npos);
+            for (const std::string key : {"extruder", "sparse_infill_density", "notes"}) {
+                CAPTURE(key);
+                CHECK(block.find("key=\"" + key + "\"") == std::string::npos);
+                CHECK(block.find("key=\"precise_seam_config:" + key + "\"") != std::string::npos);
+            }
+        }
+        pos = end + close.size();
+    }
+    REQUIRE(helpers == seam_types.size());
+    Scene destination;
+    load(file.string(), backend, destination);
+    REQUIRE(destination.model.objects.size() == 1);
+    const auto &volumes = destination.model.objects.front()->volumes;
+    REQUIRE(volumes.size() == 1 + seam_types.size());
+    CHECK(volumes.front()->is_model_part());
+    for (size_t i = 0; i < seam_types.size(); ++i) {
+        CAPTURE(i);
+        CHECK(volumes[i + 1]->type() == seam_types[i]);
+        CHECK(volumes[i + 1]->name == "helper_" + std::to_string(i));
+        check_geometry(*source.model.objects.front()->volumes[i + 1], *volumes[i + 1]);
+        check_config(*volumes[i + 1], i);
+        if (shared && backend == Backend::Bbs)
+            CHECK(volumes[i + 1]->mesh_ptr().get() == volumes[1]->mesh_ptr().get());
+        volumes[i + 1]->set_type(ModelVolumeType::PARAMETER_MODIFIER);
+    }
+
+    // Reverse conversion must survive another file round trip, not just retain config in memory.
+    destination.share_mesh = shared;
+    ScopedTemporaryFile converted_file(".3mf");
+    save(converted_file.string(), backend, destination);
+    Archive converted_archive = read_archive(converted_file.string());
+    const auto &converted_xml = model_xml(converted_archive, backend);
+    CHECK(converted_xml.find("key=\"precise_seam_type\"") == std::string::npos);
+    CHECK(converted_xml.find("key=\"precise_seam_config:") == std::string::npos);
+    Scene converted;
+    load(converted_file.string(), backend, converted);
+    REQUIRE(converted.model.objects.size() == 1);
+    const auto &converted_volumes = converted.model.objects.front()->volumes;
+    REQUIRE(converted_volumes.size() == volumes.size());
+    for (size_t i = 0; i < seam_types.size(); ++i) {
+        CAPTURE(i);
+        CHECK(converted_volumes[i + 1]->type() == ModelVolumeType::PARAMETER_MODIFIER);
+        CHECK(converted_volumes[i + 1]->name == "helper_" + std::to_string(i));
+        check_geometry(*volumes[i + 1], *converted_volumes[i + 1]);
+        check_modifier_config(*converted_volumes[i + 1], backend, i);
+    }
+}
+
+TEST_CASE("Ordinary modifier settings are escaped once in 3MF attributes", "[PreciseSeam3mf][Regression]")
+{
+    const auto backend = GENERATE(Backend::Bbs, Backend::Prusa);
+    CAPTURE(int(backend));
+    Scene source;
+    source.populate(false);
+    source.model.objects.front()->volumes[1]->set_type(ModelVolumeType::PARAMETER_MODIFIER);
+    ScopedTemporaryFile file(".3mf");
+    save(file.string(), backend, source);
+    Archive archive = read_archive(file.string());
+    const auto &xml = model_xml(archive, backend);
+    // Check the serialized value independently of the writer's escaping helper. Prusa drops
+    // ordinary notes on import, so successful loading alone would not detect double escaping.
+    const std::string encoded_notes = R"(quoted \&quot;value\&quot; &amp; &lt;tag>&#x9;column\nnext line &amp;amp; path\\file0)";
+    CHECK(xml.find("key=\"notes\" value=\"" + encoded_notes + "\"") != std::string::npos);
+    Scene destination;
+    load(file.string(), backend, destination);
+    REQUIRE(destination.model.objects.size() == 1);
+    REQUIRE(destination.model.objects.front()->volumes.size() == 2);
+    const ModelVolume &modifier = *destination.model.objects.front()->volumes[1];
+    CHECK(modifier.type() == ModelVolumeType::PARAMETER_MODIFIER);
+    check_modifier_config(modifier, backend);
+}
+
+TEST_CASE("Seam metadata restores only recognized modes on compatible base types", "[PreciseSeam3mf][Regression]")
+{
+    const auto backend = GENERATE(Backend::Bbs, Backend::Prusa);
+    // Include both XML orders, fallback cases, and the previous inline type representation.
+    const int variant = GENERATE(0, 1, 2, 3, 4, 5);
+    CAPTURE(int(backend), variant);
+    Scene source;
+    source.populate(false);
+    ScopedTemporaryFile original(".3mf"), edited(".3mf");
+    save(original.string(), backend, source);
+    Archive archive = read_archive(original.string());
+    auto &xml = model_xml(archive, backend);
+    if (variant == 0 || variant == 1) {
+        const std::string metadata = remove_mode_metadata(xml);
+        const std::string tag = backend == Backend::Bbs ? "part" : "volume";
+        // Locate the helper via its dormant config, then move its mode before/after base metadata.
+        const auto key = xml.find("key=\"precise_seam_config:extruder\"");
+        REQUIRE(key != std::string::npos);
+        const auto start = xml.rfind("<" + tag + " ", key);
+        REQUIRE(start != std::string::npos);
+        if (backend == Backend::Bbs) {
+            // Exercise base-type metadata too, not just the subtype attribute preceding all metadata.
+            const auto end = xml.find("</part>", key);
+            REQUIRE(end != std::string::npos);
+            xml.insert(end, "<metadata key=\"part_type\" value=\"modifier_part\"/>");
+        }
+        const auto opening_end = xml.find('>', start);
+        REQUIRE(opening_end != std::string::npos);
+        const auto insertion = variant == 0 ? opening_end + 1 : xml.find("</" + tag + ">", key);
+        REQUIRE(insertion != std::string::npos);
+        xml.insert(insertion, metadata);
+    } else if (variant == 2) {
+        replace_once(xml, "value=\"precise_seam_center\"", "value=\"unknown_future_seam\"");
+    } else if (variant == 3) {
+        remove_mode_metadata(xml);
+    } else if (variant == 4) {
+        // A known seam mode must not reinterpret an ordinary printable part.
+        if (backend == Backend::Bbs) replace_once(xml, "subtype=\"modifier_part\"", "subtype=\"normal_part\"");
+        else {
+            replace_once(xml, "key=\"modifier\" value=\"1\"", "key=\"modifier\" value=\"0\"");
+            replace_once(xml, "value=\"ParameterModifier\"", "value=\"ModelPart\"");
+        }
+    } else {
+        remove_mode_metadata(xml);
+        if (backend == Backend::Bbs) replace_once(xml, "subtype=\"modifier_part\"", "subtype=\"precise_seam_center\"");
+        else replace_once(xml, "value=\"ParameterModifier\"", "value=\"precise_seam_center\"");
+        // Older files stored settings as ordinary volume keys, without the new dormant namespace.
+        for (const std::string key : {"extruder", "sparse_infill_density", "notes"})
+            replace_once(xml, "key=\"precise_seam_config:" + key + "\"", "key=\"" + key + "\"");
+    }
+    write_archive(edited.string(), archive);
+    Scene destination;
+    load(edited.string(), backend, destination);
+    REQUIRE(destination.model.objects.size() == 1);
+    REQUIRE(destination.model.objects.front()->volumes.size() == 2);
+    const ModelVolume &helper = *destination.model.objects.front()->volumes[1];
+    if (variant == 0 || variant == 1 || variant == 5) {
+        CHECK(helper.type() == ModelVolumeType::PRECISE_SEAM_CENTER);
+        if (variant == 5 && backend == Backend::Prusa) {
+            // The legacy Prusa whitelist only accepted extruder among these ordinary setting keys.
+            REQUIRE(helper.config.has("extruder"));
+            CHECK(helper.config.opt_int("extruder") == 2);
+            CHECK_FALSE(helper.config.has("sparse_infill_density"));
+            CHECK_FALSE(helper.config.has("notes"));
+        } else check_config(helper);
+    } else {
+        CHECK(helper.type() == (variant == 4 ? ModelVolumeType::MODEL_PART : ModelVolumeType::PARAMETER_MODIFIER));
+        CHECK_FALSE(helper.config.has("extruder"));
+        CHECK_FALSE(helper.config.has("sparse_infill_density"));
+        CHECK_FALSE(helper.config.has("notes"));
+    }
+}

tests/slic3rutils/CMakeLists.txt

@@ -12,6 +12,8 @@ add_executable(${_TEST_NAME}_tests
     test_network_versions.cpp
     test_action_source.cpp
     test_plugin_host_api.cpp
+    # Exercise seam enums and predicates through the embedded Python host API.
+    test_precise_seam_plugin.cpp
     test_plugin_capability_config.cpp
     test_plugin_config.cpp
     test_plugin_capabilities_in_use.cpp

tests/slic3rutils/test_precise_seam_plugin.cpp

@@ -0,0 +1,61 @@
+#include <catch2/catch_all.hpp>
+
+#include "libslic3r/Model.hpp"
+#include "libslic3r/TriangleMesh.hpp"
+#include "python_test_support.hpp"
+
+#include <pybind11/embed.h>
+#include <array>
+
+namespace py = pybind11;
+using namespace Slic3r;
+
+TEST_CASE("Python exposes every seam mode and distinguishes strong weak and ordinary volumes", "[PreciseSeamPlugin][Python]")
+{
+    struct Mode {
+        const char *name;
+        ModelVolumeType type;
+        bool strong;
+        bool weak;
+    };
+    // Explicit expectations protect enum registration and both predicate groups independently.
+    const std::array<Mode, 11> modes = {{
+        {"PreciseSeamCenter", ModelVolumeType::PRECISE_SEAM_CENTER, true, false},
+        {"PreciseSeamLeft", ModelVolumeType::PRECISE_SEAM_LEFT, true, false},
+        {"PreciseSeamRight", ModelVolumeType::PRECISE_SEAM_RIGHT, true, false},
+        {"PreciseSeamEnforced", ModelVolumeType::PRECISE_SEAM_ENFORCED, false, true},
+        {"PreciseSeamBlocked", ModelVolumeType::PRECISE_SEAM_BLOCKED, false, true},
+        {"PreciseSeamNeutral", ModelVolumeType::PRECISE_SEAM_NEUTRAL, false, true},
+        {"ModelPart", ModelVolumeType::MODEL_PART, false, false},
+        {"NegativeVolume", ModelVolumeType::NEGATIVE_VOLUME, false, false},
+        {"ParameterModifier", ModelVolumeType::PARAMETER_MODIFIER, false, false},
+        {"SupportEnforcer", ModelVolumeType::SUPPORT_ENFORCER, false, false},
+        {"SupportBlocker", ModelVolumeType::SUPPORT_BLOCKER, false, false}
+    }};
+
+    // Bootstrap holds the GIL on first initialization; acquire explicitly for an already-running interpreter too.
+    ensure_python_initialized();
+    py::gil_scoped_acquire gil;
+    py::object host = import_orca_module().attr("host");
+    REQUIRE(py::hasattr(host, "ModelVolumeType"));
+    py::object enumeration = host.attr("ModelVolumeType");
+    for (const Mode &mode : modes) {
+        DYNAMIC_SECTION(mode.name) {
+            REQUIRE(py::hasattr(enumeration, mode.name));
+            CHECK(enumeration.attr(mode.name).cast<ModelVolumeType>() == mode.type);
+            Model model;
+            auto *volume = model.add_object()->add_volume(make_cube(1, 1, 1));
+            volume->set_type(mode.type);
+            // Borrow the volume: the Python reference is destroyed before its owning Model.
+            py::object py_volume = py::cast(volume, py::return_value_policy::reference);
+            REQUIRE(py::hasattr(py_volume, "is_precise_seam"));
+            REQUIRE(py::hasattr(py_volume, "is_precise_seam_strong"));
+            REQUIRE(py::hasattr(py_volume, "is_precise_seam_weak"));
+            CHECK(py_volume.attr("type")().cast<ModelVolumeType>() == mode.type);
+            const bool precise = mode.strong || mode.weak;
+            CHECK(py_volume.attr("is_precise_seam")().cast<bool>() == precise);
+            CHECK(py_volume.attr("is_precise_seam_strong")().cast<bool>() == mode.strong);
+            CHECK(py_volume.attr("is_precise_seam_weak")().cast<bool>() == mode.weak);
+        }
+    }
+}