Belt printer: retire the redundant and unused options (#16236)

Follow-up to #16195 and the review discussion on #14394 (yw4z's note
about the third column on the *Belt tilt* row). Removes the belt options
that are redundant or unused before the branch ships, so they never need
compatibility handling after a release, and fixes supports under a
leading overhang. Every removed key is on `handle_legacy()`'s ignore
list, so existing profiles and 3MFs load silently.

## Removed

- **`belt_slice_rotation_global`**, **`preslice_remap_global`**,
**`belt_preslice_global`** (*Global mesh transforms*) and
**`gcode_back_transform`** — the global mode and the back-transform are
what belt printing is; they are presumed on wherever the flags were
consulted (`PrintObjectSlice`, `BeltBackTransform`, `BeltGCode`,
`Print::process`, `PrintApply`, `GCodeViewer`). The *Belt tilt* row is
axis + angle only; the three `fdm_belt_common.json` drop the keys.
- **`preslice_remap_x/y/z`** — no profile used the pre-slice axis remap;
the belt tilt axis plus the G-code axis remap cover the machines that
exist, and its implementation only agreed with itself for a plain swap.
The forward transform is the rotation.
- **`belt_support_z_offset_mode`** and **`belt_support_floor_mode`** —
the first was never read by a generator; the second's only shipped value
(*Generator only*) is now the behaviour.
- **`first_layer_plane`**, **`first_layer_plane_offset`**,
**`first_layer_plane_thickness`** and `FirstLayerPlane.{cpp,hpp}` — the
first-layer band is measured from the belt surface and is one first
layer height thick.
- `belt_brim_instances_compatible()` and its validation warning:
instances along the belt get their brim.

## Supports under a leading overhang (the clipping at the object's local
Z = 0)

The slicing frame of a belt object started at its lowest vertex, but the
belt under the leading end of an overhang lies below that, so no
generator could reach it: normal supports stopped at the object's lowest
layer, and both tree generators carried extension hacks sized from the
pre-rotation bbox and capped at global Z = 0 (right only for the
trailing half of the belt). The frame now starts at the lowest
belt-floor point under the footprint, less a 10 mm margin along the belt
for the base of a support column, and the extensions are gone:

- **Normal supports** run in the object frame and get the global belt Z
offset shifted onto the result (as organic already did). With the offset
on the object layers, a top contact at negative Z turned the
intermediate-layer count negative and the generator allocated layers
until the kernel killed it — any overhang in the leading half of the
belt did this. The first-layer flange expansion is skipped on a belt
(the first support layer is the leading tip, not a flange).
- **Classic tree** nodes keep dropping until their whole circle is in
the belt, so a branch tapers to a tip on the belt instead of stopping a
radius above it.
- **Organic**: the belt is no longer a support blocker. A blocker is a
collision, and a branch descending onto one slides off it, down the belt
and ahead of the part; the belt is where branches end, which the
per-layer floor clipping already does.

Regression test *Belt supports reach the belt under a leading overhang*:
a cube with a fin whose underside is parallel to the layers, 20 mm ahead
of the cube and up to 41 mm of slicing Z above the belt, for normal,
organic and classic tree supports; the lowest support layer must sit on
the belt beneath its own lines.

The belt object height (the layer range) is now estimated from the box
of the mesh as placed on the bed. `raw_bounding_box()` has the
instance's Z offset removed, which was harmless for the old
rotated-extent estimate but not for one anchored at the belt floor (a
point's rotated z and the floor under it move in opposite directions
under a Z shift): with the first version of this change every part came
out as a wedge, sliced only up to its diagonal, in the GUI and CLI
alike. Caught by a GUI test pass; the leading-overhang test now also
checks that the whole part is sliced.

## Belt brim after the parallel support step

`belt_brim_obstacles()` reads every object's layers and support layers,
which another object's support step rebuilds (and now shifts) at the
same time. The brim is generated sequentially once the parallel step is
over (`PrintObject::generate_belt_brim()`). This is the race behind the
Windows arm64 segfault in *Belt brim of each object precedes its
perimeters on its own filament*.

## UI

- *Belt tilt* is two rows: the angle (Advanced) and the axis (Developer;
a profile-level kinematics choice). A shared line is shown by its first
option's mode, so they cannot share one.
- *Machine frame transforms* is five single-option rows (G-code remap X
/ Y / Z, Decouple machine-frame tilt, Machine-frame tilt angle — the
angle row only appears when decoupled) instead of two multi-column
lines; the remap fields got full labels since they stand alone now.
- The gravity indicator on the bed is a plain line along the up
direction (no cone, 60 % of the axes' length), per yw4z.
- The *Show raw G-code (belt only)* legend/canvas toggle and its `B`
shortcut are gone; the preview is the designed view.

Also carries the two-line `phong.fs` fix from #16226 (merges as a
no-op).

## Verification

- `libslic3r_tests` 1116 passed (92 648 assertions); `fff_print_tests`
351 passed (561 696 assertions).
- `scripts/clang_tidy_diff.py --base upstream/belt-printer`: no
findings.
- `scripts/orca_profile_tool.py check`: no profile references a removed
key.
- GUI target builds; a scripted GUI pass (xdotool) checked the settings
groups in every mode, slicing, export, instances, the purge tower,
calibration dialogs, the wizard, printer switching and 3MF round-trip.

The wiki pages (OrcaSlicer/OrcaSlicer_WIKI#374) get a follow-up dropping
the removed sections once this is in.

src/libslic3r/GCode/BeltBackTransform.cpp

@@ -10,19 +10,10 @@ bool BeltBackTransform::init_from_config(const PrintConfig &config)
     m_active  = false;
     m_inverse = Transform3d::Identity();
 
-    if (!config.belt_printer.value || !config.gcode_back_transform.value)
+    if (!config.belt_printer.value)
         return false;
 
-    // Require at least one active transform to proceed.
-    bool has_global_rotation = config.belt_slice_rotation_global.value
-                            && config.belt_slice_rotation.value != BeltRotationAxis::None;
-    bool has_preslice_global = config.belt_preslice_global.value
-                            || config.preslice_remap_global.value;
-    if (!has_global_rotation && !has_preslice_global
-        && !BeltTransformPipeline::has_preslice_remap(config))
-        return false;
-
-    // Build the forward pipeline (rotation * pre_remap) and store its inverse.
+    // Build the forward pipeline (the rotation) and store its inverse.
     Transform3d forward = BeltTransformPipeline::build_forward_transform(config);
     if (forward.isApprox(Transform3d::Identity()))
         return false;

src/libslic3r/GCode/BeltBackTransform.hpp

@@ -7,25 +7,21 @@
 
 namespace Slic3r {
 
-// Reverses the pre-slice remap + shear + scale transforms that
-// PrintObjectSlice.cpp applies to belt printer geometry, converting G-code
-// coordinates from the sliced (remapped/sheared/scaled) frame back to the
-// machine's real coordinate space.
+// Reverses the pre-slice rotation that PrintObjectSlice.cpp applies to belt
+// printer geometry, converting G-code coordinates from the sliced (rotated)
+// frame back to the machine's real coordinate space.
 //
 // Initialized once from PrintConfig, then applied per-point in
 // BeltKinematics::to_machine() before axis remapping.
 //
-// Active when gcode_back_transform is true AND at least one of:
-//   - a shear axis has global mode enabled, or
-//   - a pre-slice axis remap is non-identity.
+// Active on belt printers with a non-identity pre-slice rotation.
 class BeltBackTransform
 {
 public:
     BeltBackTransform() = default;
 
-    // Initialize from belt printer config.  Rebuilds the same pre-slice remap,
-    // shear, and scale matrices as PrintObjectSlice.cpp and precomputes the
-    // affine inverse.  Returns true if a non-identity back-transform was computed.
+    // Initialize from belt printer config.  Rebuilds the same pre-slice rotation
+    // as PrintObjectSlice.cpp and precomputes the affine inverse.  Returns true if a non-identity back-transform was computed.
     bool init_from_config(const PrintConfig &config);
 
     // Apply the inverse transform to a point.  Returns pos unchanged if

src/libslic3r/GCode/GCodeProcessor.cpp

@@ -2619,9 +2619,6 @@ void GCodeProcessorResult::reset() {
     machine_frame_transform_active = false;
     belt_tilt_angle = 0.f;
     belt_z_origin   = 0.f;
-    preslice_remap_x = RemapAxis::PosX;
-    preslice_remap_y = RemapAxis::PosY;
-    preslice_remap_z = RemapAxis::PosZ;
     settings_ids.reset();
     filaments_count = 0;
     backtrace_enabled = false;
@@ -3201,9 +3198,8 @@ void GCodeProcessor::apply_config(const DynamicPrintConfig& config)
         const auto *belt = config.option<ConfigOptionBool>("belt_printer");
         if (belt != nullptr) {
             static const char *belt_keys[] = {
-                "belt_printer", "belt_slice_rotation", "belt_slice_rotation_angle", "belt_slice_rotation_global",
-                "belt_preslice_global", "preslice_remap_x", "preslice_remap_y", "preslice_remap_z", "preslice_remap_global",
-                "gcode_remap_x", "gcode_remap_y", "gcode_remap_z", "gcode_back_transform",
+                "belt_printer", "belt_slice_rotation", "belt_slice_rotation_angle",
+                "gcode_remap_x", "gcode_remap_y", "gcode_remap_z",
                 "belt_frame_tilt_decouple", "belt_frame_tilt_angle",
             };
             for (const char *key : belt_keys)
@@ -4328,36 +4324,6 @@ void GCodeProcessor::process_tags(const std::string_view comment, bool producers
         } catch (...) {}
         return;
     }
-    // Belt printer: parse pre-slice axis remap from header comments.
-    {
-        auto trim = [](const std::string &s) -> std::string {
-            size_t start = s.find_first_not_of(" \t\r\n");
-            size_t end   = s.find_last_not_of(" \t\r\n");
-            return (start == std::string::npos) ? "" : s.substr(start, end - start + 1);
-        };
-        // Pre-slice axis remap
-        auto parse_remap_axis = [](const std::string &s) -> RemapAxis {
-            if (s == "pos_x") return RemapAxis::PosX;
-            if (s == "pos_y") return RemapAxis::PosY;
-            if (s == "pos_z") return RemapAxis::PosZ;
-            if (s == "neg_x") return RemapAxis::NegX;
-            if (s == "neg_y") return RemapAxis::NegY;
-            if (s == "neg_z") return RemapAxis::NegZ;
-            if (s == "rev_x") return RemapAxis::RevX;
-            if (s == "rev_y") return RemapAxis::RevY;
-            if (s == "rev_z") return RemapAxis::RevZ;
-            return RemapAxis::PosX;
-        };
-        if (boost::starts_with(comment, " preslice_remap_x = ")) {
-            m_result.preslice_remap_x = parse_remap_axis(trim(std::string(comment.substr(20)))); return;
-        }
-        if (boost::starts_with(comment, " preslice_remap_y = ")) {
-            m_result.preslice_remap_y = parse_remap_axis(trim(std::string(comment.substr(20)))); return;
-        }
-        if (boost::starts_with(comment, " preslice_remap_z = ")) {
-            m_result.preslice_remap_z = parse_remap_axis(trim(std::string(comment.substr(20)))); return;
-        }
-    }
     // wipe start tag
     if (boost::starts_with(comment, reserved_tag(ETags::Wipe_Start))) {
         m_wiping = true;

src/libslic3r/GCode/GCodeProcessor.hpp

@@ -304,9 +304,6 @@ class Print;
         // machine frame and should not be compared against `printable_height`
         // (which lives in the build-volume frame).
         bool machine_frame_transform_active{ false };
-        RemapAxis preslice_remap_x{ RemapAxis::PosX };
-        RemapAxis preslice_remap_y{ RemapAxis::PosY };
-        RemapAxis preslice_remap_z{ RemapAxis::PosZ };
         SettingsIds settings_ids;
         size_t filaments_count;
         bool backtrace_enabled;
@@ -405,9 +402,6 @@ class Print;
             belt_tilt_angle = std::forward<Other>(other).belt_tilt_angle;
             belt_z_origin = std::forward<Other>(other).belt_z_origin;
             machine_frame_transform_active = std::forward<Other>(other).machine_frame_transform_active;
-            preslice_remap_x = std::forward<Other>(other).preslice_remap_x;
-            preslice_remap_y = std::forward<Other>(other).preslice_remap_y;
-            preslice_remap_z = std::forward<Other>(other).preslice_remap_z;
 #if ENABLE_GCODE_VIEWER_STATISTICS
             time = std::forward<Other>(other).time;
 #endif