OrcaSlicer/TEXTURE_DISPLACEMENT.md

Texture Displacement - Technical Notes

Branch: feature/texture_displacement. This document is a knowledge dump of the whole feature as it stands: architecture, file map, algorithms, known bugs found and fixed (with root causes worth remembering), and what's still deferred. Written so a fresh session (or a fresh pair of eyes) can pick this up without re-deriving everything from scratch.

What it does

A paint-style gizmo (GLGizmoTextureDisplacement) that lets you:

• Paint one or more "layers" onto a model's surface, each a height-map texture with its own depth/tiling/rotation/offset/invert/tile-mode/projection-mode/blend-mode.
• Pick a texture from a shipped library (resources/textures/displacement/) or import your own (saved into <data_dir>/textures/displacement/, kept separate so app updates can't clobber it).
• Combine overlapping layers with image-editor-style blend modes (Add/Subtract/Multiply/Divide).
• Preview the true displaced result live, before baking (background job, not on the UI thread).
• Optionally preview via a fast GPU bump-map shader instead (no real geometry movement, just shading) for a lighter-weight alternative.
• Bake into real mesh geometry on demand, restricted to the painted area only.
• Subdivide a low-poly model first so there are enough vertices to show fine detail.
• Unwrap a painted patch with a real CGAL LSCM parameterization and view it in a dedicated, dockable 2D "UV Editor" pane.

Architecture

Data model (per ModelVolume)

Each of up to TEXTURE_DISPLACEMENT_MAX_LAYERS (8) layers gets its own independent FacetsAnnotation paint mask - the exact same TriangleSelector/FacetsAnnotation machinery every other paint gizmo (FdmSupports, Seam, MMU, FuzzySkin) already uses, just one full instance per layer slot instead of one per volume. This is what makes "layered/blended" painting work for free: the same triangle can be ENFORCER in layer 2's mask and layer 5's mask simultaneously, and at bake/preview time each layer displaces the surface left by the previous one (image-editor-layer semantics).

Bake algorithm (libslic3r/TextureDisplacement.cpp)

build_texture_displacement(base_mesh, layers, facets_data, options) is accumulate-then-displace, and topology-preserving: the returned mesh has exactly the input's vertices and triangles, in the same order - only the positions of displaced vertices differ.

1. its_compactify_vertices() on a copy of the input. In practice a no-op (it only drops unreferenced vertices, and preserves the order and indices of the rest). It is there to guarantee the index alignment step 3 depends on.
2. Area-weighted vertex normals of the undisplaced mesh, computed once. Every layer both projects and displaces along these, so a vertex covered by several layers moves along one single well-defined direction. Where the paint does not cover every triangle around a vertex, the normal is recomputed from the painted triangles alone (the union over all layers, so it stays one direction per vertex). On the rim of a fully painted top face the whole-mesh normal is the 45 degrees bisector it shares with the side wall, and displacing along that flares the rim outwards instead of raising it. Interior vertices are unaffected - all their triangles are painted, so the two coincide. Paint coverage per original triangle comes straight off TriangleSplittingData::triangles_to_split.
3. For each layer in slot order: deserialize its stored paint mask into a TriangleSelector against the base mesh (never against a previous layer's output), then selector.get_facets_strict(ENFORCER) → the painted patch. Two facts are exploited:
   • get_facets_strict() returns the mesh's entire referenced vertex array regardless of which state was asked for - only .indices is filtered by state. So get_facets_strict(ENFORCER) and get_facets_strict(NONE) share identical vertex indexing, which is what lets boundary detection be a plain index check instead of a position-hash lookup.
   • The selector's vertex array starts with the mesh's own vertices (extra ones created where a brush stroke split a triangle are appended after them), and get_facets_strict() emits the referenced ones in order. Combined with step 1, selector vertex index i is our vertex i. Split vertices live past the end of our array and are simply skipped - they sit on the paint boundary anyway (splitting only happens at partial coverage), so they would be pinned regardless.
4. A vertex used by at least one unpainted triangle is a border vertex. Whether it moves is TextureDisplacementOptions::displace_border, and it does by default. The original design pinned it, justified as stopping the patch tearing away from the surrounding surface - which stopped being true the moment the bake became topology-preserving. There is no seam to tear: a border vertex is one vertex shared by both regions, and moving it simply tilts the unpainted triangles that use it. What pinning actually does is clamp the outermost ring of relief to zero, so on a fully painted face the pattern collapses into a ring of steep ramps right at the edge - the reported "it doesn't extrude at the border" artifact. Pinning is kept as an option for the case where the relief must not spill past the paint at all. Either way the border still drives the edge_smoothing falloff.
5. Per interior vertex: sample the height texture (sample_layer_height(), see Projection methods) and fold height * depth_mm * (invert ? -1 : 1) into that vertex's running total via the layer's TextureBlendMode (see Blend modes). A visited set makes each layer fold in exactly once per vertex, no matter how many of the patch's triangles share it - otherwise a Multiply/Subtract layer would apply two or three times over depending on local triangle fan-out.
6. Finally, move each touched vertex along its (step 2) normal by its accumulated total.
7. Optionally (TextureDisplacementOptions::smooth_*) relax the result - see Post-process smoothing.

Post-process smoothing

smooth_mesh_vertices(mesh, movable, strength, iterations) - plain Laplacian relaxation, run after all layers have been folded in, restricted to the vertices flagged in movable. Each pass moves a movable vertex a strength fraction of the way to the average of its one-ring, read from a snapshot of the previous pass so the result does not depend on vertex order (a Gauss-Seidel sweep would smooth several times as hard at the end of the array as at the start). Neighbours come from a CSR-style adjacency built once per call.

Its job is to round off the hard steps a bitmap height map leaves behind, which is a different knob from TextureDisplacementLayer::smoothing - that blurs the height map before it is ever sampled, this relaxes the geometry afterwards. Topology-preserving, like the bake.

Two ways in, sharing one set of settings on the volume:

• The "Smooth result" checkbox + "Smoothing (%)" / "Passes" ride along with Preview and Bake. movable is exactly the set of vertices the displacement moved, so the untouched part of the model keeps its exact geometry and the ring just outside the displaced set anchors the relaxation (the relief cannot creep outward).
• "Smooth baked mesh now" (GLGizmoTextureDisplacement::smooth_model()) applies the same settings to the volume's committed geometry, for relief that is already baked in. movable there is the painted triangles' vertices. Because smoothing never touches the triangle list, this is the one geometry operation in the gizmo that keeps every paint channel verbatim - it saves and restores the eight texture-displacement masks around set_mesh() rather than remapping or dropping them.

"Ignore outer ring" (smooth_skip_border, on by default) drops the patch's own outermost ring of vertices from movable. That ring's neighbours outside the paint never move, so relaxing it drags the rim of the relief back down toward the flat surface and the pattern comes out half-melted exactly where it meets the edge - crisp everywhere else, which is what makes it look like a bug rather than a setting. Held out, the border keeps the full depth the texture asked for and only the interior relaxes. Turning it off softens the outer edge deliberately (a blunter version of the per-layer edge-smoothing falloff). Note this is the smoothing rim, a separate question from whether that rim is displaced at all (displace_border, step 4 above) - the two are independent and both default to "keep the border sharp".

Blend modes

TextureBlendMode {Add, Subtract, Multiply, Divide}, per layer, applied per vertex against the total accumulated by the layers below it (lower slots). The quantity blended is a signed displacement in mm, not a pixel value.

Add/Subtract are self-explanatory. Multiply/Divide are scaling operations and so need a unit convention: they treat the layer's own value as a factor relative to 1 mm. That makes depth_mm a gain, and - the property that makes a Multiply layer usable as a mask - a layer with depth 1 mm sampling a white (1.0) texel multiplies by exactly 1, i.e. leaves the layers below unchanged. Divide floors its divisor's magnitude at 0.05 - a black texel samples to exactly zero, so the divisor really does hit zero in ordinary use, and an unbounded 1/0 would fling vertices thousands of mm away and poison the mesh's bounding box (and every plate/print-volume check downstream). The floor doubles as a cap on how far Divide can amplify the relief beneath it: at most 20×.

The lowest painted layer ignores its blend mode: it has nothing beneath it, and Multiply/Divide against an implicit zero base would annihilate (or blow up) it. Enforced in build_texture_ displacement() (the first layer to reach a given vertex always folds in additively) and surfaced in the UI, which labels that layer "Base layer" instead of offering a control that silently does nothing.

Projection methods

Four choices per layer (TextureProjectionMethod), all funneling through apply_uv_transform() (scale by 1/tiling_scale, rotate by rotation_deg, add offset). They are dispatched by sample_layer_height(), which returns a height, not a UV - because Triplanar takes three texture samples per vertex and so has no single UV that represents it.

• Triplanar (default) - samples the texture on all three world planes ((y,z), (x,z), (x,y)) and blends the three by the vertex's own normal raised to TRIPLANAR_BLEND_SHARPNESS (4). This is the fix for a real, user-reported bug. The previous version hard-picked the single axis most aligned with the normal, which is discontinuous wherever that dominant axis flips: on a +X face the planar coordinate is (y, z), on a −Y face it is (x, z), so at the shared edge u jumps from y_edge to x_edge. On a box centred near the origin those two happen to agree at the (+,+) and (−,−) corners and differ by the full corner width at the (+,−) and (−,+) corners

  • which is exactly the "two bad corners, two good ones" symmetry that was observed. A weighted blend is continuous across the transition by construction, since the weight of the axis being left behind falls smoothly to zero. (Note this removes the hard seam; some cross-fade blurring in the band right at a 90° edge is inherent to triplanar mapping. A genuinely seam-free wrap around a box needs a real unwrap - that is what the LSCM mode is for.)

• Cylindrical - wraps around an axis through the patch centroid, axis auto-picked as the world axis least aligned with the average normal (perpendicular to the outward radial normal, as a cylinder's own axis would be). u = angle * local_radius (arc length in mm), v = distance along axis. Approximation, not an exact fit for arbitrary geometry.

• Spherical - longitude/latitude around the centroid, scaled by local radius. Same caveat.

• LSCM - real UV unwrap via MeshBoolean::cgal::parameterize_lscm() (CGAL's Surface_mesh_parameterization package, LSCM algorithm). Computed once per patch (not per-vertex like the others - it's a single global least-squares solve), then each vertex looks up its precomputed UV. Requires the patch to be a single topological disk (one connected component, one boundary loop) - compute_lscm_uvs() returns empty and the layer silently falls back to Triplanar if not (e.g. multiple disconnected painted islands, or a fully closed patch). CGAL's parameterizer needs a mesh with no isolated/unreferenced vertices, but get_facets_strict() returns the whole mesh's vertex array - so there's a compaction step (compact_patch_with_map()) that builds a clean sub-mesh + an index map back to the original (uncompacted) vertex numbering, purely local to this file.

• ViewProjected ("From view") - a flat projection along a fixed direction captured from the 3D camera, like a slide projector. capture_view_projection() takes the camera's right/up axes, transforms them into the volume's local frame (so the projection rides along if the part is later moved), and stores them as TextureDisplacementLayer::view_project_right/up (unit vectors, so the projected coordinate stays in mm and tiling_scale keeps meaning mm). sample_layer_height() projects Vec2f(dot(pos, right), dot(pos, up)). Single-valued per point, so - like LSCM but unlike blended Triplanar - the fast preview and UV-check overlay precompute it per vertex (compute_layer_vertex_uvs()) and drive the shader's use_vertex_uv path. Faces angled away from the projector smear; that is inherent to view projection, not a bug.

  Two companions to this mode:

  • Projection frame overlay (TextureProjectorFrame, see below) - a semi-transparent window dragged over the 3D view whose border becomes the projection's edge. Applying it stores an exact projective map in view_project_matrix, which supersedes the affine right/up axes above for that layer (view_project_projective).
  • "Project only on visible" (select_visible_faces()) - repaints the layer with exactly the facets the camera can see, so the projected area matches the viewpoint the projector was captured from. Two tests: a facing test (normal vs. view direction, per triangle - under perspective the view direction varies across the model, so it is taken from the eye to each centroid), then MeshRaycaster::get_unobscured_idxs() on the survivors to drop facets hidden behind other geometry, so a concave part's far inner wall is correctly excluded. One ray query per front-facing facet, hence click-driven (on the checkbox and on each "Capture current view"), never per frame. It replaces the layer's paint rather than adding to it - "project onto what I can see" would otherwise accumulate every angle the user had ever looked from.

Manual seams and island cutting

TextureDisplacementLayer::lscm_seam_edges - undirected mesh-vertex-index edge pairs the unwrap is forced to cut along, on top of the dihedral-angle seams. segment_into_charts() takes a set of these (translated from mesh → compacted-patch numbering inside compute_patch_unwrap()) and refuses to union two triangles across a marked edge whatever their angle. Both the unwrap cache key and the gizmo's UVEditorState include the seam list, so marking a seam (which leaves the paint mask untouched) still forces a re-solve. Like the paint masks, seams are mesh-index-space and so dropped on any topology change.

Two ways to write to it:

• Mark seam (manual, #9) - a "Mark seams" click mode (m_seam_edit_mode) that suppresses painting. A click raycasts the volume (m_c->raycaster()->raycasters()[idx]->unproject_on_mesh(), idx = the volume's slot among model-part volumes), finds the facet's edge nearest the hit point, and toggles it. Marked edges render as a red overlay (render_seam_overlay()), pulled toward the camera so they read on top. This is the Blender mark-seam workflow.
• Cut island (auto, #17) - cut_island() takes the selected chart's triangles (back-mapped from the unwrap via source_vertex), finds their 3D bounding box, and marks every edge that straddles the mid-plane perpendicular to the longest axis. The re-unwrap then splits the chart across its narrow waist - the "islands might be very long" case. Exposed as the UV pane's Cut button.

UV-check overlays (checker / distortion)

resources/shaders/{110,140}/texture_displacement_uvcheck.{vs,fs}, one shader with a mode uniform, drawn over the painted patch (rebuild_uvcheck_mesh()/render_uvcheck_mesh(), P3N3T2: normal.x = distortion, tex_coord = uv), pulled forward with a polygon offset. Checker (#13) samples a procedural checkerboard at the layer's uv (per-vertex for LSCM/ViewProjected, in-shader triplanar otherwise) - squares that stay square mean low distortion. Distortion (#14) colours each triangle blue→green→red by log2(uv_area / surface_area) centred on the patch's median stretch (so a globally-scaled unwrap reads as uniformly ideal and only relative stretch shows), averaged to vertices. A separate Show mesh wireframe toggle (#8) draws the whole volume's triangle edges, rebuilt only when the vertex count changes (not per stroke).

Tiling

DecodedHeightTexture::sample(uv, tile_enabled, tile_method). Two tile methods when enabled (Repeat, MirroredRepeat). When tile_enabled is false, sampling outside [0,1) returns 0 directly - clamping the coordinate into range (what an earlier version did) instead smears the border row/column of pixels outward to infinity in every direction, which is a real bug that was reported and fixed (visually: streaky lines radiating out from the painted patch).

Subdivision — two modes

Uniform (subdivide_mesh_uniform()) — whole-mesh, 1-to-4 split. Recursive edge-midpoint split with a shared per-pass midpoint cache (keyed by sorted vertex-index pair) so triangles sharing an edge get the same new vertex - capped at max_iterations (default 6). Whole-mesh so it never leaves a T-junction, at the cost of densifying everywhere. Wired as a "Subdivide steps" slider (0–5, 0 = no subdivision), Apply snaps back to 0. Drops texture-displacement paint (no remap) via the standard save_painting()/set_mesh()/restore_painting() dance; the other four channels are remapped.

Adaptive (subdivide_mesh_adaptive()) — refine only the painted area, by Rivara longest-edge bisection. This is the algorithm that was "scoped out" originally for fear of the T-junction/crack problem; it is safe because it is conformal by construction. Only terminal edges are ever bisected

• an edge that is the longest edge of every triangle sharing it - which splits both those triangles along one shared midpoint at once, so a hanging node is never created. The edge to split for a triangle that wants refining is found by longest-edge propagation (LEPP): walk to the longest edge of ever-longer-edged neighbours until a terminal one is reached, and bisect that. Edge length strictly increases along the path (ties broken by mesh-vertex key, which both sides of an edge compute identically), so the walk cannot cycle, and Rivara's result is that repeating it refines the original triangle in a bounded number of bisections. The transition triangles it pulls in just outside the painted patch are the graded band that makes the size change conformal.

The win: a small decal on a big model no longer quadruples the whole model's triangle count.

Run to completion, worst-first, against a triangle budget. The refinement loop is not a fixed number of sweeps: it holds every triangle that is over its criteria in a max-heap keyed by how many times over it is, pops the worst, walks its LEPP, bisects, and re-scores. Edge adjacency (nb[e], the triangle across each edge) is built once and maintained incrementally through each bisection, so the cost scales with the refined region rather than with the whole model. max_triangles is the only bound; stopping on it leaves a perfectly valid, still-conformal mesh that spent its budget on the largest errors.

This shape replaced a first version that ran a fixed 12 sweeps, each rebuilding a whole-mesh edge map and bisecting one terminal edge per active triangle. Two failure modes came out of that, and they are worth remembering because they look like separate bugs and are not: the sweeps were consumed grading the coarse surroundings (whose edges are the longest, so they win every terminal-edge contest), which both stopped refinement of the painted patch far short of the requested detail and left the band outside it looking wildly over-refined relative to the patch itself.

It carries the paint forward, which is what makes it usable (uniform/remesh both drop paint). Because the refinement is driven by the paint, the remap is trivial: subdivide_mesh_adaptive() fills an out_source[new_tri] = input_tri map (children inherit their parent), and the gizmo rebuilds each layer's mask on the new mesh - a new triangle is painted iff its source was fully painted in that layer. collect_paint_region() derives both:

• the union refine-region: exactly the original triangles the brush touched, read straight off TriangleSplittingData::triangles_to_split (serialize() records an entry per original triangle that is either split - i.e. partially painted, the patch boundary - or carries a non-default state). No dilation. An earlier version marked every triangle sharing a vertex with the patch, which drags in a whole fan of huge unpainted neighbours and then refines those down to the resolution floor, since the height field the detail test samples is not restricted to the painted area. The conformal closure already grades the size change outward on its own; it does not need help.
• the per-layer fully-painted-triangle sets (a get_facets_strict(ENFORCER) sub-triangle with all three original vertex indices == a whole, fully-painted original triangle; a partial stroke's sub-triangles always carry a split vertex).

The other four channels still ride the normal restore_painting() remap. Covered by a conformality unit test (every_edge_used_twice on a partially-refined cube - an exact crack detector for a closed mesh), plus tests that the target edge length is actually reached and that the budget caps the result without opening a crack.

Both share the gizmo's Preview/Apply/Done flow; the "Only painted area (adaptive)" checkbox picks the mode, and the adaptive preview follows the paint live (rebuild_preview() refreshes the wireframe while the subdivide preview is open in adaptive mode). The panel shows the previewed triangle count.

Feature-adaptive (follow texture detail). A sub-mode of adaptive (the "Follow texture detail" checkbox) that puts triangles where the displaced surface actually bends, not evenly. The insight: a flat region or a linear ramp needs no extra vertices (linear interpolation is exact for a ramp); what needs them is curvature - the second derivative, not the gradient. So the extra predicate is a chord-error test: sample the combined displacement at the triangle's three edge midpoints and its centroid (sampling the interior is what catches a bump sitting inside a triangle, the blind spot of an edge-only test) and take the largest departure from the flat triangle's barycentric interpolation. Refine while that exceeds chord_tolerance_mm ("Detail (mm)"). Zero chord error on a ramp ⇒ untouched; high on a bump/ridge/noise ⇒ refined until captured. Same conformal machinery, so still crack-free. The per-triangle error is cached and recomputed only for the children of a split.

Four knobs bracket it, and all four matter:

• "Max edge (mm)" (target_edge_length_mm) is a baseline that applies in feature mode too. Without it the chord test aliases: a big triangle over a fine pattern can sample four points that all land at similar heights, report no error, and stall before refinement ever starts. The baseline guarantees a sampling density fine enough for the curvature test to see the texture at all.
• "Detail (mm)" is the chord tolerance above.
• "Min edge (mm)" is a hard floor under both, and is what guarantees termination across a sharp texture step, where the error never falls however fine the mesh gets.
• "Added triangles (k)" is the budget, passed as max_triangles (the model's own triangle count plus the slider, so the control still means something on an already-dense model).

The height field is make_combined_displacement_sampler() - it mirrors build_texture_displacement()'s per-layer setup (decode, patch centroid, cylinder axis, blend order, "lowest layer folds additively") but evaluated per point. Two deliberate simplifications, both erring toward more detail (safe - over-refinement is never a crack): every sampleable layer is sampled at every point (no per-point paint test), and edge-smoothing falloff is ignored. Note the first one is why the refine region must not be dilated - outside the paint the sampler still reports full relief. LSCM layers are skipped (no per-point UV); a purely LSCM stack yields a null sampler and the code falls back to the length baseline alone. Per-vertex heights are sampled lazily, so a small patch on a huge model never pays for the rest of it. Covered by unit tests: a Gaussian bump refines densely at its center and leaves flat corners coarse, a linear ramp produces zero extra triangles (the case a gradient criterion would over-refine), and a flat field still honours the max-edge baseline.

Fast bump preview (GPU-only, no CPU meshing)

resources/shaders/{110,140}/texture_displacement_bump.{vs,fs}, registered as "texture_displacement_bump". Perturbs the shading normal from the height texture's local gradient instead of moving geometry - active-layer-only, toggled via a "Fast preview (normal map)" checkbox. Vertex format is GLModel::Geometry::EVertexLayout::P3N3T2: normal.x carries the per-vertex paint weight (0/1) and tex_coord carries a precomputed texture UV, so it can use GLModel normally instead of needing a hand-rolled VBO/VAO manager. Weight buffer is rebuilt at the same cadence as the true-displacement preview (stroke-end/slider-release), using the live TriangleSelector state (not the flushed model facets), so it doesn't lag by a full model round-trip.

The perturbed normal is the analytic one for a height field H = ±depth_mm · h(uv) displaced along N over any orthonormal surface tangent pair T/B:

N' = normalize(N − (dH/da)·T − (dH/db)·B),   a = dot(p,T), b = dot(p,B)

The two slopes have to be genuine mm-per-mm derivatives for the preview's apparent depth to match the bake's - see bug #13.

Two projection paths (use_vertex_uv uniform):

• Triplanar (use_vertex_uv = 0) - uv and the T/B axes are both derived in-shader from the dominant normal component, mirroring project_planar()/apply_uv_transform(), and the slope is formed analytically. T/B are the projection's axis-aligned pair, exact only when the face is axis-aligned; the shader drops the along-normal component to keep the gradient in the surface. Here one uv unit is exactly tiling_scale mm, so the 1/tiling_scale gradient factor is right.
• Precomputed UV (use_vertex_uv = 1, used for LSCM) - uv comes per-vertex from the CPU (compute_lscm_uvs(patch, layer), so island placement + tiling/rotation/offset are already folded in), and the perturbed normal is built with Mikkelsen's method ("Bump Mapping Unparametrized Surfaces on the GPU"): the surface gradient taken directly from the screen-space derivatives of the sampled height and position. This makes no uv→mm scale assumption, which is essential - the first cut used the same global 1/tiling_scale factor as triplanar and the depth came out visibly wrong, because an LSCM map is conformal, not isometric: it is globally area-scaled but the local mm-per-uv varies across the chart. dFdx(h) captures the true on-screen rate of change however the chart is stretched. This path is also what makes the fast preview follow the UV editor: move an island and its uv - hence its bump - moves with it (the bump mesh rebuilds on drag-end, since on_island_edited(finished) → rebuild_preview() → rebuild_bump_preview_mesh()). The branch is uniform (use_vertex_uv is a uniform) and the paint weight gates by multiply, so the texture derivatives stay well defined. A triangle straddling a seam has a discontinuous uv → the det≈0 guard skips it (a localised preview-only artifact, never in the bake).

Remaining deliberate approximation: the GPU sampler's wrap mode stands in for tile_enabled/tile_method, so with tiling off the GPU repeats where the CPU returns 0 outside [0,1).

On-canvas "Adjust Texture" gizmo

A per-active-layer toggle ("Adjust placement (drag on model)") that disables painting and shows a flat pan panel (free 2D drag on both axes) plus two arrows along the patch's own U/V axes (constrained single-axis drag). Anchored to the painted patch's centroid/average-normal (compute_layer_paint_anchor()). Hit-testing is screen-space distance/point-to-segment (not real 3D ray intersection against the handle geometry) - simple and good enough at this handle size.

Projection frame overlay (ViewProjected)

src/slic3r/GUI/TextureProjectorFrame.hpp/.cpp - a semi-transparent, resizable wxFrame the user drags over the 3D view, like a slide projector's gate. Whatever the model shows through it is what the texture is projected onto, and the window's border becomes the hard edge of the displacement. Press Apply projection frame and the gizmo reads the window's rectangle and commits it.

The window is deliberately dumb: it owns no placement state and reports nothing continuously. Its position and size are the placement, read on demand at Apply - which is also when the expensive visible-facet raycast runs. So dragging it is free and nothing recomputes until asked.

Plain 2D (wxPaintDC), not a wxGLCanvas: a second GL canvas would have to share the app's one real wxGLContext, the cause of bugs #10 and #14 below. It only ever draws a bitmap and a border.

The projective mapping (apply_projection_frame()). The frame defines a screen-space rectangle, but the bake samples from a local-space position, so the two have to be reconciled. view_project_right/up can only express an affine projection - exact under an orthographic camera, but wrong under perspective, where the near end of a part projects larger than the far end and no pair of axes reproduces that. So the layer instead stores a full projective map (view_project_matrix, row-major 3×4, uv = (row0·p̃/row2·p̃, row1·p̃/row2·p̃)), built like this:

• K = projection · view · (instance · volume), i.e. local → clip, the same product the renderer uses. Note Camera::get_projection_matrix() is typed Transform3d (nominally affine) but its perspective form explicitly writes a (0, 0, −1, 0) bottom row into the underlying 4×4, so clip.w = −z_eye is genuinely carried. The build therefore multiplies .matrix() products (plain Matrix4d), never Transform3d products, which would not compose that row correctly.
• Window coordinates follow igl::project's convention (as CameraUtils::project does), with y measured downward. Writing uv = (win − rect_origin) / rect_size makes u and v affine in ndc = clip.xyz / clip.w; multiplying through by clip.w leaves a plain linear combination of K's rows, which is exactly the 3×4 matrix - the perspective divide survives intact.
• w > 0 is checked rather than divided blindly. A point behind the projector has w < 0 and divides to a plausible-looking but mirrored uv - the classic way a projected decal reappears on the back of a model. project_uv_projective() returns false there and the caller treats it as no height.

The map already includes placement, so apply_uv_transform() is not applied on top of it - the window's own position and size are the placement, and the tiling/rotation/offset sliders would shove the result off the frame the user just aligned. A "Clear" button drops back to the affine path where those controls mean something again.

Apply also sets tile_enabled = false, so DecodedHeightTexture::sample() returns 0 outside [0,1) and the border is a hard edge rather than the first seam of an endless repeat, and repaints the layer via select_visible_faces(&matrix) - the frame's uv square clips the selection, which both matches the paint to the border and keeps the ray queries proportional to the framed area instead of the model.

Owned by the gizmo and destroyed (not just hidden) in on_shutdown(). Closing it only hides it, so reopening keeps it where it was left.

UV Editor pane

UVEditorCanvas (src/slic3r/GUI/UVEditorCanvas.hpp/.cpp) - a standalone wxGLCanvas rendering the flattened LSCM islands (per-island wireframe + outline + fill) over the height texture (background quad tiled across the whole unwrap), with mouse pan/zoom. It is wrapped in a UVEditorPanel (same file) that adds a button row (Frame / Snap / Average scale) and a status line along the bottom naming the current gesture and the shortcuts in play. The panel is what is registered as a wxAuiPaneInfo pane on Plater's m_aui_mgr; Plater::show_uv_editor(bool) shows/hides it (deferred via CallAfter, since the gizmo calls it mid-3D-frame), and get_uv_editor_canvas() returns the inner canvas the gizmo talks to.

Deliberately shares the app's one real wxGLContext (wxGetApp().init_glcontext(*this), the same call View3D/Preview/AssembleView make) rather than creating an independent context like SkipPartCanvas does elsewhere in this codebase - this is what lets it reuse the already-registered "flat"/"flat_texture" shaders and GLModel as-is, instead of needing its own shader compilation/VBO management.

Geometry is uploaded once, in the unwrap's own (raw, mm) coordinates, one GLModel set per island; each island is then drawn through its own 2x3 affine (island_transform_matrix() composed with the layer's tiling/rotation/offset) passed as the flat shader's view_model_matrix. A drag updates one matrix per island and touches no vertex buffer - on_island_edited(!finished) calls only set_island_transforms(), and the full set_islands() rebuild happens solely when the unwrap itself changes (unwrap_changed in update_uv_editor()).

Gestures (canvas-owned, reported to the gizmo as incremental deltas via IslandEditFn): left-drag = move, right-drag or R = rotate (hold Shift to snap to 15° steps - quantised on the cumulative rotation, not each delta, so it doesn't judder, and accumulated incrementally so it survives crossing ±180°), S = scale (R/S modal, click/Enter to confirm, Esc to cancel), wheel = zoom about the cursor, middle-drag = pan, Home/F = frame all. Scale writes TextureIsland::scale; "Average scale" (average_island_scales()) sets every island to the mean, so one island scaled by hand can be matched back to its neighbours' texel density. Snap (canvas-owned m_snap_enabled, toggled from the toolbar) sticks a dragged island's nearest boundary vertex onto a neighbouring island's at drag-end only - a magnet that re-applies mid-drag is very hard to pull out of. Toolbar commands the canvas can't service itself (Average scale) are forwarded to the gizmo via CommandFn; view-only ones (Frame, Snap) it handles directly.

Known limitations / deferred work

• No .3mf serialization for texture-displacement paint data or texture assets. A background agent attempted this in an earlier session, hit its own usage limit mid-edit, and left bbs_3mf.cpp with an undefined forward-declared function; that partial edit was reverted rather than shipped broken. Practical impact: baked geometry round-trips fine (it's just an ordinary part of the mesh via the existing mesh serialization path) - what does not survive a project save/reload is any unbaked paint stroke and texture layer definition.
• No remap-across-topology-change for texture-displacement paint (ModelObject::split(), mesh boolean ops, Simplify, uniform subdivide, and remesh all drop it via reset_extra_facets()). The other four paint channels (supported/seam/mmu/fuzzy) do get remapped in these cases. The lone exception is adaptive subdivide, which carries texture-displacement paint forward itself via its source map (see the Subdivision section) - a targeted remap that only works because the operation is driven by the paint.
• Cylindrical/Spherical axis/center are auto-picked heuristically, not user-controllable - no UI to override the auto-detected wrap axis if it picks the "wrong" one for an odd shape.
• Fast preview covers the active layer only — and is now the default view when the gizmo opens (m_use_bump_preview = true): it is the instant, no-CPU-meshing preview, so it is the better first impression while painting. The exact true-displacement view is one click away in the View row.
• Displacement resolution is capped by the mesh's own vertex density. Baking only ever moves existing vertices (it never inserts any), so a coarse patch cannot show fine texture detail no matter how high-resolution the height map is - that is what the subdivision controls are for (uniform, adaptive, or feature-adaptive; see the Subdivision section). Since the rewrite the bake is topology-preserving, so this is now a hard, explicit property rather than something partly papered over by the old per-layer re-meshing.

File map

libslic3r (core, no GUI dependency):

• src/libslic3r/TextureDisplacement.hpp/.cpp - data model, bake algorithm, projection methods, tiling, subdivision (uniform + adaptive longest-edge bisection), post-process smoothing (smooth_mesh_vertices()), and TextureDisplacementOptions (the whole-stack settings). See doc comments throughout, they're kept accurate and up to date.
• src/libslic3r/MeshBoolean.hpp/.cpp - added parameterize_lscm() and remesh_isotropic() in the cgal sub-namespace, reusing the existing CGALMesh/_EpicMesh/conversion-helper infrastructure already there for mesh boolean ops. New CGAL includes: Polygon_mesh_processing/border.h, Polygon_mesh_processing/connected_components.h, Surface_mesh_parameterization/{Error_code, LSCM_parameterizer_3, parameterize}.h. No new dependency - CGAL 5.6.3 is already vendored and the Surface_mesh_parameterization package headers were already present, just unused before now.
• src/libslic3r/Model.hpp/.cpp - the 8 named FacetsAnnotation fields + accessor, texture_displacement_layers, texture_displacement_options, and all the mirrored touch points (see Data model above).

GUI:

• src/slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.hpp/.cpp - the gizmo. Panel controls: dock/ undock toggle, brush/face/connected-area selection mode + "select whole model" button, per-layer texture picker + depth/tiling/rotation/invert/tile-mode/projection-mode/blend-mode controls, "Adjust placement" toggle (on-canvas gizmo), "Fast preview (normal map)" toggle, "Subdivide model" button, Add layer/Erase all/Bake.
• src/slic3r/GUI/TextureLibrary.hpp/.cpp - scans the shipped + user texture folders, imports an arbitrary image into the user folder (converting it to the 8-bit grayscale PNG libslic3r decodes), and loads a library file's bytes for a layer. The image→grayscale-PNG conversion lives here, on the GUI side, because libslic3r has no image toolkit; both the import path and the "pick a shipped texture" path go through the same one function.
• resources/textures/displacement/*.png - the 10 shipped height maps (Bricks, Grid, Hexagons, Knurl, Noise, Quilt, Studs, Waves, Weave, Wood Grain). All 512×512 8-bit grayscale and seamless (each is periodic over the full image in both axes, so tiling shows no seam). Generated procedurally; the whole resources/ tree is installed recursively by CMake, so a new folder under it ships with no build-system change.
• src/slic3r/GUI/Jobs/TextureDisplacementBakeJob.hpp/.cpp - background bake commit.
• src/slic3r/GUI/Jobs/TextureDisplacementPreviewJob.hpp/.cpp - background preview compute (mirrors the bake job's shape but commits nothing to the Model).
• src/slic3r/GUI/TextureProjectorFrame.hpp/.cpp - the semi-transparent projection-frame overlay for ViewProjected layers (plain 2D wxPaintDC, no GL context - see its section above).
• src/slic3r/GUI/UVEditorCanvas.hpp/.cpp - the 2D UV unwrap viewer widget.
• src/slic3r/GUI/Plater.hpp/.cpp - uv_editor_canvas member, AUI pane registration, get_uv_editor_canvas()/show_uv_editor().
• src/slic3r/GUI/GLShadersManager.cpp - registers "texture_displacement_bump".
• resources/shaders/{110,140}/texture_displacement_bump.{vs,fs} - the bump-preview shader.
• src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp - PainterGizmoType::TEXTURE_DISPLACEMENT.
• src/slic3r/GUI/Gizmos/GLGizmosManager.hpp/.cpp - EType::TextureDisplacement registration.
• src/slic3r/GUI/ImGuiWrapper.cpp - the light-mode checkmark-color fix

Tests: tests/libslic3r/test_texture_displacement.cpp - run and passing (7 cases, 116 assertions). Covers decode_height_texture round-trip, empty-layer no-op, full-cube uniform displacement, boundary-vertex pinning on a hand-built fan mesh, and - added with the bake rewrite - a regression test that a second layer over the same area actually contributes, a table-driven check of all four blend modes, and that the lowest layer ignores its blend mode. BUILD_TESTS is OFF in the checked-in build cache; flip it on to run them:

cmake -S . -B build -DBUILD_TESTS=ON
cmake --build build --config Release --target libslic3r_tests -- -m
./build/tests/libslic3r/Release/libslic3r_tests.exe "[TextureDisplacement]" --order rand