Merge remote-tracking branch 'origin/main' into feature/texdisp-model-uv

# Conflicts:
#	src/slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.cpp

resources/images/texture_displacement_map_model_uv.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16"><path d="M2.5,2.5h11v11h-11z" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/><path d="M8,2.5v11M2.5,8h11" style="fill:none;stroke:#949494;stroke-dasharray:1.5,1.5"/><path d="M4.5,11.5l3.5-6l3.5,6z" style="fill:#009688"/></svg>

src/libslic3r/Model.cpp

@@ -353,6 +353,24 @@ Model Model::read_from_file(const std::string&
         ObjInfo                 obj_info;
         ObjParser::MtlData      mtl_data;
         result = load_obj(input_file.c_str(), &model, obj_info, message, nullptr, &mtl_data);
+        // Keep the model's own UV channel on the volume, whatever the colour import then does with
+        // the mesh: it is what TextureProjectionMethod::ModelUV maps a texture-displacement layer
+        // through, so a model that arrives with a UV layout is textured on that layout instead of on
+        // a fresh unwrap. load_obj() fills obj_info.uvs per face of the mesh it built, in the same
+        // order (and only when every face named a known material, hence the size check). V is flipped
+        // on the way in, to the convention DecodedHeightTexture samples in - the same flip
+        // obj_to_textured_mesh() does for the colour importer.
+        if (result && !obj_info.uvs.empty() && !model.objects.empty() && !model.objects.back()->volumes.empty()) {
+            ModelVolume                &volume = *model.objects.back()->volumes[0];
+            const indexed_triangle_set &its    = volume.mesh().its;
+            if (obj_info.uvs.size() == its.indices.size()) {
+                volume.texture_uv.resize(its.indices.size() * 3);
+                for (size_t fi = 0; fi < obj_info.uvs.size(); ++fi)
+                    for (int k = 0; k < 3; ++k)
+                        volume.texture_uv[fi * 3 + size_t(k)] =
+                            Vec2f(obj_info.uvs[fi][size_t(k)].x(), 1.f - obj_info.uvs[fi][size_t(k)].y());
+            }
+        }
         if (result && obj_info.has_uv_png && !obj_info.uvs.empty() && !model.objects.empty()) {
             // Textured OBJ: hand the mesh + materials to the texture-to-color importer
             // instead of the flat per-face colour dialog.
@@ -2133,6 +2151,10 @@ void ModelVolume::reset_extra_facets()
     // than left referring to a mesh that no longer matches it.
     for (int i = 0; i < int(TEXTURE_DISPLACEMENT_MAX_LAYERS); ++i)
         this->texture_displacement_facet(i).reset();
+    // Same reasoning for the model's own UV channel: it is indexed by triangle, so against a mesh
+    // this no longer matches it would map the texture by another mesh's triangle numbering. A caller
+    // that can carry it across - see transfer_uv_channel() - puts it back after this.
+    this->reset_texture_uv();
 }
 
 std::optional<TriangleSelector::SavedPainting> ModelVolume::save_painting() const

src/libslic3r/Model.hpp

@@ -974,6 +974,26 @@ public:
     // they are not a property of any single layer.
     TextureDisplacementOptions texture_displacement_options;
 
+    // The model's own texture coordinates, as the file it was imported from carried them: three per
+    // triangle of mesh().its, in its triangle order (corner k of triangle i at 3i+k), V already
+    // flipped to the sampling convention (V = 0 is the image's top row). Empty when the source had
+    // none, which is every STL and every 3mf - only a textured OBJ brings one in today.
+    //
+    // Per corner rather than per vertex because that is what a UV layout is: the two sides of a seam
+    // are the same vertex with two different coordinates, and collapsing them would smear the texture
+    // across every island boundary. TextureProjectionMethod::ModelUV maps through it, which is what
+    // lets a model that already carries a UV channel be textured on the artist's own layout instead
+    // of on a fresh unwrap.
+    //
+    // Indexed by triangle, so anything that *replaces* the mesh has to bring it across (or drop it):
+    // see transfer_uv_channel(), and reset_texture_uv() for the cases that cannot.
+    std::vector<Vec2f> texture_uv;
+
+    bool has_texture_uv() const { return !texture_uv.empty() && texture_uv.size() == mesh().its.indices.size() * 3; }
+    // Forgets the channel. For a mesh change that nothing can carry it across - a split, a boolean -
+    // where leaving it would map the texture by another mesh's triangle numbering.
+    void reset_texture_uv() { texture_uv.clear(); }
+
     // Save painting data before reset_extra_facets() discards it.
     // Used for replacing mesh without losing painting data.
     // Only for model parts (not modifiers/connectors).
@@ -1236,6 +1256,7 @@ private:
         texture_displacement_facets_6(other.texture_displacement_facets_6), texture_displacement_facets_7(other.texture_displacement_facets_7),
         texture_displacement_layers(other.texture_displacement_layers),
         texture_displacement_options(other.texture_displacement_options),
+        texture_uv(other.texture_uv),
         cut_info(other.cut_info), text_configuration(other.text_configuration), emboss_shape(other.emboss_shape)
     {
 		assert(this->id().valid());
@@ -1334,7 +1355,7 @@ private:
             cereal::load_by_value(ar, f);
             mesh_changed |= tf != f.timestamp();
         }
-        ar(texture_displacement_layers, texture_displacement_options);
+        ar(texture_displacement_layers, texture_displacement_options, texture_uv);
         cereal::load_by_value(ar, config);
         cereal::load(ar, text_configuration);
         cereal::load(ar, emboss_shape);
@@ -1358,7 +1379,7 @@ private:
         cereal::save_by_value(ar, fuzzy_skin_facets);
         for (int i = 0; i < int(TEXTURE_DISPLACEMENT_MAX_LAYERS); ++i)
             cereal::save_by_value(ar, texture_displacement_facet(i));
-        ar(texture_displacement_layers, texture_displacement_options);
+        ar(texture_displacement_layers, texture_displacement_options, texture_uv);
         cereal::save_by_value(ar, config);
         cereal::save(ar, text_configuration);
         cereal::save(ar, emboss_shape);

src/libslic3r/TextureDisplacement.cpp

@@ -1571,6 +1571,61 @@ std::vector<TextureIsland> compute_connected_net(const PatchUnwrap &unwrap)
     return islands;
 }
 
+std::vector<uint8_t> transfer_facet_paint(const indexed_triangle_set                    &src_mesh,
+                                         const TriangleSelector::TriangleSplittingData &src_paint,
+                                         const indexed_triangle_set                    &dst_mesh)
+{
+    if (src_mesh.indices.empty() || dst_mesh.indices.empty() || src_paint.bitstream.empty())
+        return {};
+
+    const TriangleMesh src(src_mesh);
+    TriangleSelector   selector(src);
+    selector.deserialize(src_paint, false);
+
+    // Every sub-triangle of the source, each carrying its own state - NONE included, so the soup
+    // covers the whole surface and a foot point always lands on something. One soup rather than one
+    // tree per state: a foot point has one nearest triangle, and that triangle's state is the answer.
+    std::vector<indexed_triangle_set> per_state;
+    selector.get_facets(per_state);
+    indexed_triangle_set soup;
+    std::vector<uint8_t> label;
+    for (size_t st = 0; st < per_state.size(); ++st) {
+        const indexed_triangle_set &part = per_state[st];
+        if (part.indices.empty())
+            continue;
+        const int base = int(soup.vertices.size());
+        soup.vertices.insert(soup.vertices.end(), part.vertices.begin(), part.vertices.end());
+        for (const stl_triangle_vertex_indices &t : part.indices) {
+            soup.indices.emplace_back(t[0] + base, t[1] + base, t[2] + base);
+            label.push_back(uint8_t(st));
+        }
+    }
+    if (soup.indices.empty())
+        return {};
+
+    const AABBTreeIndirect::Tree3f tree =
+        AABBTreeIndirect::build_aabb_tree_over_indexed_triangle_set(soup.vertices, soup.indices);
+
+    std::vector<uint8_t> out(dst_mesh.indices.size(), 0);
+    tbb::parallel_for(tbb::blocked_range<size_t>(0, dst_mesh.indices.size()),
+                      [&](const tbb::blocked_range<size_t> &range) {
+        for (size_t j = range.begin(); j < range.end(); ++j) {
+            const stl_triangle_vertex_indices &t = dst_mesh.indices[j];
+            // The centroid, not a corner: a facet takes one state, and its middle is the point that
+            // is inside it whatever the triangle looks like.
+            const Vec3f centroid = (dst_mesh.vertices[size_t(t[0])] + dst_mesh.vertices[size_t(t[1])] +
+                                    dst_mesh.vertices[size_t(t[2])]) / 3.f;
+            size_t      hit = 0;
+            Vec3f       foot;
+            const float d2 = AABBTreeIndirect::squared_distance_to_indexed_triangle_set(soup.vertices, soup.indices,
+                                                                                        tree, centroid, hit, foot);
+            if (d2 >= 0.f && hit < label.size())
+                out[j] = label[hit];
+        }
+    });
+    return out;
+}
+
 void average_island_scales(std::vector<TextureIsland> &islands)
 {
     if (islands.empty())
@@ -1673,6 +1728,110 @@ std::vector<Vec2f> compute_lscm_corner_uvs(const indexed_triangle_set &patch, co
     return corner;
 }
 
+// Barycentric weights of `p` inside triangle (a, b, c), as the pair that belongs with the edges
+// (b - a) and (c - a): p ~= (1 - w1 - w2) * a + w1 * b + w2 * c. `p` is projected onto the triangle's
+// plane, so a point a hair off the surface still reports the weights of its foot point. A degenerate
+// triangle reports its centroid, the only coordinate that means anything there.
+static void barycentric_weights(const Vec3f &a, const Vec3f &b, const Vec3f &c, const Vec3f &p, float &w1, float &w2)
+{
+    const Vec3f e0 = b - a, e1 = c - a, ep = p - a;
+    const float d00 = e0.dot(e0), d01 = e0.dot(e1), d11 = e1.dot(e1), dp0 = ep.dot(e0), dp1 = ep.dot(e1);
+    const float den = d00 * d11 - d01 * d01;
+    if (std::abs(den) <= 1e-20f) {
+        w1 = w2 = 1.f / 3.f;
+        return;
+    }
+    w1 = (d11 * dp0 - d01 * dp1) / den;
+    w2 = (d00 * dp1 - d01 * dp0) / den;
+}
+
+std::vector<Vec2f> compute_model_corner_uvs(const indexed_triangle_set &base_mesh,
+                                            const std::vector<Vec2f>   &model_uv,
+                                            const indexed_triangle_set &patch,
+                                            const std::vector<int>     &patch_source)
+{
+    if (patch.indices.empty() || model_uv.size() != base_mesh.indices.size() * 3 ||
+        patch_source.size() != patch.indices.size())
+        return {};
+
+    std::vector<Vec2f> corner(patch.indices.size() * 3, Vec2f::Zero());
+    for (size_t j = 0; j < patch.indices.size(); ++j) {
+        const int S = patch_source[j];
+        if (S < 0 || size_t(S) >= base_mesh.indices.size())
+            continue; // no source triangle: leave (0, 0), which the callers read as "no placement"
+        const stl_triangle_vertex_indices &src = base_mesh.indices[size_t(S)];
+        const Vec3f &a = base_mesh.vertices[size_t(src[0])];
+        const Vec3f &b = base_mesh.vertices[size_t(src[1])];
+        const Vec3f &c = base_mesh.vertices[size_t(src[2])];
+        const Vec2f *uv = &model_uv[size_t(S) * 3];
+        const stl_triangle_vertex_indices &tri = patch.indices[j];
+        for (int k = 0; k < 3; ++k) {
+            // An unsplit patch triangle is its source triangle, so its corners land exactly on a, b
+            // and c and this reproduces their uvs unchanged; a sub-triangle the brush cut gets the
+            // interpolated coordinate, which is the whole point of going through the barycentric.
+            float w1 = 0.f, w2 = 0.f;
+            barycentric_weights(a, b, c, patch.vertices[size_t(tri[k])], w1, w2);
+            corner[j * 3 + size_t(k)] = (1.f - w1 - w2) * uv[0] + w1 * uv[1] + w2 * uv[2];
+        }
+    }
+    return corner;
+}
+
+std::vector<Vec2f> collapse_corner_uvs(const indexed_triangle_set &patch, const std::vector<Vec2f> &corner)
+{
+    if (patch.indices.empty() || corner.size() != patch.indices.size() * 3)
+        return {};
+
+    std::vector<Vec2f> per_vertex(patch.vertices.size(), Vec2f::Zero());
+    std::vector<char>  seen(patch.vertices.size(), 0);
+    for (size_t j = 0; j < patch.indices.size(); ++j)
+        for (int k = 0; k < 3; ++k) {
+            const int vi = patch.indices[j][k];
+            if (vi < 0 || size_t(vi) >= per_vertex.size() || seen[size_t(vi)])
+                continue;
+            seen[size_t(vi)]       = 1;
+            per_vertex[size_t(vi)] = corner[j * 3 + size_t(k)];
+        }
+    return per_vertex;
+}
+
+std::vector<Vec2f> transfer_uv_channel(const indexed_triangle_set &src_mesh,
+                                       const std::vector<Vec2f>   &src_uv,
+                                       const indexed_triangle_set &dst_mesh,
+                                       float                       max_distance)
+{
+    if (src_mesh.indices.empty() || dst_mesh.indices.empty() || src_uv.size() != src_mesh.indices.size() * 3)
+        return {};
+
+    const AABBTreeIndirect::Tree3f tree =
+        AABBTreeIndirect::build_aabb_tree_over_indexed_triangle_set(src_mesh.vertices, src_mesh.indices);
+    const float limit2 = max_distance >= 0.f ? max_distance * max_distance : std::numeric_limits<float>::max();
+
+    std::vector<Vec2f> out(dst_mesh.indices.size() * 3, Vec2f::Zero());
+    tbb::parallel_for(tbb::blocked_range<size_t>(0, dst_mesh.indices.size()),
+                      [&](const tbb::blocked_range<size_t> &range) {
+        for (size_t j = range.begin(); j < range.end(); ++j) {
+            const stl_triangle_vertex_indices &tri = dst_mesh.indices[j];
+            for (int k = 0; k < 3; ++k) {
+                const Vec3f &p = dst_mesh.vertices[size_t(tri[k])];
+                size_t       hit = 0;
+                Vec3f        foot;
+                const float  d2 = AABBTreeIndirect::squared_distance_to_indexed_triangle_set(
+                    src_mesh.vertices, src_mesh.indices, tree, p, hit, foot);
+                if (d2 < 0.f || d2 > limit2 || hit >= src_mesh.indices.size())
+                    continue; // nowhere near the source surface: (0, 0), as documented
+                const stl_triangle_vertex_indices &st = src_mesh.indices[hit];
+                float w1 = 0.f, w2 = 0.f;
+                barycentric_weights(src_mesh.vertices[size_t(st[0])], src_mesh.vertices[size_t(st[1])],
+                                    src_mesh.vertices[size_t(st[2])], foot, w1, w2);
+                const Vec2f *uv        = &src_uv[hit * 3];
+                out[j * 3 + size_t(k)] = (1.f - w1 - w2) * uv[0] + w1 * uv[1] + w2 * uv[2];
+            }
+        }
+    });
+    return out;
+}
+
 namespace {
 // apply_uv_transform()'s per-layer constants, worked out once. Triplanar sampling runs the transform
 // three times per point, and recomputing the rotation's cos/sin and the tiling reciprocal on every one
@@ -1682,13 +1841,16 @@ struct UVTransform
     float scale, cs, sn, aspect;
     Vec2f offset;
 
-    UVTransform(const TextureDisplacementLayer &layer, float aspect_)
+    // `normalized_uv` drops the two steps that only mean something for a coordinate measured on the
+    // model - the millimetre tiling scale and the non-square aspect correction - and keeps the
+    // rotation and offset. See apply_uv_transform(); TextureProjectionMethod::ModelUV is what needs it.
+    UVTransform(const TextureDisplacementLayer &layer, float aspect_, bool normalized_uv = false)
     {
-        scale = (layer.tiling_scale > 1e-6f) ? (1.f / layer.tiling_scale) : 1.f;
+        scale = (normalized_uv || layer.tiling_scale <= 1e-6f) ? 1.f : (1.f / layer.tiling_scale);
         const float rad = layer.rotation_deg * float(M_PI) / 180.f;
         cs     = std::cos(rad);
         sn     = std::sin(rad);
-        aspect = aspect_;
+        aspect = normalized_uv ? 1.f : aspect_;
         offset = layer.offset;
     }
     Vec2f operator()(const Vec2f &planar) const
@@ -1713,9 +1875,11 @@ inline Vec3f triplanar_weights(const Vec3f &normal)
 }
 } // namespace
 
-Vec2f apply_uv_transform(const Vec2f &planar, const TextureDisplacementLayer &layer, float aspect)
+Vec2f apply_uv_transform(const Vec2f &planar, const TextureDisplacementLayer &layer, float aspect, bool normalized_uv)
 {
-    const float scale = (layer.tiling_scale > 1e-6f) ? (1.f / layer.tiling_scale) : 1.f;
+    // A uv that is already normalized has no millimetre scale to divide by and no aspect to correct -
+    // see the header. UVTransform does the same, and the two must stay in step.
+    const float scale = (normalized_uv || layer.tiling_scale <= 1e-6f) ? 1.f : (1.f / layer.tiling_scale);
     const Vec2f scaled = planar * scale;
 
     const float rad = layer.rotation_deg * float(M_PI) / 180.f;
@@ -1733,7 +1897,7 @@ Vec2f apply_uv_transform(const Vec2f &planar, const TextureDisplacementLayer &la
     // Applied after the rotation, not before: scaling one axis of an already-rotated coordinate is a
     // shear, and doing it the other way round would make "Rotation" skew the pattern instead of
     // turning it.
-    if (aspect > 0.f && aspect != 1.f)
+    if (!normalized_uv && aspect > 0.f && aspect != 1.f)
         rotated.y() *= aspect;
 
     return rotated + layer.offset;
@@ -1758,6 +1922,9 @@ float blend_displacement(float accumulated, float value, TextureBlendMode mode)
                                                                         value;
         return accumulated / divisor;
     }
+    // Replaces instead of combining, so `accumulated` is deliberately unused: whatever the layers
+    // below put here is gone where this one is painted.
+    case TextureBlendMode::Override: return value;
     case TextureBlendMode::Add:
     default: return accumulated + value;
     }
@@ -1787,14 +1954,21 @@ float sample_layer_height(const DecodedHeightTexture &texture, const TextureDisp
     // apply_uv_transform()). Every projection except the projective "from view" one funnels through
     // here, so this one line is what makes them all aspect-correct.
     const float       aspect = (texture.height > 0) ? float(texture.width) / float(texture.height) : 1.f;
-    const UVTransform xf(layer, aspect);
+    // A ModelUV layer's precomputed uv is the model's own, already normalized; every other coordinate
+    // reaching the transform - the unwrap's, and every analytic projection's - is in millimetres. The
+    // `lscm_uv` test is what keeps the two apart: a ModelUV layer with no channel gets none, falls
+    // through to the millimetre methods below, and must be scaled like them.
+    const bool        normalized_uv = layer.projection_method == TextureProjectionMethod::ModelUV &&
+                                      lscm_uv != nullptr;
+    const UVTransform xf(layer, aspect, normalized_uv);
     auto sample_at = [&](const Vec2f &planar) {
         return texture.sample(xf(planar), layer.tile_enabled, layer.tile_method);
     };
 
-    // Precomputed per-patch LSCM solve wins over the layer's own method (see the header): the
-    // caller only passes it when the patch actually parameterized successfully, so a patch that
-    // failed to unwrap falls through to the analytic methods below as its documented fallback.
+    // A precomputed per-patch uv - the LSCM solve, or the model's own channel - wins over the layer's
+    // own method (see the header): the caller only passes it when the patch actually has one, so a
+    // patch that failed to unwrap, or a volume with no UV channel, falls through to the analytic
+    // methods below as its documented fallback.
     if (lscm_uv != nullptr)
         return sample_at(*lscm_uv);
 
@@ -1852,8 +2026,12 @@ bool sample_layer_color(const DecodedHeightTexture &texture, const TextureDispla
     // the two differ in what "nothing here" means. Height returns 0, which is a perfectly good height
     // (no displacement); colour has no such neutral value - black is a colour - so every path that
     // returns 0 there has to report false here instead, and the caller leaves the triangle uncoloured.
-    const float       aspect = (texture.height > 0) ? float(texture.width) / float(texture.height) : 1.f;
-    const UVTransform xf(layer, aspect);
+    const float       aspect        = (texture.height > 0) ? float(texture.width) / float(texture.height) : 1.f;
+    // Same reasoning as sample_layer_height(), and it has to match it exactly or a layer's colour and
+    // its relief would be read from different places in the image.
+    const bool        normalized_uv = layer.projection_method == TextureProjectionMethod::ModelUV &&
+                                      lscm_uv != nullptr;
+    const UVTransform xf(layer, aspect, normalized_uv);
     auto sample_at = [&](const Vec2f &planar) {
         return texture.sample_color(xf(planar), layer.tile_enabled, layer.tile_method);
     };
@@ -1867,6 +2045,8 @@ bool sample_layer_color(const DecodedHeightTexture &texture, const TextureDispla
     };
 
     if (lscm_uv != nullptr) {
+        // The coverage test goes through the same transform, so an untiled ModelUV layer is "there"
+        // exactly where the model's own uv lands inside the image.
         if (!covered(*lscm_uv))
             return false;
         out = sample_at(*lscm_uv);
@@ -2211,9 +2391,10 @@ indexed_triangle_set build_texture_displacement_v2(const indexed_triangle_set
                                                    const TextureColorRequest                   *color,
                                                    bool                                         flip_normals,
                                                    BakeStageRecorder                           *debug,
-                                                   TextureBakeStats                            *stats)
+                                                   TextureBakeStats                            *stats,
+                                                   const std::vector<Vec2f>                    *model_uv)
 {
-    HeightFieldSampler combined = make_combined_displacement_sampler(mesh, layers, facets_data);
+    HeightFieldSampler combined = make_combined_displacement_sampler(mesh, layers, facets_data, model_uv);
     if (!combined)
         return mesh; // nothing decodable to displace with
 
@@ -2383,7 +2564,8 @@ indexed_triangle_set build_texture_displacement_v2(const indexed_triangle_set
     // part of it (half a tile in the neighbour's colour, a tile edge that wanders).
     const bool              want_color = color != nullptr && color->out_triangle != nullptr && bool(color->quantize);
     const ColorFieldSampler color_sampler =
-        want_color ? make_combined_color_sampler(mesh, layers, facets_data, color->quantize, color->quantize_pure) : ColorFieldSampler{};
+        want_color ? make_combined_color_sampler(mesh, layers, facets_data, color->quantize, color->quantize_pure, model_uv) :
+                     ColorFieldSampler{};
     //
     // The *palette* index, not the printed filament. The decimation treats any edge whose two faces
     // differ as a crease (TextureBakeDecimate.cpp), so it must only ever see where the **perceived**
@@ -2515,7 +2697,8 @@ static indexed_triangle_set build_texture_displacement_in_place(
     const TextureColorRequest                   *color,
     bool                                         flip_normals,
     BakeStageRecorder                           *debug,
-    TextureBakeStats                            *stats)
+    TextureBakeStats                            *stats,
+    const std::vector<Vec2f>                    *model_uv)
 {
     // The classic path moves the vertices the mesh already has, so there is no budget to report on.
     (void) stats;
@@ -2552,7 +2735,7 @@ static indexed_triangle_set build_texture_displacement_in_place(
 
     if (options.pipeline_v2)
         return build_texture_displacement_v2(mesh, layers, facets_data, options, progress, color, flip_normals,
-                                             debug, stats);
+                                             debug, stats, model_uv);
 
     // Layers are combined in slot order, like stacked layers in an image editor: each one folds its
     // own displacement into the running total via its blend mode (see TextureBlendMode).
@@ -2670,13 +2853,19 @@ static indexed_triangle_set build_texture_displacement_in_place(
         selector_dirty = true;
 
         const bool       color_this_layer = want_color && layer->color_enabled;
+        // The model's own channel is per base-mesh *triangle*, so reading it needs to know which base
+        // triangle each (possibly brush-split) patch triangle came from - the colour pass's map, asked
+        // for here as well.
+        const bool       model_uv_layer   = layer->projection_method == TextureProjectionMethod::ModelUV &&
+                                            model_uv != nullptr && model_uv->size() == mesh.indices.size() * 3;
         // A flat-colour image is matched against the filaments alone (see TextureColorRequest).
         const ColorQuantizeFn &layer_quantize =
             (color_this_layer && color->quantize_pure && analyze_texture_detail(*layer).flat_colors) ? color->quantize_pure
                                                                                                      : color->quantize;
-        std::vector<int> patch_source; // sub-triangle -> base mesh triangle, only built when colouring
+        std::vector<int> patch_source; // sub-triangle -> base mesh triangle; built when colouring or mapping by ModelUV
         const indexed_triangle_set patch =
-            selector.get_facets_strict(EnforcerBlockerType::ENFORCER, color_this_layer ? &patch_source : nullptr);
+            selector.get_facets_strict(EnforcerBlockerType::ENFORCER,
+                                       (color_this_layer || model_uv_layer) ? &patch_source : nullptr);
         if (patch.indices.empty())
             continue;
         // get_facets_strict() returns the same vertex array whichever state is asked for (only the
@@ -2707,18 +2896,25 @@ static indexed_triangle_set build_texture_displacement_in_place(
         // A real unwrap of the whole patch, computed once here rather than per vertex - it is a
         // per-chart solve over the whole patch, not a per-point formula. Cached, so repeating this
         // for every slider tweak costs a hash rather than a re-solve (see compute_patch_unwrap()).
-        const std::vector<Vec2f> lscm_uvs = (layer->projection_method == TextureProjectionMethod::LSCM) ?
+        // ModelUV takes its corners from the volume's own channel instead of solving anything, and
+        // collapses them to one per vertex the same way the unwrap does.
+        const std::vector<Vec2f> model_corner_uvs = model_uv_layer ?
+                                                        compute_model_corner_uvs(mesh, *model_uv, patch, patch_source) :
+                                                        std::vector<Vec2f>{};
+        const std::vector<Vec2f> patch_uvs = model_uv_layer ? collapse_corner_uvs(patch, model_corner_uvs) :
+                                             (layer->projection_method == TextureProjectionMethod::LSCM) ?
                                                  compute_lscm_uvs(patch, *layer) :
                                                  std::vector<Vec2f>{};
         // The colour pass below samples per *triangle*, so it takes the per-corner unwrap instead: the
         // per-vertex collapse above would hand a triangle at a seam the island layout did not join its
         // neighbour's placement, painting one triangle per face from the wrong part of the texture.
-        // (The displacement itself stays on lscm_uvs - a vertex has one position, so one height.)
-        const std::vector<Vec2f> lscm_corner_uvs = (layer->projection_method == TextureProjectionMethod::LSCM) ?
+        // (The displacement itself stays on patch_uvs - a vertex has one position, so one height.)
+        const std::vector<Vec2f> patch_corner_uvs = model_uv_layer ? model_corner_uvs :
+                                                    (layer->projection_method == TextureProjectionMethod::LSCM) ?
                                                         compute_lscm_corner_uvs(patch, *layer) :
                                                         std::vector<Vec2f>{};
-        const bool               corner_uv_ok    = !lscm_corner_uvs.empty() &&
-                                                   lscm_corner_uvs.size() == patch.indices.size() * 3;
+        const bool               corner_uv_ok     = !patch_corner_uvs.empty() &&
+                                                    patch_corner_uvs.size() == patch.indices.size() * 3;
 
         // Colour, if this layer carries any. Area-weighted over each base triangle's *painted* part,
         // so a triangle the brush only clipped a corner off takes the colour of that corner rather
@@ -2748,7 +2944,7 @@ static indexed_triangle_set build_texture_displacement_in_place(
                     // the per-vertex height sampling uses, evaluated at the centroid instead.
                     Vec3f n = Vec3f::Zero();
                     Vec2f uv = Vec2f::Zero();
-                    bool  have_uv = !lscm_uvs.empty();
+                    bool  have_uv = !patch_uvs.empty();
                     for (int k = 0; k < 3; ++k) {
                         const int vi = t[k];
                         if (vi < int(vertex_normals.size()))
@@ -2756,9 +2952,9 @@ static indexed_triangle_set build_texture_displacement_in_place(
                         if (!have_uv)
                             continue;
                         if (corner_uv_ok)
-                            uv += lscm_corner_uvs[j * 3 + size_t(k)];
-                        else if (size_t(vi) < lscm_uvs.size())
-                            uv += lscm_uvs[size_t(vi)];
+                            uv += patch_corner_uvs[j * 3 + size_t(k)];
+                        else if (size_t(vi) < patch_uvs.size())
+                            uv += patch_uvs[size_t(vi)];
                         else
                             have_uv = false;
                     }
@@ -2832,7 +3028,7 @@ static indexed_triangle_set build_texture_displacement_in_place(
                           [&](const tbb::blocked_range<size_t> &range) {
             for (size_t k = range.begin(); k < range.end(); ++k) {
                 const size_t vi      = size_t(layer_vertices[k]);
-                const Vec2f *lscm_uv = lscm_uvs.empty() ? nullptr : &lscm_uvs[vi];
+                const Vec2f *lscm_uv = patch_uvs.empty() ? nullptr : &patch_uvs[vi];
                 const float  h       = sample_layer_height(height, *layer, mesh.vertices[vi], vertex_normals[vi],
                                                             patch_centroid, patch_axis, lscm_uv);
                 // midlevel is the height that means "stay put", so anything below it displaces
@@ -2965,7 +3161,8 @@ indexed_triangle_set build_texture_displacement(const indexed_triangle_set
                                                  const TextureColorRequest                   *color,
                                                  const Transform3d                           &volume_to_world,
                                                  BakeStageRecorder                           *debug,
-                                                 TextureBakeStats                            *stats)
+                                                 TextureBakeStats                            *stats,
+                                                 const std::vector<Vec2f>                    *model_uv)
 {
     // An untransformed volume on an untransformed instance is by far the common case, and the round
     // trip costs two matrix multiplies per vertex on a mesh that can carry millions of them - so take
@@ -2975,7 +3172,7 @@ indexed_triangle_set build_texture_displacement(const indexed_triangle_set
     const Transform3d frame = texture_displacement_bake_frame(volume_to_world);
     if (frame.matrix().isApprox(Transform3d::Identity().matrix()))
         return build_texture_displacement_in_place(base_mesh, layers, facets_data, options, progress, color,
-                                                   false, debug, stats);
+                                                   false, debug, stats, model_uv);
 
     const Transform3d to_local = frame.inverse();
     // A mirroring placement leaves the positions correct but every winding-derived normal pointing
@@ -2989,8 +3186,10 @@ indexed_triangle_set build_texture_displacement(const indexed_triangle_set
         v = (frame * v.cast<double>()).cast<float>();
 
     const size_t         debug_mark = (debug != nullptr) ? debug->mark() : 0;
+    // The UV channel rides along untouched: a uv is a coordinate in the texture, not on the model, so
+    // the frame the mesh is measured in does not reach it.
     indexed_triangle_set out = build_texture_displacement_in_place(world, layers, facets_data, options,
-                                                                   progress, color, mirrored, debug, stats);
+                                                                   progress, color, mirrored, debug, stats, model_uv);
     // Everything the bake recorded is in world millimetres, like `out` itself. The debug view draws in
     // the volume's local frame, so the stages are brought back the same way the result is.
     if (debug != nullptr)
@@ -3062,7 +3261,8 @@ indexed_triangle_set build_texture_displacement(const ModelVolume &volume)
 
     return build_texture_displacement(volume.mesh().its, volume.texture_displacement_layers, facets_data,
                                       volume.texture_displacement_options, {}, nullptr,
-                                      texture_displacement_volume_to_world(volume));
+                                      texture_displacement_volume_to_world(volume), nullptr, nullptr,
+                                      volume.has_texture_uv() ? &volume.texture_uv : nullptr);
 }
 
 void smooth_mesh_vertices(indexed_triangle_set &mesh, const std::vector<uint8_t> &movable, float strength,
@@ -3126,16 +3326,18 @@ void smooth_mesh_vertices(indexed_triangle_set &mesh, const std::vector<uint8_t>
 namespace {
 // One decoded texture + placement per sampleable layer, in blend (slot) order. Held by shared_ptr so
 // the returned closure owns it for as long as the subdivider keeps calling back.
-// An unwrap turned into something a *point* sampler can use. LSCM has no formula from position to
-// uv - it is a per-triangle map - so a point is placed on the painted patch (the nearest patch
-// triangle, and its barycentric coordinates there) and the uv is interpolated from that triangle's own
-// per-corner uvs. Exact for a point on the base surface, which is where both samplers are queried: the
-// displacement samples refined positions before moving them, and the colour pass samples the foot
-// point on the painted pieces.
+// A per-triangle uv map turned into something a *point* sampler can use. Neither of the two maps that
+// are not a formula from position to uv - the LSCM unwrap and the model's own UV channel - can be
+// evaluated at an arbitrary point directly, so a point is placed on the painted patch (the nearest
+// patch triangle, and its barycentric coordinates there) and the uv is interpolated from that
+// triangle's own per-corner uvs. Exact for a point on the base surface, which is where both samplers
+// are queried: the displacement samples refined positions before moving them, and the colour pass
+// samples the foot point on the painted pieces.
 struct LscmLookup {
-    indexed_triangle_set    patch;  // the layer's painted patch, as the unwrap was solved on
+    indexed_triangle_set    patch;  // the layer's painted patch, as the uvs were computed on
     AABBTreeIndirect::Tree3f tree;
-    std::vector<Vec2f>      corner; // compute_lscm_corner_uvs(patch, layer)
+    // compute_lscm_corner_uvs(patch, layer), or compute_model_corner_uvs() for a ModelUV layer
+    std::vector<Vec2f>      corner;
 
     // False when `pos` is not on this layer's patch (farther than `tol`): there is no uv there, so the
     // layer contributes nothing - the same as a non-tiled texture outside its placement.
@@ -3148,15 +3350,9 @@ struct LscmLookup {
         if (d2 < 0.f || d2 > tol * tol || hit >= patch.indices.size())
             return false;
         const stl_triangle_vertex_indices &t = patch.indices[hit];
-        const Vec3f &a = patch.vertices[size_t(t[0])], &b = patch.vertices[size_t(t[1])], &c = patch.vertices[size_t(t[2])];
-        const Vec3f  e0 = b - a, e1 = c - a, ep = foot - a;
-        const float  d00 = e0.dot(e0), d01 = e0.dot(e1), d11 = e1.dot(e1), dp0 = ep.dot(e0), dp1 = ep.dot(e1);
-        const float  den = d00 * d11 - d01 * d01;
-        float        w1 = 1.f / 3.f, w2 = 1.f / 3.f; // a degenerate triangle takes its centroid's uv
-        if (std::abs(den) > 1e-20f) {
-            w1 = (d11 * dp0 - d01 * dp1) / den;
-            w2 = (d00 * dp1 - d01 * dp0) / den;
-        }
+        float w1 = 0.f, w2 = 0.f; // a degenerate triangle takes its centroid's uv
+        barycentric_weights(patch.vertices[size_t(t[0])], patch.vertices[size_t(t[1])], patch.vertices[size_t(t[2])],
+                            foot, w1, w2);
         const Vec2f *c3 = &corner[hit * 3];
         uv = (1.f - w1 - w2) * c3[0] + w1 * c3[1] + w2 * c3[2];
         return true;
@@ -3215,7 +3411,7 @@ struct PreparedLayer {
 // where a layer is placed. `need_color` additionally drops layers that cannot contribute colour.
 std::shared_ptr<std::vector<PreparedLayer>> prepare_sampleable_layers(
     const indexed_triangle_set &base_mesh, const std::vector<TextureDisplacementLayer> &layers,
-    const TextureDisplacementFacetsData &facets_data, bool need_color)
+    const TextureDisplacementFacetsData &facets_data, bool need_color, const std::vector<Vec2f> *model_uv)
 {
     auto prepared = std::make_shared<std::vector<PreparedLayer>>();
 
@@ -3246,9 +3442,14 @@ std::shared_ptr<std::vector<PreparedLayer>> prepare_sampleable_layers(
         if (tex.empty() || (need_color && !tex.has_color()))
             continue;
 
+        const bool model_uv_layer = layer->projection_method == TextureProjectionMethod::ModelUV &&
+                                    model_uv != nullptr && model_uv->size() == base_mesh.indices.size() * 3;
+
         TriangleSelector selector(selector_mesh);
         selector.deserialize(data, false);
-        const indexed_triangle_set patch = selector.get_facets_strict(EnforcerBlockerType::ENFORCER);
+        std::vector<int>           patch_source; // only the model's own channel needs it; see the classic path
+        const indexed_triangle_set patch =
+            selector.get_facets_strict(EnforcerBlockerType::ENFORCER, model_uv_layer ? &patch_source : nullptr);
         if (patch.indices.empty())
             continue;
 
@@ -3258,12 +3459,16 @@ std::shared_ptr<std::vector<PreparedLayer>> prepare_sampleable_layers(
         texture_displacement_patch_frame(patch, vertex_normals, centroid, axis, average_normal);
 
         std::shared_ptr<const LscmLookup> lscm;
-        if (layer->projection_method == TextureProjectionMethod::LSCM) {
+        if (model_uv_layer || layer->projection_method == TextureProjectionMethod::LSCM) {
             // Solved on the very patch the classic path and the GUI solve it on (same geometry, seam
             // angle and edges), so it hits the unwrap cache and lands exactly where the UV editor
-            // shows it, hand-placed islands and UV edits included.
+            // shows it, hand-placed islands and UV edits included. A ModelUV layer needs no solve at
+            // all - its corners are read straight out of the volume's channel - but it is looked up
+            // through the same per-triangle machinery, for the same reason: there is no formula from a
+            // point to its uv.
             auto l    = std::make_shared<LscmLookup>();
-            l->corner = compute_lscm_corner_uvs(patch, *layer);
+            l->corner = model_uv_layer ? compute_model_corner_uvs(base_mesh, *model_uv, patch, patch_source) :
+                                          compute_lscm_corner_uvs(patch, *layer);
             if (l->corner.size() == patch.indices.size() * 3) {
                 l->patch = patch;
                 l->tree  = AABBTreeIndirect::build_aabb_tree_over_indexed_triangle_set(l->patch.vertices, l->patch.indices);
@@ -3289,11 +3494,12 @@ ColorFieldSampler make_combined_color_sampler(const indexed_triangle_set
                                               const std::vector<TextureDisplacementLayer> &layers,
                                               const TextureDisplacementFacetsData         &facets_data,
                                               ColorQuantizeFn                              quantize,
-                                              ColorQuantizeFn                              quantize_pure)
+                                              ColorQuantizeFn                              quantize_pure,
+                                              const std::vector<Vec2f>                    *model_uv)
 {
     if (!quantize)
         return nullptr;
-    auto prepared = prepare_sampleable_layers(base_mesh, layers, facets_data, /* need_color */ true);
+    auto prepared = prepare_sampleable_layers(base_mesh, layers, facets_data, /* need_color */ true, model_uv);
     if (prepared->empty())
         return nullptr;
     // Per layer: a flat-colour image is matched against the filaments alone, when that quantizer
@@ -3324,9 +3530,10 @@ ColorFieldSampler make_combined_color_sampler(const indexed_triangle_set
 
 HeightFieldSampler make_combined_displacement_sampler(const indexed_triangle_set                  &base_mesh,
                                                       const std::vector<TextureDisplacementLayer> &layers,
-                                                      const TextureDisplacementFacetsData         &facets_data)
+                                                      const TextureDisplacementFacetsData         &facets_data,
+                                                      const std::vector<Vec2f>                    *model_uv)
 {
-    auto prepared = prepare_sampleable_layers(base_mesh, layers, facets_data, /* need_color */ false);
+    auto prepared = prepare_sampleable_layers(base_mesh, layers, facets_data, /* need_color */ false, model_uv);
     if (prepared->empty())
         return nullptr;
 

src/libslic3r/TextureDisplacement.hpp

@@ -86,6 +86,17 @@ enum class TextureProjectionMethod : int
     // that is inherent to view projection and is the user's call, not a bug. The projector's two
     // in-plane axes live in TextureDisplacementLayer::view_project_right/up.
     ViewProjected = 4,
+    // The model's own texture coordinates, as the file it was imported from carried them (see
+    // ModelVolume::texture_uv). Nothing is unwrapped or projected: the artist's layout *is* the
+    // placement, so a texture authored against that layout - the model's own albedo map, say - lands
+    // exactly where it was painted. Automatically picked for a new layer on a volume that has a UV
+    // channel, since re-unwrapping a model that already carries one can only be worse than using it.
+    //
+    // The uv is already normalized, so unlike every method above it is not measured in millimetres:
+    // "Tile size" and the non-square aspect correction do not apply to it (the layer's rotation and
+    // offset still do). Falls back to Triplanar when the volume has no channel, exactly as LSCM falls
+    // back when a patch cannot be flattened.
+    ModelUV       = 5,
 };
 
 // Dihedral angle (degrees) above which an edge between two painted triangles becomes a chart seam
@@ -121,6 +132,11 @@ enum class TextureBlendMode : int
     Subtract = 1, // acc - value          (carve this layer's relief out of the layers below)
     Multiply = 2, // acc * (value / 1mm)  (mask/modulate the layers below by this layer)
     Divide   = 3, // acc / (value / 1mm)  (inverse mask; guarded against a zero/near-zero divisor)
+    // acc = value: this layer replaces what is under it instead of combining with it, so where it is
+    // painted only its own relief remains. The image editor's "Normal" mode, under the name the panel
+    // uses. Note the stack's *colour* already behaves this way - a higher slot's colour overwrites a
+    // lower one's where they overlap - so this is what makes the displacement agree with it.
+    Override = 4,
 };
 
 // Combines one layer's signed displacement `value` (mm) into `accumulated` (mm) per `mode`.
@@ -568,7 +584,13 @@ Vec2f project_planar(const Vec3f &position, const Vec3f &normal);
 // squeezed into a square tile: `tiling_scale` is the tile's size along u, and the tile is
 // `tiling_scale * height / width` mm along v, which keeps texels square. 1 (the default) is the
 // square case and leaves the coordinate exactly as it always was.
-Vec2f apply_uv_transform(const Vec2f &planar, const TextureDisplacementLayer &layer, float aspect = 1.f);
+//
+// `normalized_uv` says the coordinate is already a uv rather than a millimetre measurement on the
+// model - which is what TextureProjectionMethod::ModelUV hands in. Then neither the tiling scale (mm
+// per tile is meaningless for it) nor the aspect correction (the artist's layout already accounts for
+// the image's proportions) applies, and only the rotation and offset are left.
+Vec2f apply_uv_transform(const Vec2f &planar, const TextureDisplacementLayer &layer, float aspect = 1.f,
+                         bool normalized_uv = false);
 
 // Applies a row-major 3x4 projective matrix (see TextureDisplacementLayer::view_project_matrix) to a
 // local-space point, writing the resulting texture uv. Returns false - and leaves `uv` untouched -
@@ -605,6 +627,31 @@ bool sample_layer_color(const DecodedHeightTexture &texture, const TextureDispla
                         const Vec3f &patch_center = Vec3f::Zero(), const Vec3f &patch_axis = Vec3f::UnitZ(),
                         const Vec2f *lscm_uv = nullptr);
 
+// Carries a per-facet paint annotation - the colour paint above all - from a mesh onto the mesh a
+// bake produced from it: every triangle of `dst_mesh` takes the state painted at its own foot point on
+// `src_mesh`, i.e. on the nearest point of the surface it was built from. Returned as one
+// EnforcerBlockerType per `dst_mesh` triangle, 0 (NONE) where that foot point carried no paint.
+//
+// ModelVolume::restore_painting() cannot do this. Its remap pairs a source paint triangle with a
+// target triangle that *overlaps* it to within 0.01 mm, which is right for a remesh or a
+// simplification - they leave the surface where it was - and useless after a bake, which moves the
+// surface by the relief depth. Measured on a model displaced by a tenth of its own size, that remap
+// kept a quarter of the colour and left the rest unpainted; this keeps essentially all of it, because
+// a foot point is exactly as far away as the relief is deep and the lookup does not care how far that
+// is.
+//
+// The source's *sub*-triangles are what is looked up, not its triangles, so the part of a triangle a
+// brush stroke only clipped a corner off keeps its own state rather than rounding to the whole
+// triangle's. No distance limit is needed or wanted: the nearest point on the source surface is where
+// a baked triangle came from however far the relief carried it, and an unpainted origin reports NONE
+// on its own.
+//
+// Returns an empty vector when `src_paint` is empty or either mesh is - the caller then has nothing to
+// carry, which is not the same as "carry nothing".
+std::vector<uint8_t> transfer_facet_paint(const indexed_triangle_set                    &src_mesh,
+                                         const TriangleSelector::TriangleSplittingData &src_paint,
+                                         const indexed_triangle_set                    &dst_mesh);
+
 // Area-weighted centroid and average normal of a layer's currently painted patch, in mesh-local
 // coordinates - the same measurements build_texture_displacement() uses to pick its dominant
 // projection axis. Used by the GUI to anchor the on-canvas "adjust texture placement" gizmo to
@@ -718,6 +765,55 @@ std::vector<Vec2f> compute_lscm_uvs(const indexed_triangle_set &patch, const Tex
 // Returns an empty vector if the patch has no triangles or the unwrap carries no source_face map.
 std::vector<Vec2f> compute_lscm_corner_uvs(const indexed_triangle_set &patch, const TextureDisplacementLayer &layer);
 
+// Three UVs per patch triangle, read out of the model's *own* UV channel - the
+// TextureProjectionMethod::ModelUV counterpart of compute_lscm_corner_uvs(), and laid out the same way
+// (corner k of triangle i at index 3i+k).
+//
+// `model_uv` is the volume's channel: three uvs per triangle of `base_mesh`, in its triangle order
+// (see ModelVolume::texture_uv). `patch_source` maps each patch triangle back to the base-mesh
+// triangle it was cut from - TriangleSelector::get_facets_strict()'s out_source. A patch triangle is
+// always a piece of exactly one base triangle, so each of its corners is interpolated barycentrically
+// inside that triangle's three uvs. That is what keeps a sub-triangle the brush split (whose corners
+// are not base-mesh vertices at all) on the texture, and what keeps the two sides of a UV seam
+// reading from their own island instead of from one averaged coordinate.
+//
+// Returns an empty vector when the channel does not match `base_mesh`, when `patch_source` does not
+// match `patch`, or when either is empty - the callers treat that as "no placement" and fall back,
+// exactly as they do for an unwrap that failed.
+std::vector<Vec2f> compute_model_corner_uvs(const indexed_triangle_set &base_mesh,
+                                            const std::vector<Vec2f>   &model_uv,
+                                            const indexed_triangle_set &patch,
+                                            const std::vector<int>     &patch_source);
+
+// One UV per patch vertex, collapsed from a per-corner array: the first corner to reach a vertex wins.
+// This is the per-corner/per-vertex split compute_lscm_uvs() documents, applied to any corner array -
+// displacement is per-vertex, so a vertex on a seam has to settle on one of its copies.
+// Returns an empty vector if `corner` is not three per patch triangle.
+std::vector<Vec2f> collapse_corner_uvs(const indexed_triangle_set &patch, const std::vector<Vec2f> &corner);
+
+// Resamples a per-corner UV channel from `src_mesh` onto `dst_mesh`, for when something replaces a
+// volume's mesh and the model's own UV channel has to come along: the colour import's painted mesh,
+// the gizmo's remesh/subdivide, the default bake pipeline's rebuild. Each destination corner takes the
+// uv of its closest point on the source surface, interpolated barycentrically inside the source
+// triangle it lands on - the same transfer LscmLookup does per point, run over every corner.
+//
+// Exact wherever the two meshes describe the same surface, which is the case for every caller here
+// (they subdivide, repair or displace it, they do not reshape it). A corner that lands nowhere near
+// the source - farther than `max_distance` (negative: no limit) - takes (0, 0).
+//
+// One thing a positional transfer cannot reproduce: a *seam* vertex, where one position carries a
+// different uv in each island that meets there. Position alone does not say which island a corner
+// belongs to, so corners on a seam all take whichever island the nearest-triangle lookup lands on,
+// and the texture can be off by up to one triangle in a band along the island borders. Doing better
+// needs a triangle-to-triangle correspondence, which only some of the stages that replace a mesh
+// have (a remesh has none at all).
+//
+// Returns an empty vector when `src_uv` is not three per source triangle, or when either mesh is empty.
+std::vector<Vec2f> transfer_uv_channel(const indexed_triangle_set &src_mesh,
+                                       const std::vector<Vec2f>   &src_uv,
+                                       const indexed_triangle_set &dst_mesh,
+                                       float                       max_distance = -1.f);
+
 // The TextureDisplacementLayer::lscm_uv_overrides key for one unwrapped vertex (an index into PatchUnwrap::uvs).
 inline int lscm_uv_override_key(int unwrapped_vertex) { return -(unwrapped_vertex + 1); }
 
@@ -776,6 +872,10 @@ using TextureDisplacementFacetsData = std::array<TriangleSelector::TriangleSplit
 //
 // `color`, when given, also reports which filament each triangle should print in - see
 // TextureColorRequest.
+//
+// `model_uv`, when given, is the volume's own UV channel (three uvs per base-mesh triangle, see
+// ModelVolume::texture_uv) and is what a TextureProjectionMethod::ModelUV layer maps through. A layer
+// set to ModelUV without it falls back to Triplanar.
 struct TextureColorRequest
 {
     // RGB -> palette index. Supplied by the GUI, which owns both the perceptual matching and the list
@@ -850,7 +950,9 @@ indexed_triangle_set build_texture_displacement(const indexed_triangle_set
                                                  // stage, already brought back into `base_mesh`'s frame.
                                                  BakeStageRecorder                           *debug = nullptr,
                                                  // When given, receives what the bake spent.
-                                                 TextureBakeStats                            *stats = nullptr);
+                                                 TextureBakeStats                            *stats = nullptr,
+                                                 // The model's own UV channel; see above.
+                                                 const std::vector<Vec2f>                    *model_uv = nullptr);
 
 // `volume`'s mesh coordinates -> world millimetres: its first instance's transform times its own.
 // The mesh is shared by every instance, so a multi-instance object can only be baked for one of
@@ -925,11 +1027,15 @@ ColorFieldSampler make_combined_color_sampler(const indexed_triangle_set
                                               const std::vector<TextureDisplacementLayer> &layers,
                                               const TextureDisplacementFacetsData         &facets_data,
                                               ColorQuantizeFn                              quantize,
-                                              ColorQuantizeFn                              quantize_pure = nullptr);
+                                              ColorQuantizeFn                              quantize_pure = nullptr,
+                                              // The model's own UV channel, for ModelUV layers; see
+                                              // build_texture_displacement().
+                                              const std::vector<Vec2f>                    *model_uv = nullptr);
 
 HeightFieldSampler make_combined_displacement_sampler(const indexed_triangle_set                  &base_mesh,
                                                       const std::vector<TextureDisplacementLayer> &layers,
-                                                      const TextureDisplacementFacetsData         &facets_data);
+                                                      const TextureDisplacementFacetsData         &facets_data,
+                                                      const std::vector<Vec2f>                    *model_uv = nullptr);
 
 // Uniformly subdivides `mesh` (every triangle recursively split into 4 via edge midpoints, using a
 // shared cache so a midpoint is computed once and reused by both triangles on either side of that
@@ -1134,6 +1240,10 @@ struct TextureDisplacementPrepareResult
 {
     indexed_triangle_set          mesh;
     TextureDisplacementFacetsData masks;
+    // The model's own UV channel, resampled onto `mesh` (see transfer_uv_channel()). Empty when the
+    // volume had none. Like the masks it is handed back rather than left to the caller to recompute,
+    // because only the run itself knows which stages replaced the mesh.
+    std::vector<Vec2f>            uv;
     // The remesh landed but no layer's paint survived being carried onto it. Nothing is committed:
     // everything downstream is driven by that paint, so baking on would bake a flat mesh.
     bool                          paint_lost = false;

src/libslic3r/TexturePainting.cpp

@@ -25,6 +25,7 @@
 #include "TextureToColor/ColorUtils.hpp"
 
 #include "Model.hpp"
+#include "TextureDisplacement.hpp"
 #include "TextureToColor/TriMesh.hpp"
 #include "TriangleMesh.hpp"
 #include "TriangleSelector.hpp"
@@ -554,9 +555,29 @@ bool apply_painted_mesh_to_volume(
                 << ", unit_scale=" << unit_scale << ")";
     }
 
+    // The model's own UV channel is indexed by triangle, and this replaces the triangles, so it has
+    // to be resampled onto them. Kept rather than dropped because it is the layout the model was
+    // authored against: a texture-displacement layer set to ModelUV maps through it, and the colour
+    // import is exactly the path a model that carries one arrives by.
+    const indexed_triangle_set old_its = volume.has_texture_uv() ? volume.mesh().its : indexed_triangle_set{};
+    const std::vector<Vec2f>   old_uv  = volume.texture_uv;
+
     volume.set_mesh(std::move(new_mesh));
     volume.calculate_convex_hull();
 
+    // Resampled here and not further down, while the two meshes still share a frame: the painted mesh
+    // was just shifted by the same source.mesh_offset the old one was centred by, so the only thing
+    // between them is whatever drift the repair introduced. The re-centering below translates the mesh
+    // again, and a translation is exactly what a closest-point transfer cannot see through - it would
+    // read every uv off a point that far away. A uv is a coordinate in the texture, so it does not care
+    // about either shift; it just has to be read before the second one happens.
+    if (!old_its.indices.empty())
+        volume.texture_uv = transfer_uv_channel(old_its, old_uv, volume.mesh().its);
+    else
+        // No channel to carry, so make sure none is left behind either: a leftover of the right length
+        // would read as a valid channel against a mesh it was never measured on.
+        volume.reset_texture_uv();
+
     // Re-center the replaced mesh so its bbox center sits at the origin,
     // matching what center_geometry_after_creation did for the original mesh.
     // CGAL repair / subdivision may shift the bbox center (drift); without

src/slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.cpp

@@ -1231,9 +1231,33 @@ indexed_triangle_set GLGizmoTextureDisplacement::patch_in_world(const indexed_tr
     return world;
 }
 
-std::vector<Vec2f> GLGizmoTextureDisplacement::compute_layer_vertex_uvs(const indexed_triangle_set     &local_patch,
-                                                                       const TextureDisplacementLayer &layer) const
+std::vector<Vec2f> GLGizmoTextureDisplacement::model_uv_corners(const indexed_triangle_set     &local_patch,
+                                                                const TextureDisplacementLayer &layer,
+                                                                const std::vector<int>         *patch_source) const
 {
+    const ModelVolume *mv = texture_volume();
+    if (layer.projection_method != TextureProjectionMethod::ModelUV || patch_source == nullptr || mv == nullptr ||
+        !mv->has_texture_uv())
+        return {};
+    // The channel is indexed against the volume's *local* mesh, and a uv is a coordinate in the
+    // texture rather than on the model, so unlike every projected method this never goes through
+    // patch_in_world() - there is no millimetre measurement here for a world scale to change.
+    std::vector<Vec2f> uv = compute_model_corner_uvs(mv->mesh().its, mv->texture_uv, local_patch, *patch_source);
+    for (Vec2f &p : uv)
+        // Already a uv: no tiling scale, no aspect correction - see apply_uv_transform().
+        p = apply_uv_transform(p, layer, 1.f, /* normalized_uv */ true);
+    return uv;
+}
+
+std::vector<Vec2f> GLGizmoTextureDisplacement::compute_layer_vertex_uvs(const indexed_triangle_set     &local_patch,
+                                                                       const TextureDisplacementLayer &layer,
+                                                                       const std::vector<int>         *patch_source) const
+{
+    if (layer.projection_method == TextureProjectionMethod::ModelUV)
+        // Collapsed to one per vertex the same way the unwrap is - a vertex has one position, so the
+        // displacement can only read one height for it.
+        return collapse_corner_uvs(local_patch, model_uv_corners(local_patch, layer, patch_source));
+
     // The bake maps the texture in world millimetres, so "Tile size (mm)" means the same thing on a
     // scaled instance as it does on an untouched one (see build_texture_displacement()). Everything
     // that has to agree with the bake - the fast preview's uvs, the checker/distortion overlays -
@@ -1293,8 +1317,11 @@ int GLGizmoTextureDisplacement::layer_projection_frame(const indexed_triangle_se
 }
 
 std::vector<Vec2f> GLGizmoTextureDisplacement::compute_layer_corner_uvs(const indexed_triangle_set     &local_patch,
-                                                                        const TextureDisplacementLayer &layer) const
+                                                                        const TextureDisplacementLayer &layer,
+                                                                        const std::vector<int>         *patch_source) const
 {
+    if (layer.projection_method == TextureProjectionMethod::ModelUV)
+        return model_uv_corners(local_patch, layer, patch_source);
     if (layer.projection_method == TextureProjectionMethod::LSCM) {
         const indexed_triangle_set patch  = patch_in_world(local_patch);
         const float                aspect = layer_texture_aspect(layer);
@@ -1304,7 +1331,7 @@ std::vector<Vec2f> GLGizmoTextureDisplacement::compute_layer_corner_uvs(const in
         return uv;
     }
     // Single-valued per point: fan the per-vertex result out over the corners.
-    const std::vector<Vec2f> per_vertex = compute_layer_vertex_uvs(local_patch, layer);
+    const std::vector<Vec2f> per_vertex = compute_layer_vertex_uvs(local_patch, layer, patch_source);
     if (per_vertex.size() != local_patch.vertices.size())
         return {};
     std::vector<Vec2f> corner(local_patch.indices.size() * 3);
@@ -1325,7 +1352,11 @@ void GLGizmoTextureDisplacement::rebuild_shaded_preview_mesh()
     // Uses the *live* selector (not the flushed model facet data), so this reflects an in-progress
     // stroke immediately rather than only once it ends - the point of this preview mode is to be
     // the fast, no-CPU-meshing one.
-    const indexed_triangle_set patch = m_triangle_selectors[0]->get_facets_strict(EnforcerBlockerType::ENFORCER);
+    // The source map comes along because a ModelUV layer's uvs are read per base-mesh triangle; every
+    // other mapping ignores it.
+    std::vector<int>           patch_source;
+    const indexed_triangle_set patch =
+        m_triangle_selectors[0]->get_facets_strict(EnforcerBlockerType::ENFORCER, &patch_source);
     if (patch.indices.empty())
         return;
 
@@ -1349,7 +1380,8 @@ void GLGizmoTextureDisplacement::rebuild_shaded_preview_mesh()
     // Per *corner*, not per vertex: the mesh below is flat (unshared) anyway, so each triangle can
     // carry its own chart's UVs - see compute_layer_corner_uvs().
     const TextureDisplacementLayer *active = active_layer();
-    std::vector<Vec2f> corner_uv = active != nullptr ? compute_layer_corner_uvs(patch, *active) : std::vector<Vec2f>{};
+    std::vector<Vec2f> corner_uv = active != nullptr ? compute_layer_corner_uvs(patch, *active, &patch_source) :
+                                                       std::vector<Vec2f>{};
     m_shaded_preview_uses_vertex_uv = corner_uv.size() == patch.indices.size() * 3;
     if (!m_shaded_preview_uses_vertex_uv)
         corner_uv.clear();
@@ -2006,7 +2038,9 @@ void GLGizmoTextureDisplacement::rebuild_uvcheck_mesh()
     const ModelVolume *mv = texture_volume();
     if (mv == nullptr || m_triangle_selectors.empty())
         return;
-    const indexed_triangle_set patch = m_triangle_selectors[0]->get_facets_strict(EnforcerBlockerType::ENFORCER);
+    std::vector<int>           patch_source; // only ModelUV reads it; see compute_layer_vertex_uvs()
+    const indexed_triangle_set patch =
+        m_triangle_selectors[0]->get_facets_strict(EnforcerBlockerType::ENFORCER, &patch_source);
     if (patch.indices.empty())
         return;
     // Everything below works in the *patch's* vertex space, not the mesh's. Those agree only until a
@@ -2020,7 +2054,7 @@ void GLGizmoTextureDisplacement::rebuild_uvcheck_mesh()
 
     // The checker samples wherever the projection puts it; the projections the shader can't
     // reconstruct (LSCM, ViewProjected) get a precomputed per-vertex uv, the rest project in-shader.
-    std::vector<Vec2f> uv       = compute_layer_vertex_uvs(patch, *layer);
+    std::vector<Vec2f> uv       = compute_layer_vertex_uvs(patch, *layer, &patch_source);
     const bool         have_uvs = uv.size() == patch.vertices.size();
     m_uvcheck_uses_vertex_uv    = have_uvs;
     m_uvcheck_projection_mode   = have_uvs ? 0 :
@@ -2029,7 +2063,7 @@ void GLGizmoTextureDisplacement::rebuild_uvcheck_mesh()
     // different uv in each island it borders, so the shared-vertex form drew one triangle per face from
     // a neighbouring island's placement. Only the *drawing* needs this; the distortion metric below is
     // a per-vertex average by construction and keeps using `uv`.
-    const std::vector<Vec2f> corner_uv = compute_layer_corner_uvs(patch, *layer);
+    const std::vector<Vec2f> corner_uv = compute_layer_corner_uvs(patch, *layer, &patch_source);
     const bool               have_corner_uvs = have_uvs && corner_uv.size() == patch.indices.size() * 3;
 
     // Per-vertex area distortion in [0,1] (0.5 == ideal), only when both requested and possible.
@@ -2305,6 +2339,8 @@ void GLGizmoTextureDisplacement::queue_preview_job()
     if (input.options.pipeline_v2)
         input.options.v2_max_triangles_k = 0;
     input.volume_to_world = texture_displacement_volume_to_world(*mv);
+    if (mv->has_texture_uv())
+        input.model_uv = mv->texture_uv;
     for (int i = 0; i < int(TEXTURE_DISPLACEMENT_MAX_LAYERS); ++i)
         input.facets_data[size_t(i)] = mv->texture_displacement_facet(i).get_data();
     // Captured here rather than read in the handler: the palette is main-thread state, and the preview
@@ -3714,7 +3750,8 @@ void GLGizmoTextureDisplacement::ensure_panel_icons()
         // Paint / Erase, brush cursor shape, mapping, tiling, placement and the dock toggle.
         "texture_displacement_add.svg", "texture_displacement_add_negative.svg", "circle_paint.svg",
         "menu_obj_sphere.svg", "menu_obj_cube.svg", "menu_obj_cylinder.svg", "texture_displacement_map_unwrap.svg",
-        "texture_displacement_map_view.svg", "texture_displacement_tile_repeat.svg", "menu_mirror_x.svg",
+        "texture_displacement_map_view.svg", "texture_displacement_map_model_uv.svg",
+        "texture_displacement_tile_repeat.svg", "menu_mirror_x.svg",
         "texture_displacement_adjust.svg", "canvas_drag.svg", "texture_displacement_move_up.svg",
         "texture_displacement_move_down.svg", "texture_displacement_drag.svg",
         "texture_displacement_select_all.svg", "texture_displacement_erase_all.svg",
@@ -3763,6 +3800,11 @@ void GLGizmoTextureDisplacement::add_texture_layer()
 
     TextureDisplacementLayer layer;
     layer.slot = free_slot;
+    // A model that came with a UV channel is mapped by it. Re-unwrapping such a model throws away the
+    // layout it was authored against and can only be worse than using it, so the default follows the
+    // model rather than the other way round; the Mapping row still offers every other method.
+    if (mv->has_texture_uv())
+        layer.projection_method = TextureProjectionMethod::ModelUV;
     // Start the layer off on the first library texture rather than on nothing at all: a textureless
     // layer looks broken (painting on it appears to do nothing, because there is no height map to
     // displace by). The user swaps it for another from the layer's own picker.
@@ -4908,7 +4950,8 @@ ColorQuantizeFn GLGizmoTextureDisplacement::make_palette_quantizer(const std::ve
 
 TextureDisplacementPrepareResult GLGizmoTextureDisplacement::prepare_mesh(
     const indexed_triangle_set &base, const TextureDisplacementFacetsData &masks,
-    const std::vector<TextureDisplacementLayer> &layers, const TextureDisplacementPrepareParams &params,
+    const std::vector<TextureDisplacementLayer> &layers, const std::vector<Vec2f> &model_uv,
+    const TextureDisplacementPrepareParams &params,
     const TextureColorSettings &color_settings, const DisplacementProgressFn &progress,
     BakeStageRecorder *debug)
 {
@@ -4935,6 +4978,13 @@ TextureDisplacementPrepareResult GLGizmoTextureDisplacement::prepare_mesh(
     bool                          had_paint = false;
     for (const TriangleSelector::TriangleSplittingData &m : masks)
         had_paint = had_paint || TriangleSelector::has_facets(m, EnforcerBlockerType::ENFORCER);
+    // The model's own UV channel for whatever `mesh` currently is. Every stage below keeps the input's
+    // surface - it evens out, refines or cuts the triangles lying on it - so each corner reads its uv
+    // straight off `base`'s channel at its own position instead of being carried stage by stage, which
+    // would compound each stage's approximation. Untouched while nothing has replaced the mesh yet.
+    const auto uv_now = [&]() {
+        return (model_uv.empty() || !changed) ? model_uv : transfer_uv_channel(base, model_uv, mesh.its);
+    };
     if (!report(1))
         return out;
 
@@ -4981,9 +5031,11 @@ TextureDisplacementPrepareResult GLGizmoTextureDisplacement::prepare_mesh(
             // Feature-adaptive: sample the combined displacement so refinement follows texture
             // curvature. A null sampler (only LSCM layers, or nothing decodable) falls back to the
             // length baseline alone.
+            const std::vector<Vec2f> uv_stage = uv_now();
+            const std::vector<Vec2f> *uv_ptr  = uv_stage.empty() ? nullptr : &uv_stage;
             HeightFieldSampler sampler;
             if (params.subdiv_feature)
-                sampler = make_combined_displacement_sampler(mesh.its, layers, current);
+                sampler = make_combined_displacement_sampler(mesh.its, layers, current, uv_ptr);
             // Colour boundaries need triangles of their own - the chord test cannot see them, since
             // the height field is perfectly smooth across a change of filament.
             ColorFieldSampler color;
@@ -4991,7 +5043,7 @@ TextureDisplacementPrepareResult GLGizmoTextureDisplacement::prepare_mesh(
             // boundaries are refined where the bake will actually change filament.
             if (params.subdiv_color_edge_mm > 0.f && !color_settings.empty())
                 color = make_combined_color_sampler(mesh.its, layers, current, make_palette_quantizer(color_settings.palette),
-                                                    make_palette_quantizer(color_settings.palette_pure));
+                                                    make_palette_quantizer(color_settings.palette_pure), uv_ptr);
             // The refinement follows perceived colour: a mix is one colour here, however the slicer
             // interleaves its filaments layer by layer.
             // "Min edge" is a feature-mode control (it is the floor the curvature test refines down
@@ -5050,8 +5102,10 @@ TextureDisplacementPrepareResult GLGizmoTextureDisplacement::prepare_mesh(
                 std::vector<uint8_t> cut_region(mesh.its.indices.size(), 0);
                 for (size_t i = 0; i < cut_region.size() && i < source.size(); ++i)
                     cut_region[i] = (region[size_t(source[i])] & REFINE_PAINTED) ? 1 : 0;
+                const std::vector<Vec2f> uv_refined = uv_now();
                 const HeightFieldSampler refined_sampler =
-                    make_combined_displacement_sampler(mesh.its, layers, current);
+                    make_combined_displacement_sampler(mesh.its, layers, current,
+                                                        uv_refined.empty() ? nullptr : &uv_refined);
                 std::vector<int>     cut_source;
                 size_t               cuts = 0;
                 indexed_triangle_set cut = cut_mesh_at_steps(mesh.its, cut_region, refined_sampler,
@@ -5075,6 +5129,7 @@ TextureDisplacementPrepareResult GLGizmoTextureDisplacement::prepare_mesh(
     if (!report(100) || !changed)
         return out; // an empty result: nothing to commit, which is not a failure
 
+    out.uv    = uv_now();
     out.mesh  = std::move(mesh.its);
     out.masks = std::move(current);
     return out;
@@ -5726,6 +5781,8 @@ void GLGizmoTextureDisplacement::queue_prepare(const TextureDisplacementPrepareP
     input.layers        = mv->texture_displacement_layers;
     input.params        = params;
     input.snapshot_name = snapshot_name;
+    if (mv->has_texture_uv())
+        input.model_uv = mv->texture_uv;
     if (params.subdiv_color_edge_mm > 0.f)
         input.color = color_settings_for(*mv);
 
@@ -6584,10 +6641,19 @@ void GLGizmoTextureDisplacement::on_render_input_window(float x, float y, float
             m_preview_params_dirty |= float_row("##depth", _L("Depth"), &layer.depth_mm, 0.01f, 10.f, "%.3f mm", true, card_pad,
                                                 _L("Height of the relief: how far white in the texture lifts the surface, in "
                                                    "millimetres. Black does not move it at all, unless Midlevel says otherwise."));
+            // Mapped by the model's own UV layout there is no millimetre scale to set: the layout is the
+            // placement (see TextureProjectionMethod::ModelUV), so the slider would move nothing. The row
+            // states why rather than hiding: float_row()'s tooltip is shown for a disabled slider too.
+            const bool tile_size_na = layer.projection_method == TextureProjectionMethod::ModelUV;
+            m_imgui->disabled_begin(tile_size_na);
             m_preview_params_dirty |= float_row("##tile_size", _L("Tile size"), &layer.tiling_scale, 0.2f, 200.f, "%.2f mm", true, card_pad,
-                                                _L("How wide one copy of the texture is on the model. Smaller repeats the pattern "
-                                                   "more often and makes its detail finer; with Tile off, this is the size of the "
-                                                   "single copy."));
+                                                tile_size_na ?
+                                                    _L("Mapped by the model's own UV layout, which is already the placement - one "
+                                                       "copy of the texture covers it exactly, so there is no tile size to set.") :
+                                                    _L("How wide one copy of the texture is on the model. Smaller repeats the pattern "
+                                                       "more often and makes its detail finer; with Tile off, this is the size of the "
+                                                       "single copy."));
+            m_imgui->disabled_end();
             m_preview_params_dirty |= float_row("##rotation", _L("Rotation"), &layer.rotation_deg, 0.f, 360.f, "%.0f°", false, card_pad,
                                                 _L("Turns the texture on the surface, in degrees - for lining a pattern up with an "
                                                    "edge of the model."));
@@ -6693,7 +6759,7 @@ void GLGizmoTextureDisplacement::on_render_input_window(float x, float y, float
                     }
                 }
 
-                // Mapping, as five icons in TextureProjectionMethod's own order.
+                // Mapping, as one icon per TextureProjectionMethod, in the enum's own order.
                 {
                     const float x0 = ImGui::GetCursorPosX();
                     ImGui::AlignTextToFramePadding();
@@ -6721,13 +6787,26 @@ void GLGizmoTextureDisplacement::on_render_input_window(float x, float y, float
                         { "texture_displacement_map_view.svg", _L("From view"),
                           _L("From view - projects straight onto the painted area from where you are looking, like a slide "
                              "projector.") },
+                        { "texture_displacement_map_model_uv.svg", _L("Model UV"),
+                          _L("Model UV - uses the texture coordinates the model itself came with, so a texture made for "
+                             "that model lands exactly where it was painted. Nothing is unwrapped, and Tile size does "
+                             "not apply: the model's own layout is the placement.") },
                     };
+                    // Only a volume that actually carries a UV channel can be mapped by it. Shown
+                    // greyed out rather than hidden, so the reason is visible instead of the control
+                    // simply being missing.
+                    const wxString model_uv_na =
+                        (mv != nullptr && mv->has_texture_uv()) ?
+                            wxString() :
+                            _L("This model carries no texture coordinates of its own. Only a model imported with a UV "
+                               "channel - a textured OBJ, say - can be mapped by them.");
                     for (int mi = 0; mi < int(IM_ARRAYSIZE(mappings)); ++mi) {
                         if (mi > 0)
                             ImGui::SameLine(0.f, gap_s);
                         const auto method = static_cast<TextureProjectionMethod>(mi);
+                        const wxString na = (method == TextureProjectionMethod::ModelUV) ? model_uv_na : wxString();
                         if (icon_toggle(830 + mi, mappings[mi].file, layer.projection_method == method, icon_sm, mappings[mi].label,
-                                        mappings[mi].tip) &&
+                                        mappings[mi].tip, na) &&
                             layer.projection_method != method) {
                             // Capture the current view the moment "From view" is chosen, so it does something
                             // sensible immediately rather than projecting from a stale/default direction.
@@ -6857,18 +6936,33 @@ void GLGizmoTextureDisplacement::on_render_input_window(float x, float y, float
                     ImGui::SameLine();
                     ImGui::SetCursorPosX(std::max(ImGui::GetCursorPosX(), x0 + label_w));
                     const std::string blend_add      = _u8L_CONTEXT("Add", "Texture Displacement");
+                    const std::string blend_override  = _u8L_CONTEXT("Override", "Texture Displacement");
                     const std::string blend_subtract = _u8L_CONTEXT("Subtract", "Texture Displacement");
                     const std::string blend_multiply = _u8L_CONTEXT("Multiply", "Texture Displacement");
                     const std::string blend_divide   = _u8L("Divide");
-                    const char *blend_items[] = { blend_add.c_str(), blend_subtract.c_str(), blend_multiply.c_str(), blend_divide.c_str() };
+                    const char *blend_items[] = { blend_add.c_str(), blend_override.c_str(), blend_subtract.c_str(),
+                                                  blend_multiply.c_str(), blend_divide.c_str() };
+                    // The combo's order is the panel's, not the enum's - Override reads as the sibling of Add and
+                    // belongs next to it, while TextureBlendMode's numbering is fixed by what projects already
+                    // store. Hence the explicit mapping rather than a cast of the index.
+                    static const TextureBlendMode blend_modes[] = { TextureBlendMode::Add, TextureBlendMode::Override,
+                                                                    TextureBlendMode::Subtract, TextureBlendMode::Multiply,
+                                                                    TextureBlendMode::Divide };
+                    static_assert(IM_ARRAYSIZE(blend_items) == IM_ARRAYSIZE(blend_modes),
+                                  "every Blend entry needs its mode");
                     // The first layer has nothing before it to combine with - build_texture_displacement() makes it
                     // add regardless - so its combo shows Add and cannot be changed.
                     const bool base_layer = li == 0;
-                    int        blend_mode = base_layer ? 0 : static_cast<int>(layer.blend_mode);
+                    int        blend_mode = 0;
+                    if (!base_layer)
+                        for (int bi = 0; bi < int(IM_ARRAYSIZE(blend_modes)); ++bi)
+                            if (blend_modes[bi] == layer.blend_mode)
+                                blend_mode = bi;
                     ImGui::SetNextItemWidth(-(card_pad + gap_s + icon_sm));
                     m_imgui->disabled_begin(base_layer);
-                    if (scoped_combo("##blend_mode", &blend_mode, blend_items, IM_ARRAYSIZE(blend_items)) && !base_layer) {
-                        layer.blend_mode       = static_cast<TextureBlendMode>(blend_mode);
+                    if (scoped_combo("##blend_mode", &blend_mode, blend_items, IM_ARRAYSIZE(blend_items)) && !base_layer &&
+                        blend_mode >= 0 && blend_mode < int(IM_ARRAYSIZE(blend_modes))) {
+                        layer.blend_mode       = blend_modes[blend_mode];
                         m_preview_params_dirty = true;
                     }
                     m_imgui->disabled_end();
@@ -6876,10 +6970,11 @@ void GLGizmoTextureDisplacement::on_render_input_window(float x, float y, float
                         m_imgui->tooltip(base_layer ?
                                              _u8L("The base layer has nothing beneath it to combine with, so it always adds.") :
                                              _u8L("What this layer does where it overlaps the ones below. Add and "
-                                                  "Subtract pile relief on or carve it away. Multiply and Divide scale "
-                                                  "what is underneath, which turns this layer into a mask over it - "
-                                                  "there, Depth acts as a strength, and 1 mm leaves white areas "
-                                                  "untouched."),
+                                                  "Subtract pile relief on or carve it away. Override replaces them, "
+                                                  "so only this layer's relief is left where it is painted. Multiply "
+                                                  "and Divide scale what is underneath, which turns this layer into a "
+                                                  "mask over it - there, Depth acts as a strength, and 1 mm leaves "
+                                                  "white areas untouched."),
                                          wrap_w);
 
                     ImGui::SameLine(0.f, gap_s);

src/slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.hpp

@@ -61,6 +61,12 @@ public:
     static TextureDisplacementPrepareResult prepare_mesh(const indexed_triangle_set                  &base,
                                                          const TextureDisplacementFacetsData         &masks,
                                                          const std::vector<TextureDisplacementLayer> &layers,
+                                                         // `base`'s own UV channel, when it has one
+                                                         // (ModelVolume::texture_uv). Resampled onto
+                                                         // the result and reported back in it, and fed
+                                                         // to the samplers so a ModelUV layer drives
+                                                         // the refinement through its own layout.
+                                                         const std::vector<Vec2f>                    &model_uv,
                                                          const TextureDisplacementPrepareParams      &params,
                                                          const TextureColorSettings                  &color_settings,
                                                          const DisplacementProgressFn                &progress,
@@ -827,18 +833,31 @@ private:
     // Merges chart `b`'s join group into chart `a`'s (materialising `groups` to `chart_count` first).
     static void      join_island_groups(std::vector<int> &groups, int a, int b, int chart_count);
     // Final per-vertex texture uv for the projections the shader can't reconstruct itself - LSCM (an
-    // unwrap) and ViewProjected (a projector plane the shader doesn't know). One entry per patch/base
-    // vertex, already through apply_uv_transform(). Empty for Triplanar/Cylindrical/Spherical, which
-    // the shader projects on its own. Shared by the shaded preview and the UV-check overlay.
+    // unwrap), ModelUV (the model's own channel) and ViewProjected (a projector plane the shader
+    // doesn't know). One entry per patch/base vertex, already through apply_uv_transform(). Empty for
+    // Triplanar/Cylindrical/Spherical, which the shader projects on its own. Shared by the shaded
+    // preview and the UV-check overlay.
+    //
+    // `patch_source` maps each patch triangle back to its base-mesh triangle
+    // (TriangleSelector::get_facets_strict()'s out_source). Only ModelUV needs it - its channel is per
+    // base triangle - and without it a ModelUV layer reports no uvs and the preview falls back.
     std::vector<Vec2f> compute_layer_vertex_uvs(const indexed_triangle_set &patch,
-                                                const TextureDisplacementLayer &layer) const;
+                                                const TextureDisplacementLayer &layer,
+                                                const std::vector<int> *patch_source = nullptr) const;
     // The same, but three UVs per patch triangle (corner 0..2 of triangle i at 3i..3i+2). This is what
     // the flat, unshared-vertex preview meshes actually want: under LSCM a seam vertex has a different
     // UV in each island it borders, so collapsing to one per vertex handed a triangle at an unjoined
     // seam its neighbour's placement - one visibly skewed triangle per face. Every other projection is
     // single-valued per point, so there a corner's UV is just its vertex's.
     std::vector<Vec2f> compute_layer_corner_uvs(const indexed_triangle_set &patch,
-                                                const TextureDisplacementLayer &layer) const;
+                                                const TextureDisplacementLayer &layer,
+                                                const std::vector<int> *patch_source = nullptr) const;
+    // The model's own channel over `local_patch`, already through apply_uv_transform() - three per
+    // patch triangle. Empty when the volume has no channel, or when `patch_source` is missing. Shared
+    // by the two functions above so the per-vertex and per-corner forms can never disagree.
+    std::vector<Vec2f> model_uv_corners(const indexed_triangle_set &local_patch,
+                                        const TextureDisplacementLayer &layer,
+                                        const std::vector<int> *patch_source) const;
     // The in-shader projection for `layer` (0 Triplanar, 1 Cylindrical, 2 Spherical) plus, for the two
     // wrapping ones, the patch centroid and cylinder axis they wrap around - in the texture frame the
     // shaders project in. Taken from the bake's own texture_displacement_patch_frame(), so a preview

src/slic3r/GUI/Jobs/TextureDisplacementBakeJob.cpp

@@ -8,6 +8,7 @@
 #include "libslic3r/TextureDisplacement.hpp"
 #include <exception>
 #include <cstddef>
+#include <cstdint>
 #include "slic3r/GUI/Jobs/Worker.hpp"
 #include <memory>
 
@@ -26,6 +27,7 @@
 #include "slic3r/Utils/UndoRedo.hpp"
 #include "libslic3r/ObjectID.hpp"
 #include "libslic3r/TriangleMesh.hpp"
+#include <array>
 
 namespace Slic3r::GUI {
 
@@ -74,7 +76,8 @@ void TextureDisplacementBakeJob::process(Ctl &ctl)
             }
             return true;
         },
-        color, m_input.volume_to_world, nullptr, &m_stats));
+        color, m_input.volume_to_world, nullptr, &m_stats,
+        m_input.model_uv.empty() ? nullptr : &m_input.model_uv));
 
     // Always finish at 100: this is what closes the notification. Reported even on cancel, where
     // build_texture_displacement() returns an empty mesh and finalize() commits nothing.
@@ -126,16 +129,71 @@ void TextureDisplacementBakeJob::finalize(bool canceled, std::exception_ptr &ept
                                                      { mix.a_percent(), 100 - mix.a_percent() });
             });
 
+        // Whether the bake replaced the triangles or only moved the vertices it was given. The classic
+        // path is topology-preserving by contract, so every per-triangle annotation on the volume still
+        // means what it did; the default pipeline refines, cuts and simplifies, so none of them do.
+        // Both are checked: the flag is what the bake promised, the count is what it delivered. Only
+        // the triangles matter - the annotations are per facet and reference no vertex index - so a
+        // vertex count that changed on its own (the bake compacts unreferenced vertices away) is not a
+        // reason to remap anything.
+        const bool topology_changed = m_input.options.pipeline_v2 ||
+                                      m_result.its.indices.size() != m_input.base_mesh.indices.size();
+
+        // Carry the volume's existing paint - the model's own colours above all - across that rebuild.
+        // Without it a bake left the mmu_segmentation_facets bitstream pointing at a triangle set that
+        // no longer existed, which is what scrambled the colours of an imported coloured model even
+        // with colour switched off on every layer and nothing in the bake that was meant to touch them.
+        //
+        // transfer_facet_paint() rather than ModelVolume::restore_painting(): the latter's remap wants
+        // a target triangle overlapping the source paint to within 0.01 mm, which the baked surface is
+        // nowhere near - it has moved by the relief depth. Nothing to do when the topology is intact:
+        // triangle i is still triangle i, so the paint is already exactly right and carrying it at all
+        // could only lose detail.
+        std::array<TriangleSelector::TriangleSplittingData, 4> before;
+        if (topology_changed) {
+            before[0] = volume->supported_facets.get_data();
+            before[1] = volume->seam_facets.get_data();
+            before[2] = volume->mmu_segmentation_facets.get_data();
+            before[3] = volume->fuzzy_skin_facets.get_data();
+        }
+
         volume->set_mesh(std::move(m_result));
         volume->set_new_unique_id();
         volume->calculate_convex_hull();
 
+        if (topology_changed) {
+            const std::array<FacetsAnnotation *, 4> target{ &volume->supported_facets, &volume->seam_facets,
+                                                            &volume->mmu_segmentation_facets,
+                                                            &volume->fuzzy_skin_facets };
+            for (size_t ch = 0; ch < target.size(); ++ch) {
+                const std::vector<uint8_t> state =
+                    transfer_facet_paint(m_input.base_mesh, before[ch], volume->mesh().its);
+                // Nothing was painted in this channel, or there is nothing to paint it onto: either
+                // way the annotation must not keep describing the mesh that is gone.
+                if (state.empty()) {
+                    target[ch]->reset();
+                    continue;
+                }
+                TriangleSelector selector(volume->mesh());
+                bool             any = false;
+                for (size_t i = 0; i < state.size(); ++i)
+                    if (state[i] != 0) {
+                        selector.set_facet(int(i), EnforcerBlockerType(state[i]));
+                        any = true;
+                    }
+                if (any)
+                    target[ch]->set(selector);
+                else
+                    target[ch]->reset();
+            }
+        }
+
         // Colour lands in mmu_segmentation_facets, merged *over* whatever is already painted there
         // rather than replacing it: a triangle the texture does not colour keeps its existing filament,
         // and one the user never painted at all stays at NONE, which already means "the volume's own
         // filament". That is what confines the effect to the painted area without having to invent a
-        // colour for everything outside it. Safe to index straight onto the new mesh - the bake is
-        // topology-preserving, so triangle i is still triangle i.
+        // colour for everything outside it. What was already painted is whatever the transfer above
+        // left in the annotation, so this bake's colour lands on top of it rather than instead of it.
         if (!m_triangle_color.empty() && m_triangle_color.size() == volume->mesh().its.indices.size()) {
             TriangleSelector selector(volume->mesh());
             const TriangleSelector::TriangleSplittingData &existing = volume->mmu_segmentation_facets.get_data();
@@ -159,7 +217,6 @@ void TextureDisplacementBakeJob::finalize(bool canceled, std::exception_ptr &ept
         // pipeline, which rebuilds and then simplifies the mesh. A mask left behind against the old
         // topology is exactly what makes the gizmo reload an empty selector over a non-empty mask and
         // then erase it on the next flush - see GLGizmoTextureDisplacement::update_from_model_object().
-        const bool topology_changed = m_input.base_mesh.indices.size() != volume->mesh().its.indices.size();
         for (int slot = 0; slot < int(TEXTURE_DISPLACEMENT_MAX_LAYERS); ++slot) {
             const auto it = std::find_if(m_input.layers.begin(), m_input.layers.end(),
                                          [slot](const TextureDisplacementLayer &l) { return l.slot == slot; });
@@ -167,6 +224,14 @@ void TextureDisplacementBakeJob::finalize(bool canceled, std::exception_ptr &ept
                 volume->texture_displacement_facet(slot).reset();
         }
 
+        // The model's own UV channel. Resampled onto a rebuilt mesh from the surface the bake started
+        // from: a displaced vertex came from its foot point on that surface, which is the position its
+        // uv belongs to - so no distance limit, the relief depth is exactly how far it has moved.
+        if (!m_input.model_uv.empty())
+            volume->texture_uv = topology_changed ? transfer_uv_channel(m_input.base_mesh, m_input.model_uv,
+                                                                        volume->mesh().its) :
+                                                     m_input.model_uv;
+
         ModelObject *object = volume->get_object();
         if (object == nullptr)
             return;
@@ -219,6 +284,8 @@ void queue_texture_displacement_bake(const ModelVolume &volume, const TextureCol
     input.take_snapshot = take_snapshot;
     input.volume_id  = volume.id();
     input.base_mesh  = volume.mesh().its;
+    if (volume.has_texture_uv())
+        input.model_uv = volume.texture_uv;
     input.layers     = volume.texture_displacement_layers;
     input.options    = volume.texture_displacement_options;
     input.volume_to_world = texture_displacement_volume_to_world(volume);

src/slic3r/GUI/Jobs/TextureDisplacementBakeJob.hpp

@@ -34,6 +34,9 @@ struct TextureDisplacementBakeInput
     // writes the volume's mmu_segmentation_facets - the same per-triangle filament assignment the MMU
     // paint gizmo writes - alongside the displaced geometry.
     TextureColorSettings                  color;
+    // The volume's own UV channel (ModelVolume::texture_uv), empty when it has none: what a ModelUV
+    // layer maps through, and what the commit puts back on the baked mesh.
+    std::vector<Vec2f>                    model_uv;
     // Whether this job pushes its own undo step when it commits. False when the caller has already
     // taken one that is meant to cover the displacement as well - Standard mode's Bake, which remeshes
     // and subdivides first and has to undo as a single action.

src/slic3r/GUI/Jobs/TextureDisplacementDebugJob.cpp

@@ -17,6 +17,7 @@
 #include "slic3r/GUI/Jobs/Worker.hpp"
 #include <memory>
 #include "libslic3r/ObjectID.hpp"
+#include "libslic3r/Point.hpp"
 #include "libslic3r/TriangleMesh.hpp"
 
 namespace Slic3r::GUI {
@@ -52,13 +53,14 @@ void TextureDisplacementDebugJob::process(Ctl &ctl)
 
     indexed_triangle_set          mesh  = m_input.base_mesh;
     TextureDisplacementFacetsData masks = m_input.facets_data;
+    std::vector<Vec2f>            uv    = m_input.model_uv;
 
     // The default pipeline can only move vertices the mesh already has, so its recipe starts with the
     // preparation - which is where "first it remeshes, then it refines" actually happens. The
     // experimental pipeline refines as part of the bake and has nothing to prepare.
     if (m_input.run_prepare && !m_input.options.pipeline_v2) {
         const TextureDisplacementPrepareResult prepared =
-            GLGizmoTextureDisplacement::prepare_mesh(mesh, masks, m_input.layers, m_input.prepare_params,
+            GLGizmoTextureDisplacement::prepare_mesh(mesh, masks, m_input.layers, uv, m_input.prepare_params,
                                                      m_input.color,
                                                      // Preparation is roughly half the run; the bake
                                                      // takes the progress bar from there.
@@ -77,12 +79,14 @@ void TextureDisplacementDebugJob::process(Ctl &ctl)
         if (!prepared.mesh.indices.empty()) {
             mesh  = prepared.mesh;
             masks = prepared.masks;
+            uv    = prepared.uv; // resampled onto the prepared mesh, as the prepare commit stores it
         }
     }
 
     build_texture_displacement(mesh, m_input.layers, masks, m_input.options,
                                [&report](int pct) { return report(50 + pct / 2); },
-                               /* color */ nullptr, m_input.volume_to_world, &recorder);
+                               /* color */ nullptr, m_input.volume_to_world, &recorder, /* stats */ nullptr,
+                               uv.empty() ? nullptr : &uv);
 
     m_stages = recorder.take();
     ctl.update_status(100, status); // always finish at 100: this is what closes the notification
@@ -115,6 +119,8 @@ void queue_texture_displacement_debug(const ModelVolume &volume, const TextureCo
     input.check_topology  = check_topology;
     input.color           = color;
     input.volume_to_world = texture_displacement_volume_to_world(volume);
+    if (volume.has_texture_uv())
+        input.model_uv = volume.texture_uv;
     for (int i = 0; i < int(TEXTURE_DISPLACEMENT_MAX_LAYERS); ++i)
         input.facets_data[size_t(i)] = volume.texture_displacement_facet(i).get_data();
 

src/slic3r/GUI/Jobs/TextureDisplacementDebugJob.hpp

@@ -47,6 +47,9 @@ struct TextureDisplacementDebugInput
     // Only the quantizer is used, and only by the refinement's colour criterion - the debug run
     // produces no colour of its own.
     TextureColorSettings                  color;
+    // The volume's own UV channel (ModelVolume::texture_uv), so the recipe captured here is the one a
+    // ModelUV layer actually bakes through. Empty when the volume has none.
+    std::vector<Vec2f>                    model_uv;
 
     // The edge scan behind the open / non-manifold counts is a sort over every half-edge, which on a
     // multi-million triangle stage costs more than the stage did. On by default because a stage that

src/slic3r/GUI/Jobs/TextureDisplacementPrepareJob.cpp

@@ -46,7 +46,7 @@ void TextureDisplacementPrepareJob::process(Ctl &ctl)
     // idle loop.
     int last_reported = 1;
     m_result = GLGizmoTextureDisplacement::prepare_mesh(m_input.base_mesh, m_input.masks, m_input.layers,
-                                                        m_input.params, m_input.color,
+                                                        m_input.model_uv, m_input.params, m_input.color,
                                                         [&ctl, &status, &last_reported](int percent) {
                                                             if (ctl.was_canceled())
                                                                 return false;
@@ -106,6 +106,10 @@ void TextureDisplacementPrepareJob::finalize(bool canceled, std::exception_ptr &
         volume->restore_painting(saved_painting);
         for (int i = 0; i < int(TEXTURE_DISPLACEMENT_MAX_LAYERS); ++i)
             volume->texture_displacement_facet(i).set_data(std::move(m_result.masks[size_t(i)]));
+        // The model's own UV channel, resampled onto the new mesh by the run. Also put back after
+        // restore_painting(), which resets it along with the extra facets for the same reason: it is
+        // indexed by triangle and the triangles have changed.
+        volume->texture_uv = std::move(m_result.uv);
 
         if (ObjectList *obj_list = wxGetApp().obj_list()) {
             const ModelObjectPtrs &objs = plater->model().objects;

src/slic3r/GUI/Jobs/TextureDisplacementPrepareJob.hpp

@@ -9,6 +9,7 @@
 #include "Job.hpp"
 
 #include "libslic3r/ObjectID.hpp"
+#include "libslic3r/Point.hpp"
 #include "libslic3r/TextureDisplacement.hpp"
 
 namespace Slic3r {
@@ -47,6 +48,10 @@ struct TextureDisplacementPrepareInput
     // skips the colour criterion entirely. Only the *quantizer* is used here: refinement follows
     // perceived colour, never the interleaving that realises a mix.
     TextureColorSettings                  color;
+    // The volume's own UV channel (ModelVolume::texture_uv), empty when it has none. prepare_mesh()
+    // resamples it onto the mesh it produces, and the commit puts it back - like the masks, it is
+    // indexed by triangle and so cannot survive the stages on its own.
+    std::vector<Vec2f>                    model_uv;
     // The undo step the commit opens. Standard mode's Bake names it after the bake, because the
     // displacement job that follows commits into this same step rather than pushing its own.
     std::string                           snapshot_name;

src/slic3r/GUI/Jobs/TextureDisplacementPreviewJob.cpp

@@ -48,7 +48,8 @@ void TextureDisplacementPreviewJob::process(Ctl &ctl)
                                                      (!m_current_generation ||
                                                       m_current_generation->load() == m_generation);
                                           },
-                                          color, m_input.volume_to_world);
+                                          color, m_input.volume_to_world, nullptr, nullptr,
+                                          m_input.model_uv.empty() ? nullptr : &m_input.model_uv);
 }
 
 void TextureDisplacementPreviewJob::finalize(bool canceled, std::exception_ptr &eptr)

src/slic3r/GUI/Jobs/TextureDisplacementPreviewJob.hpp

@@ -29,6 +29,9 @@ struct TextureDisplacementPreviewInput
     // Empty unless a layer is colouring, in which case the preview reports the palette entry per
     // triangle alongside the mesh, so the Normal view shows the colours the bake will produce.
     TextureColorSettings                  color;
+    // The volume's own UV channel (ModelVolume::texture_uv), so a ModelUV layer previews through the
+    // same layout it will bake through. Empty when the volume has none.
+    std::vector<Vec2f>                    model_uv;
 };
 
 // A preview result: the displaced mesh, and - when the input carried a palette - one palette entry per

tests/libslic3r/test_texture_displacement.cpp

@@ -2346,3 +2346,353 @@ TEST_CASE("whether a layer's texture has colour agrees with its decode", "[Textu
 
     CHECK_FALSE(height_texture_has_color(TextureDisplacementLayer{}));
 }
+
+// ---------------------------------------------------------------------------------------------
+// The model's own UV channel, and the Override blend mode
+// ---------------------------------------------------------------------------------------------
+
+// A grayscale image that is black in its left half and white in its right half, so where a sample
+// lands along u is readable straight off the height it comes back with.
+static std::shared_ptr<std::vector<unsigned char>> make_left_right_split_png(size_t w = 16, size_t h = 16)
+{
+    std::vector<uint8_t> pixels(w * h);
+    for (size_t y = 0; y < h; ++y)
+        for (size_t x = 0; x < w; ++x)
+            pixels[y * w + x] = (x < w / 2) ? 0 : 255;
+    const boost::filesystem::path tmp_path = boost::filesystem::temp_directory_path()
+        / boost::filesystem::unique_path("texdisp_test_%%%%%%%%.png");
+    REQUIRE(Slic3r::png::write_gray_to_file(tmp_path.string(), w, h, pixels));
+
+    std::vector<unsigned char> bytes;
+    {
+        std::ifstream ifs(tmp_path.string(), std::ios::binary);
+        bytes.assign(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());
+    }
+    boost::system::error_code ec;
+    boost::filesystem::remove(tmp_path, ec);
+
+    REQUIRE_FALSE(bytes.empty());
+    return std::make_shared<std::vector<unsigned char>>(std::move(bytes));
+}
+
+// Where a sheet coordinate lands in uv: across [UV_LO, UV_LO + UV_SPAN] rather than the full [0, 1].
+// The inset keeps the tests off the exact edge of an untiled placement, where there is deliberately no
+// texture at all rather than its border texel (see DecodedHeightTexture::sample()) - a real model's
+// channel does reach 1.0, and a vertex sitting exactly there displaces by nothing on purpose.
+static constexpr float UV_LO   = 0.1f;
+static constexpr float UV_SPAN = 0.8f;
+
+static float sheet_uv_at(float coord, float size, bool mirror = false)
+{
+    const float t = coord / size;
+    return UV_LO + UV_SPAN * (mirror ? 1.f - t : t);
+}
+
+// One UV per triangle corner: u follows x, v follows y. `mirror_u` mirrors u, which is what the tests
+// use to show the bake is really reading the channel rather than projecting the position.
+static std::vector<Vec2f> sheet_uv_channel(const indexed_triangle_set &sheet, float size, bool mirror_u = false)
+{
+    std::vector<Vec2f> uv(sheet.indices.size() * 3);
+    for (size_t f = 0; f < sheet.indices.size(); ++f)
+        for (int k = 0; k < 3; ++k) {
+            const Vec3f &p = sheet.vertices[size_t(sheet.indices[f][k])];
+            uv[f * 3 + size_t(k)] = Vec2f(sheet_uv_at(p.x(), size, mirror_u), sheet_uv_at(p.y(), size));
+        }
+    return uv;
+}
+
+TEST_CASE("Override blend replaces the displacement under it instead of combining with it",
+          "[TextureDisplacement]")
+{
+    CHECK_THAT(blend_displacement(3.f, 1.f, TextureBlendMode::Override), WithinAbs(1.f, 1e-6f));
+    // Nothing underneath yet is still just the value, so an Override layer at the bottom of the
+    // stack behaves exactly like the Add the bake forces there.
+    CHECK_THAT(blend_displacement(0.f, 1.f, TextureBlendMode::Override), WithinAbs(1.f, 1e-6f));
+    // The sign carries, and a zero sample really does flatten what is under it - that is the point.
+    CHECK_THAT(blend_displacement(3.f, -1.f, TextureBlendMode::Override), WithinAbs(-1.f, 1e-6f));
+    CHECK_THAT(blend_displacement(3.f, 0.f, TextureBlendMode::Override), WithinAbs(0.f, 1e-6f));
+}
+
+TEST_CASE("an Override layer leaves only its own relief where two layers overlap", "[TextureDisplacement]")
+{
+    const indexed_triangle_set cube = its_make_cube(10., 10., 10.);
+
+    TextureDisplacementLayer base;
+    base.slot         = 0;
+    base.depth_mm     = 2.0f;
+    base.tiling_scale = 5.0f;
+    base.image_data   = make_flat_gray_png(255); // full depth everywhere
+
+    TextureDisplacementLayer second = base;
+    second.slot       = 1;
+    second.depth_mm   = 0.5f;
+    second.blend_mode = TextureBlendMode::Override;
+
+    TextureDisplacementFacetsData facets{};
+    facets[0] = paint_whole_mesh(cube);
+    facets[1] = facets[0];
+
+    const indexed_triangle_set overridden = build_texture_displacement(cube, {base, second}, facets, classic_options());
+    REQUIRE(overridden.vertices.size() == cube.vertices.size());
+
+    // Add would have piled the two up to 2.5 mm; Override keeps the upper layer's 0.5 mm alone.
+    TextureDisplacementLayer added = second;
+    added.blend_mode               = TextureBlendMode::Add;
+    const indexed_triangle_set summed = build_texture_displacement(cube, {base, added}, facets, classic_options());
+    REQUIRE(summed.vertices.size() == cube.vertices.size());
+
+    for (size_t i = 0; i < cube.vertices.size(); ++i) {
+        if (!above_resting_plane(cube, i))
+            continue;
+        CHECK_THAT((overridden.vertices[i] - cube.vertices[i]).norm(), WithinAbs(0.5f, 1e-3f));
+        CHECK_THAT((summed.vertices[i] - cube.vertices[i]).norm(), WithinAbs(2.5f, 1e-3f));
+    }
+}
+
+TEST_CASE("the ModelUV mapping displaces through the model's own UV channel", "[TextureDisplacement]")
+{
+    constexpr float            size  = 10.f;
+    const indexed_triangle_set sheet = make_flat_sheet(size, 1.f);
+
+    TextureDisplacementLayer layer;
+    layer.slot         = 0;
+    layer.depth_mm     = 1.0f;
+    layer.image_data   = make_left_right_split_png();
+    layer.tile_enabled = false; // the layout is the placement; nothing should repeat
+    layer.projection_method = TextureProjectionMethod::ModelUV;
+
+    TextureDisplacementFacetsData facets{};
+    facets[0] = paint_whole_mesh(sheet);
+
+    const std::vector<Vec2f> uv      = sheet_uv_channel(sheet, size);
+    const std::vector<Vec2f> flipped = sheet_uv_channel(sheet, size, /* mirror_u */ true);
+
+    const indexed_triangle_set out = build_texture_displacement(sheet, {layer}, facets, classic_options(), {}, nullptr,
+                                                                 Transform3d::Identity(), nullptr, nullptr, &uv);
+    REQUIRE(out.vertices.size() == sheet.vertices.size());
+
+    // The image is black for u < 0.5 and white above it, and the channel's u rises with x across the
+    // sheet, so it comes out flat on its left half and raised by the full depth on its right.
+    size_t low = 0, high = 0;
+    for (size_t i = 0; i < sheet.vertices.size(); ++i) {
+        const float x    = sheet.vertices[i].x();
+        const float dz   = out.vertices[i].z() - sheet.vertices[i].z();
+        if (x < 0.4f * size) {
+            CHECK_THAT(dz, WithinAbs(0.f, 1e-3f));
+            ++low;
+        } else if (x > 0.6f * size) {
+            CHECK_THAT(dz, WithinAbs(layer.depth_mm, 1e-3f));
+            ++high;
+        }
+    }
+    REQUIRE(low > 0);
+    REQUIRE(high > 0);
+
+    // Mirroring the channel mirrors the relief: the mapping really is the channel's, not a planar
+    // projection of the position (which would be unchanged by it).
+    const indexed_triangle_set mirrored = build_texture_displacement(sheet, {layer}, facets, classic_options(), {},
+                                                                      nullptr, Transform3d::Identity(), nullptr, nullptr,
+                                                                      &flipped);
+    REQUIRE(mirrored.vertices.size() == sheet.vertices.size());
+    for (size_t i = 0; i < sheet.vertices.size(); ++i) {
+        const float x = sheet.vertices[i].x();
+        if (x < 0.4f * size)
+            CHECK_THAT(mirrored.vertices[i].z() - sheet.vertices[i].z(), WithinAbs(layer.depth_mm, 1e-3f));
+        else if (x > 0.6f * size)
+            CHECK_THAT(mirrored.vertices[i].z() - sheet.vertices[i].z(), WithinAbs(0.f, 1e-3f));
+    }
+}
+
+TEST_CASE("a ModelUV layer falls back to Triplanar when the volume carries no UV channel",
+          "[TextureDisplacement]")
+{
+    constexpr float            size  = 10.f;
+    const indexed_triangle_set sheet = make_flat_sheet(size, 2.f);
+
+    TextureDisplacementLayer layer;
+    layer.slot         = 0;
+    layer.depth_mm     = 1.0f;
+    layer.tiling_scale = 5.0f;
+    layer.image_data   = make_flat_gray_png(255);
+
+    TextureDisplacementFacetsData facets{};
+    facets[0] = paint_whole_mesh(sheet);
+
+    layer.projection_method                 = TextureProjectionMethod::Triplanar;
+    const indexed_triangle_set as_triplanar = build_texture_displacement(sheet, {layer}, facets, classic_options());
+    layer.projection_method                 = TextureProjectionMethod::ModelUV;
+    const indexed_triangle_set no_channel    = build_texture_displacement(sheet, {layer}, facets, classic_options());
+
+    REQUIRE(no_channel.vertices.size() == as_triplanar.vertices.size());
+    for (size_t i = 0; i < as_triplanar.vertices.size(); ++i)
+        CHECK_THAT((no_channel.vertices[i] - as_triplanar.vertices[i]).norm(), WithinAbs(0.f, 1e-5f));
+}
+
+TEST_CASE("a bake with colour off on every layer reports no colour at all", "[TextureDisplacement]")
+{
+    const indexed_triangle_set cube = its_make_cube(10., 10., 10.);
+
+    TextureDisplacementLayer layer;
+    layer.slot          = 0;
+    layer.depth_mm      = 1.0f;
+    layer.tiling_scale  = 5.0f;
+    layer.image_data    = make_flat_rgb_png(200, 40, 10); // has colour to give...
+    layer.color_enabled = false;                          // ...but is not asked for it
+
+    TextureDisplacementFacetsData facets{};
+    facets[0] = paint_whole_mesh(cube);
+
+    std::vector<uint8_t> triangle_color;
+    TextureColorRequest  request;
+    request.quantize     = [](const Vec3f &) { return 0; }; // would colour everything, if asked
+    request.out_triangle = &triangle_color;
+
+    const indexed_triangle_set out = build_texture_displacement(cube, {layer}, facets, classic_options(), {}, &request);
+    REQUIRE_FALSE(out.vertices.empty());
+    // Every entry 0 ("this triangle takes no colour from the texture"), so the commit leaves whatever
+    // the model was already painted with exactly as it was.
+    for (const uint8_t entry : triangle_color)
+        CHECK(entry == 0);
+}
+
+TEST_CASE("transfer_uv_channel resamples a UV channel onto a subdivided copy of the same surface",
+          "[TextureDisplacement]")
+{
+    constexpr float            size   = 10.f;
+    const indexed_triangle_set coarse = make_flat_sheet(size, 5.f);
+    const indexed_triangle_set fine   = make_flat_sheet(size, 1.f);
+    const std::vector<Vec2f>   uv     = sheet_uv_channel(coarse, size);
+
+    const std::vector<Vec2f> moved = transfer_uv_channel(coarse, uv, fine);
+    REQUIRE(moved.size() == fine.indices.size() * 3);
+
+    // The channel is affine in position here (u = x/size, v = y/size), so a correct transfer
+    // reproduces it exactly at every corner of the finer mesh.
+    for (size_t f = 0; f < fine.indices.size(); ++f)
+        for (int k = 0; k < 3; ++k) {
+            const Vec3f &p = fine.vertices[size_t(fine.indices[f][k])];
+            CHECK_THAT(moved[f * 3 + size_t(k)].x(), WithinAbs(sheet_uv_at(p.x(), size), 1e-4f));
+            CHECK_THAT(moved[f * 3 + size_t(k)].y(), WithinAbs(sheet_uv_at(p.y(), size), 1e-4f));
+        }
+
+    // A mismatched channel is refused rather than read past its end.
+    CHECK(transfer_uv_channel(coarse, {Vec2f::Zero()}, fine).empty());
+    CHECK(transfer_uv_channel(coarse, uv, indexed_triangle_set{}).empty());
+}
+
+TEST_CASE("the model's per-corner UVs survive a brush split and collapse to one per vertex",
+          "[TextureDisplacement]")
+{
+    constexpr float            size  = 10.f;
+    const indexed_triangle_set sheet = make_flat_sheet(size, 5.f);
+    const std::vector<Vec2f>   uv    = sheet_uv_channel(sheet, size);
+
+    // The whole sheet as a patch, with the source map the channel is read through.
+    const TriangleMesh tm(sheet);
+    TriangleSelector   selector(tm);
+    for (int f = 0; f < int(sheet.indices.size()); ++f)
+        selector.set_facet(f, EnforcerBlockerType::ENFORCER);
+    std::vector<int>           source;
+    const indexed_triangle_set patch = selector.get_facets_strict(EnforcerBlockerType::ENFORCER, &source);
+    REQUIRE(patch.indices.size() == source.size());
+
+    const std::vector<Vec2f> corner = compute_model_corner_uvs(sheet, uv, patch, source);
+    REQUIRE(corner.size() == patch.indices.size() * 3);
+    // An unsplit patch triangle *is* its source triangle, so its corners reproduce the channel.
+    for (size_t f = 0; f < patch.indices.size(); ++f)
+        for (int k = 0; k < 3; ++k) {
+            const Vec3f &p = patch.vertices[size_t(patch.indices[f][k])];
+            CHECK_THAT(corner[f * 3 + size_t(k)].x(), WithinAbs(sheet_uv_at(p.x(), size), 1e-4f));
+            CHECK_THAT(corner[f * 3 + size_t(k)].y(), WithinAbs(sheet_uv_at(p.y(), size), 1e-4f));
+        }
+
+    const std::vector<Vec2f> per_vertex = collapse_corner_uvs(patch, corner);
+    REQUIRE(per_vertex.size() == patch.vertices.size());
+    for (size_t i = 0; i < patch.vertices.size(); ++i) {
+        CHECK_THAT(per_vertex[i].x(), WithinAbs(sheet_uv_at(patch.vertices[i].x(), size), 1e-4f));
+        CHECK_THAT(per_vertex[i].y(), WithinAbs(sheet_uv_at(patch.vertices[i].y(), size), 1e-4f));
+    }
+
+    // Mismatched inputs report nothing rather than guessing.
+    CHECK(compute_model_corner_uvs(sheet, {Vec2f::Zero()}, patch, source).empty());
+    CHECK(compute_model_corner_uvs(sheet, uv, patch, {}).empty());
+    CHECK(collapse_corner_uvs(patch, {Vec2f::Zero()}).empty());
+}
+
+TEST_CASE("facet paint is carried onto a rebuilt mesh by each facet's foot point", "[TextureDisplacement]")
+{
+    constexpr float            size   = 10.f;
+    const indexed_triangle_set coarse = make_flat_sheet(size, 2.5f);
+
+    // Two filaments, split down the middle - the pattern the transfer has to reproduce.
+    const TriangleMesh coarse_mesh(coarse);
+    TriangleSelector   painter(coarse_mesh);
+    for (size_t t = 0; t < coarse.indices.size(); ++t) {
+        const stl_triangle_vertex_indices &tri = coarse.indices[t];
+        const float x = (coarse.vertices[size_t(tri[0])].x() + coarse.vertices[size_t(tri[1])].x() +
+                         coarse.vertices[size_t(tri[2])].x()) / 3.f;
+        painter.set_facet(int(t), x < 0.5f * size ? EnforcerBlockerType::Extruder1 : EnforcerBlockerType::Extruder2);
+    }
+    const TriangleSelector::TriangleSplittingData paint = painter.serialize();
+
+    // A finer mesh, lifted clear of the original surface the way a bake's relief lifts it. This is
+    // what ModelVolume::restore_painting()'s 0.01 mm overlap remap cannot follow.
+    indexed_triangle_set lifted = make_flat_sheet(size, 1.f);
+    for (Vec3f &v : lifted.vertices)
+        v.z() += 0.5f;
+
+    const std::vector<uint8_t> state = transfer_facet_paint(coarse, paint, lifted);
+    REQUIRE(state.size() == lifted.indices.size());
+
+    size_t left = 0, right = 0;
+    for (size_t t = 0; t < lifted.indices.size(); ++t) {
+        const stl_triangle_vertex_indices &tri = lifted.indices[t];
+        const float x = (lifted.vertices[size_t(tri[0])].x() + lifted.vertices[size_t(tri[1])].x() +
+                         lifted.vertices[size_t(tri[2])].x()) / 3.f;
+        // Only away from the split: a facet straddling it may legitimately land on either side.
+        if (x < 0.4f * size) {
+            CHECK(state[t] == uint8_t(EnforcerBlockerType::Extruder1));
+            ++left;
+        } else if (x > 0.6f * size) {
+            CHECK(state[t] == uint8_t(EnforcerBlockerType::Extruder2));
+            ++right;
+        }
+    }
+    REQUIRE(left > 0);
+    REQUIRE(right > 0);
+
+    // An unpainted source has nothing to carry, which is not the same as carrying NONE everywhere:
+    // the caller has to tell those apart to decide between resetting the annotation and writing one.
+    CHECK(transfer_facet_paint(coarse, TriangleSelector::TriangleSplittingData{}, lifted).empty());
+    CHECK(transfer_facet_paint(coarse, paint, indexed_triangle_set{}).empty());
+}
+
+TEST_CASE("an unpainted part of the source carries no paint onto the rebuilt mesh", "[TextureDisplacement]")
+{
+    constexpr float            size   = 10.f;
+    const indexed_triangle_set coarse = make_flat_sheet(size, 2.5f);
+
+    // Only the left half is painted at all; the right half must come back NONE rather than borrowing
+    // the nearest painted colour from across the model.
+    const TriangleMesh coarse_mesh(coarse);
+    TriangleSelector   painter(coarse_mesh);
+    for (size_t t = 0; t < coarse.indices.size(); ++t) {
+        const stl_triangle_vertex_indices &tri = coarse.indices[t];
+        const float x = (coarse.vertices[size_t(tri[0])].x() + coarse.vertices[size_t(tri[1])].x() +
+                         coarse.vertices[size_t(tri[2])].x()) / 3.f;
+        if (x < 0.5f * size)
+            painter.set_facet(int(t), EnforcerBlockerType::Extruder3);
+    }
+
+    const std::vector<uint8_t> state = transfer_facet_paint(coarse, painter.serialize(), coarse);
+    REQUIRE(state.size() == coarse.indices.size());
+    for (size_t t = 0; t < coarse.indices.size(); ++t) {
+        const stl_triangle_vertex_indices &tri = coarse.indices[t];
+        const float x = (coarse.vertices[size_t(tri[0])].x() + coarse.vertices[size_t(tri[1])].x() +
+                         coarse.vertices[size_t(tri[2])].x()) / 3.f;
+        if (x < 0.4f * size)
+            CHECK(state[t] == uint8_t(EnforcerBlockerType::Extruder3));
+        else if (x > 0.6f * size)
+            CHECK(state[t] == uint8_t(EnforcerBlockerType::NONE));
+    }
+}