Strip zero-area faces and restitch the T-joints they leave

bindings/module.cpp

@@ -247,6 +247,24 @@ nb::list csgTreeToPy(const std::vector<std::unique_ptr<oscadeval::CSGNode>>& tre
 // Exposed so a front end that writes its own files -- BelfrySCAD's GUI has
 // its own exporters -- can run the same check the CLI does rather than a
 // second, drifting implementation.
+// Exposed for the export path: strip zero-area faces and repair the
+// T-joints their removal exposes. Returns (verts, tris, report).
+nb::tuple stripSliversPy(const std::vector<float>& verts, const std::vector<uint32_t>& tris) {
+    manifold::MeshGL m;
+    m.numProp = 3;
+    m.vertProperties = verts;
+    m.triVerts = tris;
+    oscadeval::SliverStripReport r;
+    manifold::MeshGL out = oscadeval::stripSlivers(m, r);
+    nb::dict rep;
+    rep["removed"] = r.removed;
+    rep["restitched"] = r.restitched;
+    rep["needles"] = r.needles;
+    rep["left_behind"] = r.leftBehind;
+    rep["passes"] = r.passes;
+    return nb::make_tuple(out.vertProperties, out.triVerts, rep);
+}
+
 nb::dict checkMeshPy(const std::vector<float>& verts, const std::vector<uint32_t>& tris) {
     manifold::MeshGL m;
     m.numProp = 3;
@@ -708,6 +726,9 @@ NB_MODULE(_openscad_cpp_evaluator, m) {
           "Evaluate a .scad file; return (bodies, echoes, id_to_node, csg_tree, profile_result, dyn, dyn_explicit).");
     m.def("parse_decls", &parseDecls, nb::arg("path"),
           "Parse a .scad file; return top-level declaration (namespace, name, start, end, line, column, origin) tuples.");
+    m.def("strip_slivers", &stripSliversPy, nb::arg("verts"), nb::arg("tris"),
+          "Remove zero-area faces and repair the T-joints their removal "
+          "exposes. Returns (verts, tris, report).");
     m.def("check_mesh", &checkMeshPy, nb::arg("verts"), nb::arg("tris"),
           "Diagnose a triangle mesh against the manifoldness conditions. "
           "verts is a flat [x,y,z,...] list, tris a flat index list. Returns "

include/openscad_cpp_evaluator/mesh_check.hpp

@@ -63,10 +63,11 @@ struct MeshRepairReport {
     size_t filledTriangles = 0;
     size_t splitVertices = 0;
     size_t unfilledHoles = 0;      // boundary loops it could not close
+    size_t strippedSlivers = 0;    // zero-area faces removed and restitched
 
     bool didAnything() const {
         return weldedVertices || droppedDegenerate || droppedDuplicate || reversedFaces
-               || filledHoles || splitVertices;
+               || filledHoles || splitVertices || strippedSlivers;
     }
     // "welded 12 vertices, filled 1 hole (4 triangles)" -- empty if nothing.
     std::string summary() const;
@@ -82,4 +83,26 @@ struct MeshRepairReport {
 // improved as far as it got, with the remainder reported in `report`.
 manifold::MeshGL repairMesh(const manifold::MeshGL& mesh, MeshRepairReport& report);
 
+struct SliverStripReport {
+    size_t removed = 0;        // zero-area faces taken out
+    size_t restitched = 0;     // neighbours split to close the gap they left
+    // Faces with two corners at one position. These need no restitching:
+    // their two long edges run between the same two points, so the faces on
+    // either side already meet once the needle is gone.
+    size_t needles = 0;
+    size_t leftBehind = 0;     // slivers whose neighbour could not be found
+    size_t passes = 0;         // removing one can expose another
+};
+
+// Remove zero-area faces and repair the T-joints their removal exposes.
+//
+// A sliver's three vertices are collinear, so one lies between the other
+// two. Dropping the face leaves that middle vertex sitting on the interior
+// of the neighbour's edge -- a T-joint -- and the two sides no longer share
+// an edge, which reads as a hole. Splitting the neighbour at the middle
+// vertex restores the match without moving any geometry.
+//
+// The mesh is returned unchanged if it has no slivers.
+manifold::MeshGL stripSlivers(const manifold::MeshGL& mesh, SliverStripReport& report);
+
 }  // namespace oscadeval

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "scikit_build_core.build"
 
 [project]
 name = "openscad_cpp_evaluator"
-version = "0.24.0"
+version = "0.25.0"
 description = "C++ OpenSCAD evaluator with Python bindings"
 readme = "README.md"
 requires-python = ">=3.12"

python/openscad_cpp_evaluator/__init__.py

@@ -18,7 +18,7 @@
 __all__ = [
     "Evaluator", "ColoredBody", "EvalError", "ParseError", "OscObject", "parse", "to_renderable_bodies",
     "ManifoldCache", "CallSiteProfile", "ProfileResult", "format_csg_tree", "bodies_from_dicts",
-    "FastContinueSignal", "parse_ast", "parse_ast_string", "check_mesh",
+    "FastContinueSignal", "parse_ast", "parse_ast_string", "check_mesh", "strip_slivers",
 ]
 
 
@@ -274,6 +274,20 @@ def parse(path: str) -> RootScope:
     return RootScope(decls)
 
 
+def strip_slivers(verts, tris):
+    """Remove zero-area faces and repair the T-joints removal exposes.
+
+    A sliver's three vertices are collinear, so one lies between the other
+    two. Dropping the face leaves that middle vertex on the interior of the
+    neighbour's edge, and the two sides no longer share an edge -- which
+    reads as a hole. Splitting the neighbour there restores the match
+    without moving any geometry.
+
+    Returns (verts, tris, report).
+    """
+    return _ext.strip_slivers([float(v) for v in verts], [int(t) for t in tris])
+
+
 def check_mesh(verts, tris) -> dict:
     """Diagnose a triangle mesh against the manifoldness conditions.
 

src/mesh_check.cpp

@@ -204,6 +204,8 @@ std::string MeshRepairReport::summary() const {
                         + (filledHoles == 1 ? " hole filled (" : " holes filled (")
                         + std::to_string(filledTriangles) + " triangles)");
     }
+    add(strippedSlivers, strippedSlivers == 1 ? "zero-area face stripped"
+                                              : "zero-area faces stripped");
     add(unfilledHoles, unfilledHoles == 1 ? "hole left open" : "holes left open");
     std::string out;
     for (size_t i = 0; i < parts.size(); ++i) {
@@ -426,6 +428,206 @@ manifold::MeshGL repairMesh(const manifold::MeshGL& mesh, MeshRepairReport& repo
     out.runTransform.clear();
     out.mergeFromVert.clear();
     out.mergeToVert.clear();
+
+    // Zero-area faces last. Welding already disposes of needles -- it
+    // collapses the coincident pair, leaving a face that names a vertex
+    // twice, which step 2 drops -- but a sliver whose three corners are
+    // distinct and collinear survives all of the above untouched, and an
+    // imported STL is exactly where those turn up.
+    SliverStripReport strip;
+    out = stripSlivers(out, strip);
+    report.strippedSlivers = strip.removed;
+    return out;
+}
+
+
+namespace {
+
+// Which two corners of a zero-area face sit at the same position, if any.
+// Such a face is a needle rather than a T-joint: its two long edges run
+// between the same two points, so once it is gone the faces on either side
+// already meet along one edge and nothing needs splitting. The coincident
+// pair is merged so that is true by index as well as by position.
+bool coincidentPair(const manifold::MeshGL& m, const std::array<Vert, 3>& f,
+                    Vert& keep, Vert& drop) {
+    for (int i = 0; i < 3; ++i) {
+        const Vert a = f[i], b = f[(i + 1) % 3];
+        if (a == b) { keep = a; drop = b; return true; }
+        double p[3], q[3];
+        pos(m, a, p); pos(m, b, q);
+        if (llround(p[0] * 1e6) == llround(q[0] * 1e6)
+            && llround(p[1] * 1e6) == llround(q[1] * 1e6)
+            && llround(p[2] * 1e6) == llround(q[2] * 1e6)) {
+            keep = std::min(a, b);
+            drop = std::max(a, b);
+            return keep != drop;
+        }
+    }
+    return false;
+}
+
+// Index of the vertex opposite the longest edge -- for three collinear
+// points that is the one in the middle.
+int middleOfCollinear(const manifold::MeshGL& m, const std::array<Vert, 3>& f) {
+    double p[3][3];
+    for (int i = 0; i < 3; ++i) pos(m, f[i], p[i]);
+    auto d2 = [&](int a, int b) {
+        double s = 0;
+        for (int k = 0; k < 3; ++k) { const double d = p[a][k] - p[b][k]; s += d * d; }
+        return s;
+    };
+    const double opp[3] = {d2(1, 2), d2(0, 2), d2(0, 1)};
+    int best = 0;
+    for (int i = 1; i < 3; ++i) if (opp[i] > opp[best]) best = i;
+    return best;
+}
+
+}  // namespace
+
+manifold::MeshGL stripSlivers(const manifold::MeshGL& mesh, SliverStripReport& report) {
+    report = SliverStripReport{};
+    std::vector<std::array<Vert, 3>> tris;
+    tris.reserve(triCount(mesh));
+    for (size_t t = 0; t < triCount(mesh); ++t) {
+        Vert v[3];
+        triVerts(mesh, t, v);
+        tris.push_back({v[0], v[1], v[2]});
+    }
+
+    // Removing a sliver can leave its neighbour split into pieces that are
+    // themselves slivers, so this repeats. Bounded because each pass must
+    // remove at least one face to continue.
+    const int kMaxPasses = 12;
+    for (int pass = 0; pass < kMaxPasses; ++pass) {
+        std::vector<size_t> slivers;
+        for (size_t i = 0; i < tris.size(); ++i) {
+            const auto& f = tris[i];
+            if (f[0] == f[1] || f[1] == f[2] || f[0] == f[2]) continue;  // handled elsewhere
+            double a[3], b[3], c[3];
+            pos(mesh, f[0], a); pos(mesh, f[1], b); pos(mesh, f[2], c);
+            const double ux = b[0] - a[0], uy = b[1] - a[1], uz = b[2] - a[2];
+            const double wx = c[0] - a[0], wy = c[1] - a[1], wz = c[2] - a[2];
+            const double cx = uy * wz - uz * wy, cy = uz * wx - ux * wz, cz = ux * wy - uy * wx;
+            if ((cx * cx + cy * cy + cz * cz) < 1e-24) slivers.push_back(i);
+        }
+        if (slivers.empty()) break;
+        ++report.passes;
+
+        // Which face owns each edge, so a sliver's long-edge neighbour can
+        // be found. Rebuilt per pass: splitting changes it.
+        std::map<Edge, std::vector<size_t>> owners;
+        for (size_t i = 0; i < tris.size(); ++i) {
+            for (int e = 0; e < 3; ++e) {
+                owners[undirected(tris[i][e], tris[i][(e + 1) % 3])].push_back(i);
+            }
+        }
+
+        std::vector<char> isSliver(tris.size(), 0);
+        for (size_t i : slivers) isSliver[i] = 1;
+
+        std::vector<char> dead(tris.size(), 0);
+        std::vector<std::array<Vert, 3>> added;
+        std::map<Vert, Vert> merge;      // needle corners to fold together
+        for (size_t si : slivers) {
+            if (dead[si]) continue;
+            const auto f = tris[si];
+
+            // A needle: two corners at one point. Nothing to restitch --
+            // the faces on either side already share an edge positionally,
+            // and merging the pair makes them share it by index too.
+            Vert keep = 0, drop = 0;
+            if (coincidentPair(mesh, f, keep, drop)) {
+                dead[si] = 1;
+                if (keep != drop) merge[drop] = keep;
+                ++report.removed;
+                ++report.needles;
+                continue;
+            }
+
+            const int mid = middleOfCollinear(mesh, f);
+            const Vert m = f[mid], a = f[(mid + 1) % 3], b = f[(mid + 2) % 3];
+
+            // The neighbour across the long edge a-b, which is the one the
+            // middle vertex now sits inside.
+            // Prefer a neighbour that is not itself a sliver. Two slivers
+            // sharing their long edge are each other's only candidate, and
+            // splitting one into the other just moves the problem around --
+            // a level-4 Menger sponge has exactly one such pair, and it was
+            // what stopped the last two from ever clearing.
+            size_t nb = SIZE_MAX, fallback = SIZE_MAX;
+            for (size_t cand : owners[undirected(a, b)]) {
+                if (cand == si || dead[cand]) continue;
+                if (isSliver[cand]) {
+                    if (fallback == SIZE_MAX) fallback = cand;
+                    continue;
+                }
+                nb = cand;
+                break;
+            }
+            // A non-sliver neighbour is preferred but not required. Two
+            // slivers sharing their long edge are each other's only
+            // candidate -- a level-4 Menger sponge has one such pair, and
+            // refusing to split into a sliver leaves them forever. Splitting
+            // into one still makes progress, because the halves are smaller
+            // and the next pass reconsiders them.
+            if (nb == SIZE_MAX) nb = fallback;
+            if (nb == SIZE_MAX) { ++report.leftBehind; continue; }
+
+            // Split the neighbour at m, keeping its winding: the edge a-b
+            // appears in it in some direction, and the two pieces must
+            // traverse it the same way round.
+            const auto& n = tris[nb];
+            int e = -1;
+            for (int i = 0; i < 3; ++i) {
+                const Vert x = n[i], y = n[(i + 1) % 3];
+                if ((x == a && y == b) || (x == b && y == a)) { e = i; break; }
+            }
+            if (e < 0) { ++report.leftBehind; continue; }
+            const Vert x = n[e], y = n[(e + 1) % 3], apex = n[(e + 2) % 3];
+
+            dead[si] = 1;
+            dead[nb] = 1;
+            added.push_back({x, m, apex});
+            added.push_back({m, y, apex});
+            ++report.removed;
+            ++report.restitched;
+        }
+
+        std::vector<std::array<Vert, 3>> next;
+        next.reserve(tris.size() + added.size());
+        auto resolve = [&](Vert v) {
+            for (int hop = 0; hop < 8; ++hop) {      // chains are short
+                auto it = merge.find(v);
+                if (it == merge.end()) break;
+                v = it->second;
+            }
+            return v;
+        };
+        for (size_t i = 0; i < tris.size(); ++i) {
+            if (dead[i]) continue;
+            const auto& f = tris[i];
+            next.push_back({resolve(f[0]), resolve(f[1]), resolve(f[2])});
+        }
+        for (const auto& f : added) {
+            next.push_back({resolve(f[0]), resolve(f[1]), resolve(f[2])});
+        }
+        if (next.size() == tris.size() && added.empty()) break;
+        tris.swap(next);
+    }
+
+    manifold::MeshGL out = mesh;
+    out.triVerts.clear();
+    out.triVerts.reserve(tris.size() * 3);
+    for (const auto& f : tris) {
+        out.triVerts.push_back(f[0]);
+        out.triVerts.push_back(f[1]);
+        out.triVerts.push_back(f[2]);
+    }
+    // These describe the old triangle list; a stale one is worse than none.
+    out.runIndex.clear();
+    out.runOriginalID.clear();
+    out.faceID.clear();
+    out.runTransform.clear();
     return out;
 }