BRepFilletAPI_MakeFillet2d - fails to fillet a vertex after BRepFill_OffsetWire

[gsegon]

Hi,

For a sample problem consider 2 cases of operations performed:
A) Operations with offset:

1. Make test face,
2. Offset test face inward by offset, (using BRepFill_OffsetWire)
3. Make mask face,
4. Clip offset-face by mask face (using BRepAlgoAPI_Common)
5. Fillet vertices on the cut line. <--- (Fillet2d failed, status=6, ChFi2d_ComputationError)

B) Operation without offset:

1. Make test face,
2. skip this step, no offseting
3. Make mask face,
4. Clip test face by mask face (using BRepAlgoAPI_Common)
5. Fillet vertices on the cut line. <--- (Success)

Self-contained MWE to reproduce the issue:

#include <string>

#include <gtest/gtest.h>

#include <TopoDS.hxx>
#include <TopoDS_Face.hxx>
#include <TopoDS_Wire.hxx>
#include <TopExp_Explorer.hxx>

#include <gp_Pln.hxx>
#include <GeomAbs_JoinType.hxx>
#include <GeomAdaptor_Curve.hxx>
#include <GeomFill_Pipe.hxx>
#include <Geom_Circle.hxx>

#include <BRepTools.hxx>
#include <BRepAlgoAPI_Common.hxx>

#include <BRep_Builder.hxx>
#include <BRepBuilderAPI_MakeEdge.hxx>
#include <BRepBuilderAPI_MakeFace.hxx>
#include <BRepBuilderAPI_MakeWire.hxx>
#include <BRepCheck_Analyzer.hxx>
#include <BRepFill_OffsetWire.hxx>
#include <BRepFilletAPI_MakeFillet2d.hxx>
#include <BRepLib.hxx>


namespace {

    auto dumpFaceVerts(const TopoDS_Face& f, const char* label) {
        std::cout << "\n--- " << label << " ---\n";
        if (f.IsNull()) { std::cout << "Face is NULL\n"; return; }
        const TopoDS_Wire outer = BRepTools::OuterWire(f);
        int i = 0;
        for (TopExp_Explorer ex(outer, TopAbs_VERTEX); ex.More(); ex.Next(), ++i) {
            const TopoDS_Vertex v = TopoDS::Vertex(ex.Current());
            const gp_Pnt p = BRep_Tool::Pnt(v);
            std::cout << "v[" << i << "] = (" << p.X() << "," << p.Y() << "," << p.Z() << ")\n";
        }
    };

    auto pickClipCornerVertices(const TopoDS_Face& f, double xCut, double xTol) {
        // Pick vertices on the clipped boundary x ~= xCut and return lowest/highest Y.
        std::vector<TopoDS_Vertex> vs;
        const TopoDS_Wire outer = BRepTools::OuterWire(f);
        for (TopExp_Explorer ex(outer, TopAbs_VERTEX); ex.More(); ex.Next()) {
            const TopoDS_Vertex v = TopoDS::Vertex(ex.Current());
            const gp_Pnt p = BRep_Tool::Pnt(v);
            if (std::abs(p.X() - xCut) <= xTol) vs.push_back(v);
        }
        std::ranges::sort(vs, [](const TopoDS_Vertex& a, const TopoDS_Vertex& b) {
            return BRep_Tool::Pnt(a).Y() < BRep_Tool::Pnt(b).Y();
        });

        // Dedup by Y (quick and dirty for an MWE)
        std::vector<TopoDS_Vertex> uniq;
        for (const auto& v : vs) {
            const double y = BRep_Tool::Pnt(v).Y();
            if (uniq.empty() || std::abs(y - BRep_Tool::Pnt(uniq.back()).Y()) > 1e-9) uniq.push_back(v);
        }
        return uniq;
    };

    const gp_Pln pln(gp_Pnt(0,0,0), gp_Dir(0,0,1));

    TopoDS_Face make_test_face() {

        // 1) Construct a section of a hollow 2D cylinder: annular sector on XY plane.
        const double rIn = 10.0;
        const double rOut = 20.0;
        const double ang0 = 0.0;
        const double ang1 = 1.8;

        const gp_Ax2 ax2(gp_Pnt(0,0,0), gp_Dir(0,0,1));

        const gp_Circ cOut(ax2, rOut);
        const gp_Circ cIn(ax2, rIn);

        const gp_Pnt pOut0(rOut*std::cos(ang0), rOut*std::sin(ang0), 0);
        const gp_Pnt pOut1(rOut*std::cos(ang1), rOut*std::sin(ang1), 0);
        const gp_Pnt pIn0 (rIn *std::cos(ang0), rIn *std::sin(ang0), 0);
        const gp_Pnt pIn1 (rIn *std::cos(ang1), rIn *std::sin(ang1), 0);

        // Outer arc (ang0 -> ang1)
        TopoDS_Edge eOut = BRepBuilderAPI_MakeEdge(cOut, ang0, ang1).Edge();
        // Radial line at ang1 (outer -> inner)
        TopoDS_Edge eRad1 = BRepBuilderAPI_MakeEdge(pOut1, pIn1).Edge();
        // Inner arc (ang1 -> ang0) reversed direction to close sector
        TopoDS_Edge eIn  = BRepBuilderAPI_MakeEdge(cIn, ang0, ang1).Edge();
        // Radial line at ang0 (inner -> outer)
        TopoDS_Edge eRad0 = BRepBuilderAPI_MakeEdge(pIn0, pOut0).Edge();

        BRepBuilderAPI_MakeWire mkW0;
        mkW0.Add(eOut);
        mkW0.Add(eRad1);
        mkW0.Add(eIn);
        mkW0.Add(eRad0);
        const TopoDS_Wire w0 = mkW0.Wire();

        BRepBuilderAPI_MakeFace mkF0(pln, w0, /*OnlyPlane=*/true);
        return mkF0.Face();
    }

    TopoDS_Face offset_face(const TopoDS_Face &face, double off) {
        std::cout << "offsetting face by " << off << "\n";
        BRepFill_OffsetWire mkOffset(face, GeomAbs_Arc);
        mkOffset.Perform(off);

        const TopoDS_Shape offShape = mkOffset.Shape();
        const TopoDS_Wire wOff = TopoDS::Wire(offShape);
        std::cout << "offset wire valid? " << (BRepCheck_Analyzer(wOff).IsValid() ? "yes" : "no") << "\n";

        // Build face from offset wire on the same plane
        BRepBuilderAPI_MakeFace mkF1(pln, wOff, /*OnlyPlane=*/true);
        mkF1.Build();
        return mkF1.Face();
    }

    TopoDS_Face clip_with_mask(const TopoDS_Face& face, const TopoDS_Face& mask) {
        BRepAlgoAPI_Common common(face, mask);
        common.SetFuzzyValue(1e-7);
        common.Build();

        // Extract first face
        TopoDS_Face clipped;
        for (TopExp_Explorer ex(common.Shape(), TopAbs_FACE); ex.More(); ex.Next()) {
            clipped = TopoDS::Face(ex.Current());
            break;
        }
        return clipped;
    }

    void export_to_brep(const TopoDS_Shape& shape, const std::string& filename) {
        std::ofstream file;
        file.open(filename);
        BRepTools::Write(shape, file, false, false, TopTools_FormatVersion_VERSION_2);
        file.close();
    }


}

TEST(Test2DFilletAfterOffset, with_offset) {

    // 1. Make test face (section of 2D hollow cylider)
    const TopoDS_Face& face0 = make_test_face();
    export_to_brep(face0, "test_with_offset_1_face0.brep");
    dumpFaceVerts(face0, "BASE FACE");

    // 2. Offset the face inward by 0.5
    auto face1 = offset_face(face0, 0.5);
    export_to_brep(face0, "test_with_offset_2_face1.brep");
    dumpFaceVerts(face1, "OFFSET FACE");

    // 3) Make a mask for clipping
    const double xCut = 0.0;
    const double pad = 200.0; // big enough but not insane
    BRepBuilderAPI_MakeFace mkMask(pln, -pad, xCut, -pad, pad);
    const TopoDS_Face& mask = mkMask.Face();
    export_to_brep(mask, "test_with_offset_3_mask.brep");


    // 4) Clip the offseted face with the mask.
    auto clipped = clip_with_mask(face1, mask);
    export_to_brep(clipped, "test_with_offset_4_clipped.brep");
    dumpFaceVerts(clipped, "CLIPPED FACE");


    // 4) Apply fillets to the edge of the clip (pick vertices on x=xCut)
    auto clip_verts = pickClipCornerVertices(clipped, xCut, 1e-7);
    constexpr double filletR = 0.1;
    BRepFilletAPI_MakeFillet2d fillet(clipped);
    fillet.AddFillet(clip_verts[0], filletR);
    fillet.AddFillet(clip_verts[1], filletR);
    fillet.Build();

    // This assertion is intentionally loose: the MWE is to reproduce the failure.
    // If it succeeds on your machine, change parameters (ang1/off/pad) until it fails.
    EXPECT_TRUE(fillet.IsDone()) << "Fillet2d failed, status=" << (int)fillet.Status();
    export_to_brep(fillet.Shape(), "test_with_offset_5_filleted.brep");

}

TEST(Test2DFilletAfterOffset, without_offset) {

    // 1. Make test face (section of 2D hollow cylider)
    const TopoDS_Face& face0 = make_test_face();
    export_to_brep(face0, "test_without_offset_1_face0.brep");
    dumpFaceVerts(face0, "BASE FACE");

    // 2. Skipping offset
        // Do nothing

    // 3. Make a mask for clipping
    const double xCut = 0.0;
    const double pad = 200.0; // big enough but not insane
    BRepBuilderAPI_MakeFace mkMask(pln, -pad, xCut, -pad, pad);
    const TopoDS_Face& mask = mkMask.Face();
    export_to_brep(mask, "test_without_offset_3_mask.brep");


    // 4. Clip the base face with the mask.
    auto clipped = clip_with_mask(face0, mask);
    export_to_brep(clipped, "test_without_offset_4_clipped.brep");
    dumpFaceVerts(clipped, "CLIPPED FACE");


    // 5. Apply fillets to the edge of the clip (pick vertices on x=xCut)
    auto clip_verts = pickClipCornerVertices(clipped, xCut, 1e-7);
    constexpr double filletR = 0.1;
    BRepFilletAPI_MakeFillet2d fillet(clipped);
    fillet.AddFillet(clip_verts[0], filletR);
    fillet.AddFillet(clip_verts[1], filletR);
    fillet.Build();

    // This assertion is intentionally loose: the MWE is to reproduce the failure.
    // If it succeeds on your machine, change parameters (ang1/off/pad) until it fails.
    EXPECT_TRUE(fillet.IsDone()) << "Fillet2d failed, status=" << (int)fillet.Status();
    export_to_brep(fillet.Shape(), "test_without_offset_5_filleted.brep");
}

Notes:

I did try to debug up to a point but couldn't find the root cause. Obviously, the suspicion is that the offset operation breaks some toplogy and/or geometry consistency. Same behavior observed for other GeomAbs_JoinType values of BRepFill_OffsetWire. In some cases, when attempting to fillet a line-line corner on the offseted-and-clipped-face, the fillet is successful. Behavior is deterministic, not random. Debug output also provides:

122: ...
122: offsetting face by 0.5
122: BRepFill_TrimEdgeTool: incoherent intersection. Try with a greater tolerance
122: BRepFill_TrimEdgeTool: incoherent intersection. Try with a greater tolerance
122: BRepFill_TrimEdgeTool: incoherent intersection. Try with a greater tolerance
122: BRepFill_TrimEdgeTool: incoherent intersection. Try with a greater tolerance
122: BRepFill_TrimEdgeTool: incoherent intersection. Try with a greater tolerance
122: BRepFill_TrimEdgeTool: incoherent intersection. Try with a greater tolerance
122: BRepFill_TrimEdgeTool: incoherent intersection. Try with a greater tolerance
122: SeanceDeRattrapage = 7
122: BRepFill_TrimEdgeTool: incoherent intersection
122: ...

but BRepFill_OffsetWire does not provide tolerance control.

I would appreciate a quick sanity-check if perhaps I am doing something that's not intended by design on this operations. I'm sure I can work around this issue, but it seems more like a bug to me worth fixing.

Thank you!
~ Gordan