G4OCCT_STEPSolid_impl.cc

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// SPDX-License-Identifier: LGPL-2.1-or-later
// Copyright (C) 2026 G4OCCT Contributors
 
 
#include "G4OCCT_STEPSolid_impl.hh"
 
#include "G4OCCT/G4OCCTSolid.hh"
 
#include <BRepMesh_IncrementalMesh.hxx>
#include <BRep_Tool.hxx>
#include <Poly_Triangulation.hxx>
#include <TopAbs_Orientation.hxx>
#include <TopExp_Explorer.hxx>
#include <TopoDS.hxx>
#include <TopoDS_Face.hxx>
 
#include <stdexcept>
#include <string>
 
G4OCCT_STEPSolidGeometry G4OCCT_ImportSTEPSolid(const std::string& name, const std::string& path) {
  G4OCCTSolid* solid = nullptr;
  try {
    solid = G4OCCTSolid::FromSTEP(name, path);
  } catch (const std::exception& ex) {
    throw std::runtime_error("G4OCCT_STEPSolid: failed to import '" + path + "' (" + ex.what() +
                             ")");
  }
 
  // Copy the shape out before deleting the solid so we don't hold a reference
  // into a deleted object.  G4OCCTSolid::FromSTEP() returns a heap-allocated
  // object owned by the caller; the Geant4 solid store keeps a separate entry.
  const TopoDS_Shape shape = solid->GetOCCTShape();
  delete solid;
  solid = nullptr;
 
  // Tessellate with 1 % relative deflection, matching G4OCCTSolid::CreatePolyhedron().
  static constexpr double kRelativeDeflection = 0.01;
  BRepMesh_IncrementalMesh mesher(shape, kRelativeDeflection,
                                  /*isRelative=*/Standard_True);
  (void)mesher;
 
  G4OCCT_STEPSolidGeometry result;
 
  for (TopExp_Explorer explorer(shape, TopAbs_FACE); explorer.More(); explorer.Next()) {
    const TopoDS_Face& face = TopoDS::Face(explorer.Current());
    TopLoc_Location location;
    const Handle(Poly_Triangulation) & tri = BRep_Tool::Triangulation(face, location);
    if (tri.IsNull() || tri->NbTriangles() == 0) {
      continue;
    }
 
    const gp_Trsf& transform  = location.Transformation();
    const bool reverseWinding = (face.Orientation() == TopAbs_REVERSED);
 
    for (Standard_Integer i = 1; i <= tri->NbTriangles(); ++i) {
      Standard_Integer n1, n2, n3;
      tri->Triangle(i).Get(n1, n2, n3);
      if (reverseWinding) {
        std::swap(n2, n3);
      }
 
      const gp_Pnt p1 = tri->Node(n1).Transformed(transform);
      const gp_Pnt p2 = tri->Node(n2).Transformed(transform);
      const gp_Pnt p3 = tri->Node(n3).Transformed(transform);
 
      G4OCCT_Triangle facet;
      facet.v[0] = {p1.X(), p1.Y(), p1.Z()};
      facet.v[1] = {p2.X(), p2.Y(), p2.Z()};
      facet.v[2] = {p3.X(), p3.Y(), p3.Z()};
      result.triangles.push_back(facet);
    }
  }
 
  if (result.triangles.empty()) {
    throw std::runtime_error("G4OCCT_STEPSolid: tessellation of '" + path +
                             "' produced no triangles");
  }
  return result;
}