Fix remeshing of open meshes (#2672)

Libs/Mesh/Mesh.cpp

@@ -490,7 +490,24 @@ Mesh& Mesh::remesh(int numVertices, double adaptivity) {
 
   numVertices = std::max<int>(numVertices, 1);
 
-  remesh->SetForceManifold(true);
+  // ACVD's ForceManifold treats clusters on an open boundary as non-manifold and keeps splitting
+  // them, piling extra vertices and sliver triangles along the boundary (e.g. a mesh clipped by a
+  // cutting plane ends up with more vertices than requested). Only force manifold on closed meshes.
+  // geometry only: vtkFeatureEdges copies point data, which reads out of bounds on a field that
+  // doesn't have one value per point
+  auto geometry = vtkSmartPointer<vtkPolyData>::New();
+  geometry->SetPoints(this->poly_data_->GetPoints());
+  geometry->SetPolys(this->poly_data_->GetPolys());
+  auto boundary_edges = vtkSmartPointer<vtkFeatureEdges>::New();
+  boundary_edges->SetInputData(geometry);
+  boundary_edges->BoundaryEdgesOn();
+  boundary_edges->FeatureEdgesOff();
+  boundary_edges->NonManifoldEdgesOff();
+  boundary_edges->ManifoldEdgesOff();
+  boundary_edges->Update();
+  bool is_closed = boundary_edges->GetOutput()->GetNumberOfCells() == 0;
+
+  remesh->SetForceManifold(is_closed);
   remesh->SetInput(surf);
   remesh->SetFileLoadSaveOption(0);
   remesh->SetConsoleOutput(0);

Testing/MeshTests/MeshTests.cpp

@@ -359,6 +359,16 @@ TEST(MeshTests, remeshPercentTest2) {
   ASSERT_TRUE(femur == ground_truth);
 }
 
+// Remeshing an open (clipped) mesh should hit the requested vertex count instead of piling extra
+// vertices along the open boundary (#2672)
+TEST(MeshTests, remeshPercentOpenMeshTest) {
+  Mesh femur(std::string(TEST_DATA_DIR) + "/femur.vtk");
+  femur.clip(makePlane(Point3({-91.0, 0.0, 1230.0}), makeVector({0.0, 0.0, 1.0})));
+  auto num_points = femur.numPoints();
+  femur.remeshPercent(0.40, 1.0);
+  ASSERT_NEAR(femur.numPoints(), num_points * 0.40, num_points * 0.02);
+}
+
 TEST(MeshTests, fieldGradientFloatTest) {
   // fields arrive on meshes as floats as often as doubles, and the gradient has to be computed for
   // either: a missing gradient array used to be read as a null pointer