Warn if alpha shape reconstruction has alpha too small for point scal…

…e and results in empty mesh (#6998)

CHANGELOG.md

@@ -47,6 +47,7 @@
 - Fix projection of point cloud to Depth/RGBD image if no position attribute is provided (PR #6880)
 - Support lowercase types when reading PCD files (PR #6930)
 - Fix visualization/draw ICP example and add warnings (PR #6933)
+- Fix alpha shape reconstruction if alpha too small for point scale (PR #6998)
 
 ## 0.13
 

cpp/open3d/geometry/SurfaceReconstructionAlphaShape.cpp

@@ -158,28 +158,37 @@ std::shared_ptr<TriangleMesh> TriangleMesh::CreateFromPointCloudAlphaShape(
  "[CreateFromPointCloudAlphaShape] done remove duplicate triangles "
  "and unreferenced vertices");
 
- auto tmesh = t::geometry::TriangleMesh::FromLegacy(*mesh);
-
- // use new object tmesh2 here even if some arrays share memory with tmesh.
- // We don't want to replace the blobs in tmesh.
- auto tmesh2 = t::geometry::vtkutils::ComputeNormals(
- tmesh, /*vertex_normals=*/true, /*face_normals=*/false,
- /*consistency=*/true, /*auto_orient_normals=*/true,
- /*splitting=*/false);
-
- mesh->vertices_ = core::eigen_converter::TensorToEigenVector3dVector(
- tmesh2.GetVertexPositions());
- mesh->triangles_ = core::eigen_converter::TensorToEigenVector3iVector(
- tmesh2.GetTriangleIndices());
- if (mesh->HasVertexColors()) {
- mesh->vertex_colors_ =
- core::eigen_converter::TensorToEigenVector3dVector(
- tmesh2.GetVertexColors());
- }
- if (mesh->HasVertexNormals()) {
- mesh->vertex_normals_ =
- core::eigen_converter::TensorToEigenVector3dVector(
- tmesh2.GetVertexNormals());
+ if (mesh->vertices_.size() > 0) {
+ auto tmesh = t::geometry::TriangleMesh::FromLegacy(*mesh);
+
+ // use new object tmesh2 here even if some arrays share memory with
+ // tmesh. We don't want to replace the blobs in tmesh.
+ auto tmesh2 = t::geometry::vtkutils::ComputeNormals(
+ tmesh, /*vertex_normals=*/true, /*face_normals=*/false,
+ /*consistency=*/true, /*auto_orient_normals=*/true,
+ /*splitting=*/false);
+
+ mesh->vertices_ = core::eigen_converter::TensorToEigenVector3dVector(
+ tmesh2.GetVertexPositions());
+ mesh->triangles_ = core::eigen_converter::TensorToEigenVector3iVector(
+ tmesh2.GetTriangleIndices());
+ if (mesh->HasVertexColors()) {
+ mesh->vertex_colors_ =
+ core::eigen_converter::TensorToEigenVector3dVector(
+ tmesh2.GetVertexColors());
+ }
+ if (mesh->HasVertexNormals()) {
+ mesh->vertex_normals_ =
+ core::eigen_converter::TensorToEigenVector3dVector(
+ tmesh2.GetVertexNormals());
+ }
+ } else {
+ utility::LogWarning(
+ fmt::format("[CreateFromPointCloudAlphaShape] alpha shape "
+ "reconstruction resulted in empty mesh. Alpha "
+ "value ({}) could be too small.",
+ alpha)
+ .c_str());
  }
 
  return mesh;