fix alpha shape reconstruction if alpha too small for point scale

CHANGELOG.md

@@ -46,6 +46,7 @@
 -   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;