From 023465bfe6fd51000f5c560c391683d1079ea621 Mon Sep 17 00:00:00 2001
From: Benjamin Ummenhofer <benjamin.ummenhofer@intel.com>
Date: Tue, 11 Jun 2024 15:19:32 +0200
Subject: [PATCH] add special case for += operator with empty mesh

---
 cpp/open3d/geometry/TriangleMesh.cpp | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/cpp/open3d/geometry/TriangleMesh.cpp b/cpp/open3d/geometry/TriangleMesh.cpp
index b199edff4de..90c393b5ea3 100644
--- a/cpp/open3d/geometry/TriangleMesh.cpp
+++ b/cpp/open3d/geometry/TriangleMesh.cpp
@@ -52,6 +52,7 @@ TriangleMesh &TriangleMesh::Rotate(const Eigen::Matrix3d &R,
 
 TriangleMesh &TriangleMesh::operator+=(const TriangleMesh &mesh) {
     if (mesh.IsEmpty()) return (*this);
+    bool is_empty = IsEmpty();
     size_t old_vert_num = vertices_.size();
     MeshBase::operator+=(mesh);
     size_t old_tri_num = triangles_.size();
@@ -74,21 +75,22 @@ TriangleMesh &TriangleMesh::operator+=(const TriangleMesh &mesh) {
     if (HasAdjacencyList()) {
         ComputeAdjacencyList();
     }
-    if (mesh.HasTriangleUvs()) {
+    if (mesh.HasTriangleUvs() && (HasTriangleUvs() || is_empty)) {
         size_t old_tri_uv_num = triangle_uvs_.size();
         triangle_uvs_.resize(old_tri_uv_num + mesh.triangle_uvs_.size());
         for (size_t i = 0; i < mesh.triangle_uvs_.size(); i++) {
             triangle_uvs_[old_tri_uv_num + i] = mesh.triangle_uvs_[i];
         }
     }
-    if (mesh.HasTextures()) {
+    if (mesh.HasTextures() && (HasTextures() || is_empty)) {
         size_t old_tex_num = textures_.size();
         textures_.resize(old_tex_num + mesh.textures_.size());
         for (size_t i = 0; i < mesh.textures_.size(); i++) {
             textures_[old_tex_num + i] = mesh.textures_[i];
         }
     }
-    if (mesh.HasTriangleMaterialIds()) {
+    if (mesh.HasTriangleMaterialIds() &&
+        (HasTriangleMaterialIds() || is_empty)) {
         size_t old_tex_num = textures_.size();
         size_t old_mat_id_num = triangle_material_ids_.size();
         triangle_material_ids_.resize(old_mat_id_num +