[Mesh] TriangleMesh's "+=" operator appends UVs regardless of the pre…

…sence of existing features (#6728)

* [Mesh] TriangleMesh's "+=" operator applies regardless of the presence of existing features.

* added change log

* fixed changelog

* add special case for += operator with empty mesh

* remove blank line in changelog

* adding unit test to test += operator for triangle mesh

---------

Co-authored-by: Benjamin Ummenhofer <benjamin.ummenhofer@intel.com>

CHANGELOG.md

@@ -37,6 +37,7 @@
 -   Fix KDTreeFlann possibly using a dangling pointer instead of internal storage and simplified its members (PR #6734)
 -   Fix RANSAC early stop if no inliers in a specific iteration (PR #6789)
 -   Fix segmentation fault (infinite recursion) of DetectPlanarPatches if multiple points have same coordinates (PR #6794)
+-   `TriangleMesh`'s `+=` operator appends UVs regardless of the presence of existing features (PR #6728)
 -   Fix build with fmt v10.2.0 (#6783)
 -   Fix segmentation fault (lambda reference capture) of VisualizerWithCustomAnimation::Play (PR #6804)
 -   Add O3DVisualizer API to enable collapse control of verts in the side panel (PR #6865)

cpp/open3d/geometry/TriangleMesh.cpp

@@ -52,9 +52,12 @@ TriangleMesh &TriangleMesh::Rotate(const Eigen::Matrix3d &R,
 
 TriangleMesh &TriangleMesh::operator+=(const TriangleMesh &mesh) {
     if (mesh.IsEmpty()) return (*this);
-    bool add_textures = HasTriangleUvs() && HasTextures() &&
-                        HasTriangleMaterialIds() && mesh.HasTriangleUvs() &&
-                        mesh.HasTextures() && mesh.HasTriangleMaterialIds();
+    bool is_empty = IsEmpty();
+    bool add_triangle_uvs =
+            mesh.HasTriangleUvs() && (HasTriangleUvs() || is_empty);
+    bool add_textures_and_material_ids =
+            mesh.HasTextures() && mesh.HasTriangleMaterialIds() &&
+            ((HasTextures() && HasTriangleMaterialIds()) || is_empty);
     size_t old_vert_num = vertices_.size();
     MeshBase::operator+=(mesh);
     size_t old_tri_num = triangles_.size();
@@ -77,19 +80,21 @@ TriangleMesh &TriangleMesh::operator+=(const TriangleMesh &mesh) {
     if (HasAdjacencyList()) {
         ComputeAdjacencyList();
     }
-    if (add_textures) {
+    if (add_triangle_uvs) {
         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];
         }
-
+    } else {
+        triangle_uvs_.clear();
+    }
+    if (add_textures_and_material_ids) {
         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];
         }
-
         size_t old_mat_id_num = triangle_material_ids_.size();
         triangle_material_ids_.resize(old_mat_id_num +
                                       mesh.triangle_material_ids_.size());
@@ -98,7 +103,6 @@ TriangleMesh &TriangleMesh::operator+=(const TriangleMesh &mesh) {
                     mesh.triangle_material_ids_[i] + (int)old_tex_num;
         }
     } else {
-        triangle_uvs_.clear();
         textures_.clear();
         triangle_material_ids_.clear();
     }

cpp/tests/geometry/TriangleMesh.cpp

@@ -22,8 +22,8 @@ namespace tests {
 /// \param eq_triangle_vertex_order If true then triangles are only equal if the
 /// order of the vertices is the same. If false any permutation of the triangle
 /// indices is allowed.
-void ExpectMeshEQ(const open3d::geometry::TriangleMesh& mesh0,
-                  const open3d::geometry::TriangleMesh& mesh1,
+void ExpectMeshEQ(const open3d::geometry::TriangleMesh &mesh0,
+                  const open3d::geometry::TriangleMesh &mesh1,
                   double threshold = 1e-6,
                   bool eq_triangle_vertex_order = true) {
     ExpectEQ(mesh0.vertices_, mesh1.vertices_, threshold);
@@ -599,6 +599,125 @@ TEST(TriangleMesh, OperatorADD) {
     }
 }
 
+TEST(TriangleMesh, OperatorAdditionAssignment) {
+    const size_t size = 100;
+    const size_t texture_size = 10;
+
+    // Define the minimum and maximum bounds for random data generation
+    Eigen::Vector3d dmin(0.0, 0.0, 0.0);
+    Eigen::Vector3d dmax(1000.0, 1000.0, 1000.0);
+    Eigen::Vector3i imin(0, 0, 0);
+    Eigen::Vector3i imax(size - 1, size - 1, size - 1);
+    Eigen::Vector2d uvmin(0.0, 0.0);
+    Eigen::Vector2d uvmax(1.0, 1.0);
+
+    // Mesh 0 contains only UVs but no textures and material ids
+    geometry::TriangleMesh tm0;
+    tm0.vertices_.resize(size);
+    tm0.triangles_.resize(size);
+    tm0.triangle_uvs_.resize(3 * size);
+
+    // Mesh 1 contains all of UVS, textures and material ids
+    geometry::TriangleMesh tm1;
+    tm1.vertices_.resize(size);
+    tm1.triangles_.resize(size);
+    tm1.triangle_uvs_.resize(3 * size);
+    tm1.triangle_material_ids_.resize(size);
+    tm1.textures_.resize(texture_size);
+
+    // Mesh 2 does not contains any of UVs, textures or material ids
+    geometry::TriangleMesh tm2;
+    tm2.vertices_.resize(size);
+    tm2.triangles_.resize(size);
+
+    // Randomly generate data for tm0, tm1 and tm2
+    Rand(tm0.vertices_, dmin, dmax, 0);
+    Rand(tm0.triangles_, imin, imax, 0);
+    std::vector<Eigen::Vector2d, open3d::utility::Vector2d_allocator>
+            tm0_triangle_uvs(3 * size);
+    Rand(tm0_triangle_uvs, uvmin, uvmax, 0);
+    tm0.triangle_uvs_ = std::vector<Eigen::Vector2d>(tm0_triangle_uvs.begin(),
+                                                     tm0_triangle_uvs.end());
+
+    Rand(tm1.vertices_, dmin, dmax, 0);
+    Rand(tm1.triangles_, imin, imax, 0);
+    std::vector<Eigen::Vector2d, open3d::utility::Vector2d_allocator>
+            tm1_triangle_uvs(3 * size);
+    Rand(tm1_triangle_uvs, uvmin, uvmax, 0);
+    tm1.triangle_uvs_ = std::vector<Eigen::Vector2d>(tm1_triangle_uvs.begin(),
+                                                     tm1_triangle_uvs.end());
+    for (size_t i = 0; i < texture_size; i++) {
+        geometry::Image texture_image;
+        texture_image.Prepare(100, 100, 3, 1);
+        Rand(texture_image.data_, 0, 255, 0);
+        tm1.textures_[i] = texture_image;
+    }
+    Rand(tm1.triangle_material_ids_, 0, texture_size, 0);
+
+    Rand(tm2.vertices_, dmin, dmax, 0);
+    Rand(tm2.triangles_, imin, imax, 0);
+
+    geometry::TriangleMesh temp_tm0 = tm0;
+    geometry::TriangleMesh temp_tm1 = tm1;
+    geometry::TriangleMesh temp_tm2 = tm2;
+    // Function to reset values of temp_tm0, temp_tm1 and temp_tm2
+    auto reset_values = [&tm0, &tm1, &tm2](geometry::TriangleMesh &temp_tm0,
+                                           geometry::TriangleMesh &temp_tm1,
+                                           geometry::TriangleMesh &temp_tm2) {
+        temp_tm0 = tm0;
+        temp_tm1 = tm1;
+        temp_tm2 = tm2;
+    };
+
+    // Test addition of tm1 with itself
+    temp_tm1 += tm1;
+    EXPECT_EQ(temp_tm1.triangles_.size(), 2 * size);
+    EXPECT_EQ(temp_tm1.triangle_uvs_.size(), 6 * size);
+    EXPECT_EQ(temp_tm1.vertices_.size(), 2 * size);
+    EXPECT_EQ(temp_tm1.textures_.size(), 2 * texture_size);
+    EXPECT_EQ(temp_tm1.triangle_material_ids_.size(), 2 * size);
+    for (size_t i = 0; i < size; i++) {
+        ExpectEQ(temp_tm1.vertices_[i], temp_tm1.vertices_[i + size]);
+        ExpectEQ(temp_tm1.triangles_[i],
+                 (Eigen::Vector3i)(temp_tm1.triangles_[i + size] -
+                                   Eigen::Vector3i(size, size, size)));
+        EXPECT_EQ(temp_tm1.triangle_material_ids_[i],
+                  temp_tm1.triangle_material_ids_[i + size] - texture_size);
+    }
+    for (size_t i = 0; i < 3 * size; i++) {
+        ExpectEQ(temp_tm1.triangle_uvs_[i],
+                 temp_tm1.triangle_uvs_[i + 3 * size]);
+    }
+    for (size_t i = 0; i < texture_size; i++) {
+        ExpectEQ(temp_tm1.textures_[i].data_,
+                 temp_tm1.textures_[i + texture_size].data_);
+    }
+
+    // Test addition of tm0 and tm1
+    reset_values(temp_tm0, temp_tm1, temp_tm2);
+    temp_tm0 += tm1;
+    temp_tm1 += tm0;
+    ExpectMeshEQ(temp_tm0, temp_tm1);
+    EXPECT_EQ(temp_tm0.textures_.size(), 0);
+    EXPECT_EQ(temp_tm0.triangle_material_ids_.size(), 0);
+
+    // Test addition of tm1 and tm2
+    reset_values(temp_tm0, temp_tm1, temp_tm2);
+    temp_tm1 += tm2;
+    temp_tm2 += tm1;
+    ExpectMeshEQ(temp_tm1, temp_tm2);
+    EXPECT_EQ(temp_tm0.textures_.size(), 0);
+    EXPECT_EQ(temp_tm0.triangle_material_ids_.size(), 0);
+
+    // Test addition of tm2 and tm0
+    reset_values(temp_tm0, temp_tm1, temp_tm2);
+    temp_tm2 += tm0;
+    temp_tm0 += tm2;
+    ExpectMeshEQ(temp_tm2, temp_tm0);
+    EXPECT_EQ(temp_tm0.textures_.size(), 0);
+    EXPECT_EQ(temp_tm0.triangle_material_ids_.size(), 0);
+}
+
 TEST(TriangleMesh, ComputeTriangleNormals) {
     std::vector<Eigen::Vector3d> ref = {{-0.119231, 0.738792, 0.663303},
                                         {-0.115181, 0.730934, 0.672658},