Fix for Issue #5924:

CHANGELOG.md

@@ -15,6 +15,7 @@
 * Fix raycasting scene: Allow setting of number of threads that are used for building a raycasting scene
 * Fix Python bindings for CUDA device synchronization, voxel grid saving (PR #5425)
 * Support msgpack versions without cmake
+* Changed TriangleMesh to store materials in a list so they can be accessed by the material index (PR #5938)
 * Support multi-threading in the RayCastingScene function to commit scene (PR #6051).
 * Fix some bad triangle generation in TriangleMesh::SimplifyQuadricDecimation
 

cpp/open3d/geometry/TriangleMesh.h

@@ -853,7 +853,7 @@ class TriangleMesh : public MeshBase {
         std::unordered_map<std::string, Image> additionalMaps;
     };
 
-    std::unordered_map<std::string, Material> materials_;
+    std::vector<std::pair<std::string, Material>> materials_;
 
     /// List of material ids.
     std::vector<int> triangle_material_ids_;

cpp/open3d/io/file_format/FileASSIMP.cpp

@@ -237,12 +237,13 @@ bool ReadTriangleMeshUsingASSIMP(
     }
 
     // Now load the materials
+    mesh.materials_.resize(scene->mNumMaterials);
     for (size_t i = 0; i < scene->mNumMaterials; ++i) {
         auto* mat = scene->mMaterials[i];
 
-        // create material structure to match this name
-        auto& mesh_material =
-                mesh.materials_[std::string(mat->GetName().C_Str())];
+        // Set the material structure to match this name
+        auto& mesh_material = mesh.materials_[i].second;
+        mesh.materials_[i].first = mat->GetName().C_Str();
 
         using MaterialParameter =
                 geometry::TriangleMesh::Material::MaterialParameter;
@@ -277,9 +278,9 @@ bool ReadTriangleMeshUsingASSIMP(
 
         // For legacy visualization support
         if (mesh_material.albedo) {
-            mesh.textures_.push_back(*mesh_material.albedo->FlipVertical());
+            mesh.textures_.emplace_back(*mesh_material.albedo->FlipVertical());
         } else {
-            mesh.textures_.push_back(geometry::Image());
+            mesh.textures_.emplace_back();
         }
     }
 

cpp/open3d/io/file_format/FileOBJ.cpp

@@ -140,8 +140,11 @@ bool ReadTriangleMeshFromOBJ(const std::string& filename,
     using MaterialParameter =
             geometry::TriangleMesh::Material::MaterialParameter;
 
-    for (auto& material : materials) {
-        auto& meshMaterial = mesh.materials_[material.name];
+    mesh.materials_.resize(materials.size());
+    for (std::size_t i = 0; i < materials.size(); ++i) {
+        auto& material = materials[i];
+        mesh.materials_[i].first = material.name;
+        auto& meshMaterial = mesh.materials_[i].second;
 
         meshMaterial.baseColor = MaterialParameter::CreateRGB(
                 material.diffuse[0], material.diffuse[1], material.diffuse[2]);

cpp/open3d/t/geometry/TriangleMesh.cpp

@@ -409,7 +409,9 @@ open3d::geometry::TriangleMesh TriangleMesh::ToLegacy() const {
     // Convert material if the t geometry has a valid one
     auto &tmat = GetMaterial();
     if (tmat.IsValid()) {
-        auto &legacy_mat = mesh_legacy.materials_["Mat1"];
+        mesh_legacy.materials_.emplace_back();
+        mesh_legacy.materials_.front().first = "Mat1";
+        auto &legacy_mat = mesh_legacy.materials_.front().second;
         // Convert scalar properties
         if (tmat.HasBaseColor()) {
             legacy_mat.baseColor.f4[0] = tmat.GetBaseColor().x();

cpp/open3d/visualization/visualizer/O3DVisualizer.cpp

@@ -941,11 +941,24 @@ struct O3DVisualizer::Impl {
 
             // Finally assign material properties if geometry is a triangle mesh
             if (tmesh && tmesh->materials_.size() > 0) {
-                // Only a single material is supported for TriangleMesh so we
-                // just grab the first one we find. Users should be using
-                // TriangleMeshModel if they have a model with multiple
-                // materials.
-                auto &mesh_material = tmesh->materials_.begin()->second;
+                std::size_t material_index;
+                if (tmesh->HasTriangleMaterialIds()) {
+                    auto minmax_it = std::minmax_element(
+                            tmesh->triangle_material_ids_.begin(),
+                            tmesh->triangle_material_ids_.end());
+                    if (*minmax_it.first != *minmax_it.second) {
+                        utility::LogWarning(
+                                "Only a single material is "
+                                "supported for TriangleMesh visualization, "
+                                "only the first referenced material will be "
+                                "used. Use TriangleMeshModel if more than one "
+                                "material is required.");
+                    }
+                    material_index = *minmax_it.first;
+                } else {
+                    material_index = 0;
+                }
+                auto &mesh_material = tmesh->materials_[material_index].second;
                 mat.base_color = {mesh_material.baseColor.r(),
                                   mesh_material.baseColor.g(),
                                   mesh_material.baseColor.b(),

cpp/tests/t/geometry/TriangleMesh.cpp

@@ -394,7 +394,9 @@ TEST_P(TriangleMeshPermuteDevices, FromLegacy) {
             Eigen::Vector2d(0.4, 0.5), Eigen::Vector2d(0.6, 0.7),
             Eigen::Vector2d(0.8, 0.9), Eigen::Vector2d(1.0, 1.1)};
 
-    auto& mat = legacy_mesh.materials_["Mat1"];
+    legacy_mesh.materials_.emplace_back();
+    legacy_mesh.materials_.front().first = "Mat1";
+    auto& mat = legacy_mesh.materials_.front().second;
     mat.baseColor = mat.baseColor.CreateRGB(1, 1, 1);
 
     core::Dtype float_dtype = core::Float32;
@@ -497,8 +499,11 @@ TEST_P(TriangleMeshPermuteDevices, ToLegacy) {
                                   Pointwise(FloatEq(), {0.8, 0.9}),
                                   Pointwise(FloatEq(), {1.0, 1.1})}));
 
-    EXPECT_TRUE(legacy_mesh.materials_.count("Mat1") > 0);
-    auto& mat = legacy_mesh.materials_["Mat1"];
+    auto mat_iterator = std::find_if(
+            legacy_mesh.materials_.begin(), legacy_mesh.materials_.end(),
+            [](const auto& pair) -> bool { return pair.first == "Mat1"; });
+    EXPECT_TRUE(mat_iterator != legacy_mesh.materials_.end());
+    auto& mat = mat_iterator->second;
     EXPECT_TRUE(Eigen::Vector4f(mat.baseColor.f4) ==
                 Eigen::Vector4f(1, 1, 1, 1));
     EXPECT_TRUE(mat.baseMetallic == 0.0);