Support OBJ specified only a part of vertex colors

The omitted vertex colors are treated as (0, 0, 0, 1).

e.g

v  0.0  0.0  0.0
v  0.0  0.0  1.0  0.0  0.0  0.0
v  0.0  1.0  0.0
v  1.0  0.0  0.0  1.0  0.6  0.3
v  1.0  1.0  0.0

code/AssetLib/Obj/ObjFileData.h

@@ -239,8 +239,6 @@ struct Mesh {
     unsigned int m_uiMaterialIndex;
     /// True, if normals are stored.
     bool m_hasNormals;
-    /// True, if vertex colors are stored.
-    bool m_hasVertexColors;
 
     /// Constructor
     explicit Mesh(const std::string &name) :

code/AssetLib/Obj/ObjFileParser.cpp

@@ -156,9 +156,17 @@ void ObjFileParser::parseFile(IOStreamBuffer<char> &streamBuffer) {
                     // read in vertex definition (homogeneous coords)
                     getHomogeneousVector3(m_pModel->mVertices);
                 } else if (numComponents == 6) {
+                    // fill previous omitted vertex-colors by default
+                    if (m_pModel->mVertexColors.size() < m_pModel->mVertices.size()) {
+                        m_pModel->mVertexColors.resize(m_pModel->mVertices.size(), aiVector3D(0, 0, 0));
+                    }
                     // read vertex and vertex-color
                     getTwoVectors3(m_pModel->mVertices, m_pModel->mVertexColors);
                 }
+                // append omitted vertex-colors as default for the end if any vertex-color exists
+                if (!m_pModel->mVertexColors.empty() && m_pModel->mVertexColors.size() < m_pModel->mVertices.size()) {
+                    m_pModel->mVertexColors.resize(m_pModel->mVertices.size(), aiVector3D(0, 0, 0));
+                }
             } else if (*m_DataIt == 't') {
                 // read in texture coordinate ( 2D or 3D )
                 ++m_DataIt;

test/models/OBJ/only_a_part_of_vertexcolors.obj

@@ -0,0 +1,11 @@
+g cube
+
+v  0.0  0.0  0.0
+v  0.0  0.0  1.0  0.0  0.0  1.0
+v  0.0  1.0  0.0
+v  1.0  0.0  0.0  1.0  0.6  0.3
+v  1.0  1.0  0.0
+
+f 1 2 3
+f 1 4 3
+f 2 5 4

test/unit/utObjImportExport.cpp

@@ -286,6 +286,54 @@ TEST_F(utObjImportExport, issue1923_vertex_color_Test) {
     delete scene;
 }
 
+TEST_F(utObjImportExport, only_a_part_of_vertex_colors_Test) {
+    ::Assimp::Importer importer;
+    const aiScene *const scene = importer.ReadFile(ASSIMP_TEST_MODELS_DIR "/OBJ/only_a_part_of_vertexcolors.obj", aiProcess_ValidateDataStructure);
+    EXPECT_NE(nullptr, scene);
+
+    EXPECT_EQ(scene->mNumMeshes, 1U);
+    const aiMesh *const mesh = scene->mMeshes[0];
+    EXPECT_EQ(mesh->mNumVertices, 9U);
+    EXPECT_EQ(mesh->mNumFaces, 3U);
+    EXPECT_TRUE(mesh->HasVertexColors(0));
+
+    const aiVector3D *const vertices = mesh->mVertices;
+    const aiColor4D *const colors = mesh->mColors[0];
+    EXPECT_EQ(aiVector3D(0.0f, 0.0f, 0.0f), vertices[0]);
+    EXPECT_EQ(aiColor4D(0.0f, 0.0f, 0.0f, 1.0f), colors[0]);
+    EXPECT_EQ(aiVector3D(0.0f, 0.0f, 1.0f), vertices[1]);
+    EXPECT_EQ(aiColor4D(0.0f, 0.0f, 1.0f, 1.0f), colors[1]);
+    EXPECT_EQ(aiVector3D(0.0f, 1.0f, 0.0f), vertices[2]);
+    EXPECT_EQ(aiColor4D(0.0f, 0.0f, 0.0f, 1.0f), colors[2]);
+    EXPECT_EQ(aiVector3D(0.0f, 0.0f, 0.0f), vertices[3]);
+    EXPECT_EQ(aiColor4D(0.0f, 0.0f, 0.0f, 1.0f), colors[3]);
+    EXPECT_EQ(aiVector3D(1.0f, 0.0f, 0.0f), vertices[4]);
+    EXPECT_EQ(aiColor4D(1.0f, 0.6f, 0.3f, 1.0f), colors[4]);
+    EXPECT_EQ(aiVector3D(0.0f, 1.0f, 0.0f), vertices[5]);
+    EXPECT_EQ(aiColor4D(0.0f, 0.0f, 0.0f, 1.0f), colors[5]);
+    EXPECT_EQ(aiVector3D(0.0f, 0.0f, 1.0f), vertices[6]);
+    EXPECT_EQ(aiColor4D(0.0f, 0.0f, 1.0f, 1.0f), colors[6]);
+    EXPECT_EQ(aiVector3D(1.0f, 1.0f, 0.0f), vertices[7]);
+    EXPECT_EQ(aiColor4D(0.0f, 0.0f, 0.0f, 1.0f), colors[7]);
+    EXPECT_EQ(aiVector3D(1.0f, 0.0f, 0.0f), vertices[8]);
+    EXPECT_EQ(aiColor4D(1.0f, 0.6f, 0.3f, 1.0f), colors[8]);
+
+#ifndef ASSIMP_BUILD_NO_EXPORT
+    ::Assimp::Exporter exporter;
+    EXPECT_EQ(aiReturn_SUCCESS, exporter.Export(scene, "obj", ASSIMP_TEST_MODELS_DIR "/OBJ/test_out.obj"));
+#endif // ASSIMP_BUILD_NO_EXPORT
+}
+
+TEST_F(utObjImportExport, no_vertex_colors_Test) {
+    ::Assimp::Importer importer;
+    const aiScene *const scene = importer.ReadFile(ASSIMP_TEST_MODELS_DIR "/OBJ/box.obj", aiProcess_ValidateDataStructure);
+    EXPECT_NE(nullptr, scene);
+
+    EXPECT_EQ(scene->mNumMeshes, 1U);
+    const aiMesh *const mesh = scene->mMeshes[0];
+    EXPECT_FALSE(mesh->HasVertexColors(0));
+}
+
 TEST_F(utObjImportExport, issue1453_segfault) {
     static const char *curObjModel =
             "v  0.0  0.0  0.0\n"