Fix material index off-by-one error in some OBJ files (seen in a C4D export)

code/ObjFileMtlImporter.cpp

@@ -303,11 +303,12 @@ void ObjFileMtlImporter::createMaterial()
         // New Material created
         m_pModel->m_pCurrentMaterial = new ObjFile::Material();
         m_pModel->m_pCurrentMaterial->MaterialName.Set( name );
+        m_pModel->m_MaterialLib.push_back( name );
+        m_pModel->m_MaterialMap[ name ] = m_pModel->m_pCurrentMaterial;
+
         if (m_pModel->m_pCurrentMesh) {
             m_pModel->m_pCurrentMesh->m_uiMaterialIndex = static_cast<unsigned int>(m_pModel->m_MaterialLib.size() - 1);
         }
-        m_pModel->m_MaterialLib.push_back( name );
-        m_pModel->m_MaterialMap[ name ] = m_pModel->m_pCurrentMaterial;
     } else {
         // Use older material
         m_pModel->m_pCurrentMaterial = (*it).second;

test/models/OBJ/cube_mtllib_after_g.mtl

@@ -0,0 +1,5 @@
+newmtl MyMaterial
+Ka 1.000 1.000 1.000
+Kd 1.000 1.000 1.000
+Ns 200.000
+Ks 0.050 0.050 0.050

test/models/OBJ/cube_mtllib_after_g.obj

@@ -0,0 +1,32 @@
+g Object
+mtllib cube_mtllib_after_g.mat
+usemtl MyMaterial
+
+v  0.0  0.0  0.0
+v  0.0  0.0  1.0
+v  0.0  1.0  0.0
+v  0.0  1.0  1.0
+v  1.0  0.0  0.0
+v  1.0  0.0  1.0
+v  1.0  1.0  0.0
+v  1.0  1.0  1.0
+
+vn  0.0  0.0  1.0
+vn  0.0  0.0 -1.0
+vn  0.0  1.0  0.0
+vn  0.0 -1.0  0.0
+vn  1.0  0.0  0.0
+vn -1.0  0.0  0.0
+
+f  1//2  7//2  5//2
+f  1//2  3//2  7//2 
+f  1//6  4//6  3//6 
+f  1//6  2//6  4//6 
+f  3//3  8//3  7//3 
+f  3//3  4//3  8//3 
+f  5//5  7//5  8//5 
+f  5//5  8//5  6//5 
+f  1//4  5//4  6//4 
+f  1//4  6//4  2//4 
+f  2//1  6//1  8//1 
+f  2//1  8//1  4//1 

test/unit/utObjImportExport.cpp

@@ -354,3 +354,16 @@ TEST_F(utObjImportExport, 0based_array_Test) {
     const aiScene *scene = myimporter.ReadFileFromMemory(ObjModel.c_str(), ObjModel.size(), 0);
     EXPECT_EQ(nullptr, scene);
 }
+
+TEST_F( utObjImportExport, mtllib_after_g ) {
+    ::Assimp::Importer importer;
+    const aiScene *scene = importer.ReadFile( ASSIMP_TEST_MODELS_DIR "/OBJ/cube_mtllib_after_g.obj", aiProcess_ValidateDataStructure );
+    ASSERT_NE( nullptr, scene );
+
+    EXPECT_EQ(scene->mNumMeshes, 1U);
+    const aiMesh *mesh = scene->mMeshes[0];
+    const aiMaterial* mat = scene->mMaterials[mesh->mMaterialIndex];
+    aiString name;
+    ASSERT_EQ(aiReturn_SUCCESS, mat->Get(AI_MATKEY_NAME, name));
+    EXPECT_STREQ("MyMaterial", name.C_Str());
+}