USD: Add tests for multiple materials assigned to single mesh

This includes tests for a "static" mesh where the materials have been
assigned and are expected to remain the same over time.

The test scenario also includes a "dynamic" mesh where both new faces
and new materials are assigned to the same mesh over time (inspired by
#118754). However, that cannot be tested right now due to missing
features of both the MeshSequenceCache and our USD IO code. No testing
will occur for that case until the features are implemented.

Pull Request: https://projects.blender.org/blender/blender/pulls/128341

tests/python/bl_usd_export_test.py

@@ -221,6 +221,43 @@ class USDExportTest(AbstractUSDTest):
         self.assertEqual(opacity_input.HasConnectedSource(), True, "Alpha input should be connected")
         self.assertAlmostEqual(opacity_thresh_input.Get(), 0.2, 2, "Opacity threshold input should be 0.2")
 
+    def test_export_material_subsets(self):
+        """Validate multiple materials assigned to the same mesh work correctly."""
+
+        bpy.ops.wm.open_mainfile(filepath=str(self.testdir / "usd_materials_multi.blend"))
+
+        # Ensure the simulation zone data is baked for all relevant frames...
+        for frame in range(1, 5):
+            bpy.context.scene.frame_set(frame)
+        bpy.context.scene.frame_set(1)
+
+        export_path = self.tempdir / "usd_materials_multi.usda"
+        res = bpy.ops.wm.usd_export(filepath=str(export_path), export_animation=True, evaluation_mode="RENDER")
+        self.assertEqual({'FINISHED'}, res, f"Unable to export to {export_path}")
+
+        stage = Usd.Stage.Open(str(export_path))
+
+        # The static mesh should have 4 materials each assigned to 4 faces (16 faces total)
+        static_mesh_prim = UsdGeom.Mesh(stage.GetPrimAtPath("/root/static_mesh/static_mesh"))
+        geom_subsets = UsdGeom.Subset.GetGeomSubsets(static_mesh_prim)
+        self.assertEqual(len(geom_subsets), 4)
+
+        unique_face_indices = set()
+        for subset in geom_subsets:
+            face_indices = subset.GetIndicesAttr().Get()
+            self.assertEqual(len(face_indices), 4)
+            unique_face_indices.update(face_indices)
+        self.assertEqual(len(unique_face_indices), 16)
+
+        # The dynamic mesh varies over time (currently blocked, see #124554 and #118754)
+        #  - Frame 1: 1 face and 1 material [mat2]
+        #  - Frame 2: 2 faces and 2 materials [mat2, mat3]
+        #  - Frame 3: 4 faces and 3 materials [mat2, mat3, mat2, mat1]
+        #  - Frame 4: 4 faces and 2 materials [mat2, mat3, mat2, mat3]
+        dynamic_mesh_prim = UsdGeom.Mesh(stage.GetPrimAtPath("/root/dynamic_mesh/dynamic_mesh"))
+        geom_subsets = UsdGeom.Subset.GetGeomSubsets(dynamic_mesh_prim)
+        self.assertEqual(len(geom_subsets), 0)
+
     def check_primvar(self, prim, pv_name, pv_typeName, pv_interp, elements_len):
         pv = UsdGeom.PrimvarsAPI(prim).GetPrimvar(pv_name)
         self.assertTrue(pv.HasValue())

tests/python/bl_usd_import_test.py

@@ -264,6 +264,37 @@ class USDImportTest(AbstractUSDTest):
         self.assertEqual(self.round_vector(node.inputs[1].default_value), [2, -2, 2])
         self.assertEqual(self.round_vector(node.inputs[2].default_value), [-1, 1, -1])
 
+    def test_import_material_subsets(self):
+        """Validate multiple materials assigned to the same mesh work correctly."""
+
+        # Use the existing materials test file to create the USD file
+        # for import. It is validated as part of the bl_usd_export test.
+        bpy.ops.wm.open_mainfile(filepath=str(self.testdir / "usd_materials_multi.blend"))
+        # Ensure the simulation zone data is baked for all relevant frames...
+        for frame in range(1, 5):
+            bpy.context.scene.frame_set(frame)
+        bpy.context.scene.frame_set(1)
+
+        testfile = str(self.tempdir / "usd_materials_multi.usda")
+        res = bpy.ops.wm.usd_export(filepath=testfile, export_animation=True, evaluation_mode="RENDER")
+        self.assertEqual({'FINISHED'}, res, f"Unable to export to {testfile}")
+
+        # Reload the empty file and import back in
+        bpy.ops.wm.open_mainfile(filepath=str(self.testdir / "empty.blend"))
+        res = bpy.ops.wm.usd_import(filepath=testfile)
+        self.assertEqual({'FINISHED'}, res, f"Unable to import USD file {testfile}")
+
+        # The static mesh should have 4 materials each assigned to 4 faces (16 faces total)
+        static_mesh = bpy.data.objects["static_mesh"].data
+        material_index_attr = static_mesh.attributes["material_index"]
+        self.assertEqual(len(static_mesh.materials), 4)
+        self.assertEqual(len(static_mesh.polygons), 16)
+        self.assertEqual(len(material_index_attr.data), 16)
+
+        for mat_index in range(0, 4):
+            face_indices = [i for i, d in enumerate(material_index_attr.data) if d.value == mat_index]
+            self.assertEqual(len(face_indices), 4, f"Incorrect number of faces with material index {mat_index}")
+
     def test_import_shader_varname_with_connection(self):
         """Test importing USD shader where uv primvar is a connection"""