Alembic: import and export of subdivision parameters

This adds support to import and to export subdivision parameters from
subsurface modifiers in Alembic.

As Alembic is using the OpenSubDiv 2.0 API to define the parameters, we
need to convert to and from the OpenSubDiv 3.0 API which Blender uses.
Since Alembic does not define proper enumerations for these values,
this adds relevant enumerations whose names derive from the USD Alembic
exporter to avoid using magic values.

On top of that we also export and import the subdivision levels for
viewport and render so those values get preserved when reimporting in
Blender.

For import, this also automatically adds a subdivision surface modifier
to subdivision meshes.

Pull Request: https://projects.blender.org/blender/blender/pulls/160260

tests/python/bl_alembic_io_test.py

@@ -305,7 +305,7 @@ class CameraExportImportTest(unittest.TestCase):
 
         # A MeshSequenceCache modifier should be present on every imported object
         for blender_mesh in [blender_mesh1, blender_mesh2]:
-            self.assertTrue(len(blender_mesh.modifiers) == 1 and blender_mesh.modifiers[0].type ==
+            self.assertTrue(len(blender_mesh.modifiers) >= 1 and blender_mesh.modifiers[0].type ==
                             'MESH_SEQUENCE_CACHE', f"{blender_mesh.name} has incorrect modifiers")
 
         # Conversion from USD to Blender convention
@@ -320,8 +320,10 @@ class CameraExportImportTest(unittest.TestCase):
             blender_mesh2_eval = bpy.data.objects["mesh_vert_crease"].evaluated_get(depsgraph)
 
             # The file was written using a simple formula for each frame's crease value
-            expected_edge_creases = [round(frame / 24.0, 3)] * 12
-            expected_vert_creases = [round(frame / 24.0, 3)] * 4
+            expected_edge_creases = [round(frame / 24.0, 3)] * 48
+            # Creases are not propagated to edges that do not come from a coarse edge
+            expected_edge_creases.extend([0.0] * 144)
+            expected_vert_creases = [round(frame / 24.0, 3)] * 25
 
             # Check crease values
             blender_crease_data = [round(d.value, 3) for d in blender_mesh1_eval.data.attributes["crease_edge"].data]
@@ -336,6 +338,56 @@ class CameraExportImportTest(unittest.TestCase):
                 expected_vert_creases,
                 f"Frame {frame}: {blender_mesh2_eval.name} crease values do not match")
 
+    def test_mesh_subd_params(self):
+        """Test export of subdivision parameters."""
+
+        abc_path = str(self.tempdir / "mesh_subd_params.abc")
+
+        # Export
+        bpy.ops.wm.open_mainfile(filepath=str(args.testdir / "mesh_subd_params.blend"))
+        self.assertIn('FINISHED', bpy.ops.wm.alembic_export(
+            filepath=abc_path,
+            subdiv_schema=True
+        ))
+
+        # Re-import what we just exported into an empty file.
+        bpy.ops.wm.open_mainfile(filepath=str(args.testdir / "empty.blend"))
+        bpy.ops.wm.alembic_import(
+            filepath=abc_path,
+            as_background_job=False)
+
+        uv_smooth_options = [
+            'NONE',
+            'PRESERVE_CORNERS',
+            # NOTE: in Blender this should be PRESERVE_CORNERS_AND_JUNCTIONS, however
+            # Alembic does not have this concept.
+            'PRESERVE_CORNERS',
+            'PRESERVE_CORNERS_JUNCTIONS_AND_CONCAVE',
+            'PRESERVE_BOUNDARIES',
+            'SMOOTH_ALL'
+        ]
+
+        boundary_smooth_options = [
+            'PRESERVE_CORNERS',
+            'ALL'
+        ]
+
+        for uv_smooth_index in range(len(uv_smooth_options)):
+            uv_smooth = uv_smooth_options[uv_smooth_index]
+
+            for boundary_smooth_index in range(len(boundary_smooth_options)):
+                boundary_smooth = boundary_smooth_options[boundary_smooth_index]
+
+                obj_name = 'obj{}{}'.format(uv_smooth_index, boundary_smooth_index)
+
+                obj = bpy.data.objects[obj_name]
+                modifier = obj.modifiers['Subdivision']
+
+                self.assertEqual(modifier.uv_smooth, uv_smooth)
+                self.assertEqual(modifier.boundary_smooth, boundary_smooth)
+                self.assertEqual(modifier.levels, 1 + uv_smooth_index)
+                self.assertEqual(modifier.render_levels, 2 + uv_smooth_index)
+
     def do_export_import_test(self, *, flatten: bool):
         bpy.ops.wm.open_mainfile(filepath=str(args.testdir / "camera_transforms.blend"))