Allow saving OBJ UVs without a texture map (#2047)

Summary:
Closes https://github.com/facebookresearch/pytorch3d/issues/2001.

`save_obj` currently omits UV coordinates and face UV indices unless `texture_map` is also supplied. This loses UV topology when exporting mesh optimization snapshots or meshes whose UVs describe non-color data, unless a redundant texture image and material file are written for every OBJ.

Save UVs whenever both `verts_uvs` and `faces_uvs` are supplied. Material headers and `.mtl`/`.png` files still require a texture map. The public signature is unchanged, and the docstring describes the independent UV export.

The regression test round-trips UV coordinates and distinct face UV indices, both with and without normals, and checks that no material references or sidecar files are created.

## Validation

- Confirmed both regression subcases fail on unchanged main because exported faces omit UV indices.
- Six targeted CPU OBJ IO tests pass: the new regression, existing texture export, normal export, combined normal/texture export, invalid texture inputs, and the pluggable IO texture export.
- Black, flake8, and `git diff --check` pass for the changed files.
- Native extension/full-suite validation was unavailable: the installed MSVC 14.20 toolchain fails while compiling the installed PyTorch headers (C2642/C2737). The targeted tests ran the actual Python OBJ IO code with a local import-only placeholder for `_C`; it exposes only two Pulsar default constants and raises on any native API access. No native operations were exercised. CI should verify the standard test runner with a built extension.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/2047

Reviewed By: MichaelRamamonjisoa

Differential Revision: D119957610

Pulled By: bottler

fbshipit-source-id: 8ddbbf42b13c264a8f93d699fbbfe96eaa2c0650

pytorch3d/io/obj_io.py

@@ -712,6 +712,7 @@ def save_obj(
         faces_normals_idx: LongTensor of shape (F, 3) giving the index into
             normals for each vertex in the face.
         verts_uvs: FloatTensor of shape (V, 2) giving the uv coordinate per vertex.
+            When provided with faces_uvs, UVs are saved even without a texture_map.
         faces_uvs: LongTensor of shape (F, 3) giving the index into verts_uvs for
             each vertex in the face.
         texture_map: FloatTensor of shape (H, W, 3) representing the texture map
@@ -757,7 +758,8 @@ def save_obj(
     if path_manager is None:
         path_manager = PathManager()
 
-    save_texture = all(t is not None for t in [faces_uvs, verts_uvs, texture_map])
+    save_uvs = faces_uvs is not None and verts_uvs is not None
+    save_texture = save_uvs and texture_map is not None
     output_path = Path(f)
 
     # Save the .obj file
@@ -778,7 +780,7 @@ def save_obj(
             faces_normals_idx=faces_normals_idx,
             verts_uvs=verts_uvs,
             faces_uvs=faces_uvs,
-            save_texture=save_texture,
+            save_uvs=save_uvs,
             save_normals=normals is not None,
         )
 
@@ -816,7 +818,7 @@ def _save(
     faces_normals_idx: Optional[torch.Tensor] = None,
     verts_uvs: Optional[torch.Tensor] = None,
     faces_uvs: Optional[torch.Tensor] = None,
-    save_texture: bool = False,
+    save_uvs: bool = False,
     save_normals: bool = False,
 ) -> None:
     if len(verts) and (verts.dim() != 2 or verts.size(1) != 3):
@@ -851,7 +853,7 @@ def _save(
         assert faces_normals_idx is not None
         lines += _write_normals(normals, faces_normals_idx, float_str)
 
-    if save_texture:
+    if save_uvs:
         assert faces_uvs is not None
         assert verts_uvs is not None
 
@@ -881,7 +883,7 @@ def _save(
         _write_faces(
             f,
             faces,
-            faces_uvs if save_texture else None,
+            faces_uvs if save_uvs else None,
             faces_normals_idx if save_normals else None,
         )
 

tests/test_io_obj.py

@@ -955,6 +955,60 @@ class TestMeshObjIO(TestCaseMixin, unittest.TestCase):
             with open(obj_file, "r") as actual_file:
                 self.assertEqual(actual_file.read(), expected_obj_file)
 
+    def test_save_obj_with_uvs_without_texture_map(self):
+        verts = torch.tensor(
+            [[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [1.0, 1.0, 0.0]],
+            dtype=torch.float32,
+        )
+        faces = torch.tensor([[0, 2, 1]], dtype=torch.int64)
+        verts_uvs = torch.tensor(
+            [[0.0, 0.0], [1.0, 0.0], [1.0, 1.0], [0.0, 1.0]],
+            dtype=torch.float32,
+        )
+        faces_uvs = torch.tensor([[3, 1, 2]], dtype=torch.int64)
+        normals = torch.tensor([[0.0, 0.0, 1.0], [0.0, 1.0, 0.0]])
+        faces_normals_idx = torch.tensor([[1, 0, 1]], dtype=torch.int64)
+
+        for save_normals in (False, True):
+            with (
+                self.subTest(save_normals=save_normals),
+                TemporaryDirectory() as temp_dir,
+            ):
+                obj_file = Path(temp_dir) / "mesh.obj"
+                save_obj(
+                    obj_file,
+                    verts,
+                    faces,
+                    verts_uvs=verts_uvs,
+                    faces_uvs=faces_uvs,
+                    normals=normals if save_normals else None,
+                    faces_normals_idx=faces_normals_idx if save_normals else None,
+                )
+
+                self.assertEqual(os.listdir(temp_dir), ["mesh.obj"])
+                obj_text = obj_file.read_text()
+                self.assertNotIn("mtllib", obj_text)
+                self.assertNotIn("usemtl", obj_text)
+                expected_face = (
+                    "f 1/4/2 3/2/1 2/3/2" if save_normals else "f 1/4 3/2 2/3"
+                )
+                self.assertIn(expected_face, obj_text)
+
+                loaded_verts, loaded_faces, aux = load_obj(
+                    obj_file, load_textures=False
+                )
+                self.assertClose(loaded_verts, verts)
+                self.assertClose(loaded_faces.verts_idx, faces)
+                self.assertClose(aux.verts_uvs, verts_uvs)
+                self.assertClose(loaded_faces.textures_idx, faces_uvs)
+                self.assertIsNone(aux.material_colors)
+                self.assertIsNone(aux.texture_images)
+                if save_normals:
+                    self.assertClose(aux.normals, normals)
+                    self.assertClose(loaded_faces.normals_idx, faces_normals_idx)
+                else:
+                    self.assertIsNone(aux.normals)
+
     def test_save_obj_with_texture(self):
         verts = torch.tensor(
             [[0.01, 0.2, 0.301], [0.2, 0.03, 0.408], [0.3, 0.4, 0.05], [0.6, 0.7, 0.8]],