Importing obj files without usemtl

Summary:
When there is no "usemtl" statement in the .obj file use material from .mtl if there is one.
https://github.com/facebookresearch/pytorch3d/issues/1068

Reviewed By: bottler

Differential Revision: D34141152

fbshipit-source-id: 7a5b5cc3f0bb287dc617f68de2cd085db8f7ad94

tests/data/missing_usemtl/README.md

@@ -0,0 +1,7 @@
+# Acknowledgements
+
+This is copied version of docs/tutorials/data/cow_mesh with removed line 6159 (usemtl material_1) to test behavior without usemtl material_1 declaration.
+
+Thank you to Keenen Crane for allowing the cow mesh model to be used freely in the public domain.
+
+###### Source: http://www.cs.cmu.edu/~kmcrane/Projects/ModelRepository/

tests/data/missing_usemtl/cow.mtl

@@ -0,0 +1,9 @@
+newmtl material_1
+map_Kd cow_texture.png
+
+# Test colors
+
+Ka 1.000 1.000 1.000  # white
+Kd 1.000 1.000 1.000  # white
+Ks 0.000 0.000 0.000  # black
+Ns 10.0

tests/test_io_obj.py

@@ -658,6 +658,18 @@ class TestMeshObjIO(TestCaseMixin, unittest.TestCase):
         self.assertTrue(aux.material_colors is None)
         self.assertTrue(aux.texture_images is None)
 
+    def test_load_no_usemtl(self):
+        obj_filename = "missing_usemtl/cow.obj"
+        # obj_filename has no "usemtl material_1" line
+        filename = os.path.join(DATA_DIR, obj_filename)
+        # TexturesUV type
+        mesh = IO().load_mesh(filename)
+        self.assertIsNotNone(mesh.textures)
+
+        verts, faces, aux = load_obj(filename)
+        self.assertTrue("material_1" in aux.material_colors)
+        self.assertTrue("material_1" in aux.texture_images)
+
     def test_load_mtl_fail(self):
         # Faces have a material
         obj_file = "\n".join(