Importing obj files without usemtl

Summary:
When there is no "usemtl" statement in the .obj file use material from .mtl if there is one.
#1068

Reviewed By: bottler

Differential Revision: D34141152

fbshipit-source-id: 7a5b5cc3f0bb287dc617f68de2cd085db8f7ad94

pytorch3d/io/mtl_io.py

@@ -454,8 +454,18 @@ def _load_texture_images(
     final_material_properties = {}
     texture_images = {}
 
+    used_material_names = list(material_names)
+    if not used_material_names and material_properties:
+        if len(material_properties) > 1:
+            raise ValueError(
+                "Multiple materials but no usemtl declarations in the obj file"
+            )
+        # No materials were specified in obj file and only one is in the
+        # specified .mtl file, so we use it.
+        used_material_names.append(next(iter(material_properties.keys())))
+
     # Only keep the materials referenced in the obj.
-    for material_name in material_names:
+    for material_name in used_material_names:
         if material_name in texture_files:
             # Load the texture image.
             path = os.path.join(data_dir, texture_files[material_name])

pytorch3d/io/obj_io.py

@@ -526,9 +526,8 @@ def _load_materials(
     if not load_textures:
         return None, None
 
-    if not material_names or f is None:
-        if material_names:
-            warnings.warn("No mtl file provided")
+    if f is None:
+        warnings.warn("No mtl file provided")
         return None, None
 
     if not path_manager.exists(f):
@@ -620,6 +619,13 @@ def _load_obj(
         device=device,
     )
 
+    if material_colors and not material_names:
+        # usemtl was not present but single material was present in the .mtl file
+        material_names.append(next(iter(material_colors.keys())))
+        # replace all -1 by 0 material idx
+        if torch.is_tensor(faces_materials_idx):
+            faces_materials_idx.clamp_(min=0)
+
     if create_texture_atlas:
         # Using the images and properties from the
         # material file make a per face texture map.

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