Fix saving / loading empty OBJ files

Summary:
OBJ files without vertices or faces should be allowed:
- an OBJ with only vertices can represent a point cloud
- an OBJ without any vertex or face is just empty
- an OBJ with faces referencing inexistent vertices has invalid data

Reviewed By: gkioxari

Differential Revision: D20392526

fbshipit-source-id: e72c846ff1e5787fb11d527af3fefa261f9eb0ee

pytorch3d/io/obj_io.py

@@ -15,6 +15,17 @@ from PIL import Image
 from pytorch3d.structures import Meshes, Textures, join_meshes
 
 
+def _make_tensor(data, cols: int, dtype: torch.dtype) -> torch.Tensor:
+    """
+    Return a 2D tensor with the specified cols and dtype filled with data,
+    even when data is empty.
+    """
+    if not data:
+        return torch.zeros((0, cols), dtype=dtype)
+
+    return torch.tensor(data, dtype=dtype)
+
+
 def _read_image(file_name: str, format=None):
     """
     Read an image from a file using Pillow.
@@ -61,7 +72,7 @@ def _format_faces_indices(faces_indices, max_index):
     Raises:
         ValueError if indices are not in a valid range.
     """
-    faces_indices = torch.tensor(faces_indices, dtype=torch.int64)
+    faces_indices = _make_tensor(faces_indices, cols=3, dtype=torch.int64)
 
     # Change to 0 based indexing.
     faces_indices[(faces_indices > 0)] -= 1
@@ -70,10 +81,8 @@ def _format_faces_indices(faces_indices, max_index):
     faces_indices[(faces_indices < 0)] += max_index
 
     # Check indices are valid.
-    if not (
-        torch.all(faces_indices < max_index) and torch.all(faces_indices >= 0)
-    ):
-        raise ValueError("Faces have invalid indices.")
+    if torch.any(faces_indices >= max_index) or torch.any(faces_indices < 0):
+        warnings.warn("Faces have invalid indices")
 
     return faces_indices
 
@@ -333,7 +342,7 @@ def _load(f_obj, data_dir, load_textures=True):
 
     # startswith expects each line to be a string. If the file is read in as
     # bytes then first decode to strings.
-    if isinstance(lines[0], bytes):
+    if lines and isinstance(lines[0], bytes):
         lines = [l.decode("utf-8") for l in lines]
 
     for line in lines:
@@ -380,9 +389,9 @@ def _load(f_obj, data_dir, load_textures=True):
                 faces_materials_idx,
             )
 
-    verts = torch.tensor(verts)  # (V, 3)
-    normals = torch.tensor(normals)  # (N, 3)
-    verts_uvs = torch.tensor(verts_uvs)  # (T, 3)
+    verts = _make_tensor(verts, cols=3, dtype=torch.float32)  # (V, 3)
+    normals = _make_tensor(normals, cols=3, dtype=torch.float32)  # (N, 3)
+    verts_uvs = _make_tensor(verts_uvs, cols=2, dtype=torch.float32)  # (T, 2)
 
     faces_verts_idx = _format_faces_indices(faces_verts_idx, verts.shape[0])
 
@@ -544,30 +553,46 @@ def save_obj(f, verts, faces, decimal_places: Optional[int] = None):
 
 # TODO (nikhilar) Speed up this function.
 def _save(f, verts, faces, decimal_places: Optional[int] = None):
-    if verts.dim() != 2 or verts.size(1) != 3:
-        raise ValueError("Argument 'verts' should be of shape (num_verts, 3).")
-    if faces.dim() != 2 or faces.size(1) != 3:
-        raise ValueError("Argument 'faces' should be of shape (num_faces, 3).")
+    if not (len(verts) or len(faces)):
+        warnings.warn("Empty 'verts' and 'faces' arguments provided")
+        return
+
+    if len(verts) and not (verts.dim() == 2 and verts.size(1) == 3):
+        raise ValueError(
+            "Argument 'verts' should either be empty or of shape (num_verts, 3)."
+        )
+
+    if len(faces) and not (faces.dim() == 2 and faces.size(1) == 3):
+        raise ValueError(
+            "Argument 'faces' should either be empty or of shape (num_faces, 3)."
+        )
+
     verts, faces = verts.cpu(), faces.cpu()
 
-    if decimal_places is None:
-        float_str = "%f"
-    else:
-        float_str = "%" + ".%df" % decimal_places
-
     lines = ""
-    V, D = verts.shape
-    for i in range(V):
-        vert = [float_str % verts[i, j] for j in range(D)]
-        lines += "v %s\n" % " ".join(vert)
 
-    F, P = faces.shape
-    for i in range(F):
-        face = ["%d" % (faces[i, j] + 1) for j in range(P)]
-        if i + 1 < F:
-            lines += "f %s\n" % " ".join(face)
-        elif i + 1 == F:
-            # No newline at the end of the file.
-            lines += "f %s" % " ".join(face)
+    if len(verts):
+        if decimal_places is None:
+            float_str = "%f"
+        else:
+            float_str = "%" + ".%df" % decimal_places
+
+        V, D = verts.shape
+        for i in range(V):
+            vert = [float_str % verts[i, j] for j in range(D)]
+            lines += "v %s\n" % " ".join(vert)
+
+    if torch.any(faces >= verts.shape[0]) or torch.any(faces < 0):
+        warnings.warn("Faces have invalid indices")
+
+    if len(faces):
+        F, P = faces.shape
+        for i in range(F):
+            face = ["%d" % (faces[i, j] + 1) for j in range(P)]
+            if i + 1 < F:
+                lines += "f %s\n" % " ".join(face)
+            elif i + 1 == F:
+                # No newline at the end of the file.
+                lines += "f %s" % " ".join(face)
 
     f.write(lines)