fix saved glb length

Summary: Make GLB files report their own length correctly. They were off by 28.

Reviewed By: davidsonic

Differential Revision: D41838340

fbshipit-source-id: 9cd66e8337c142298d5ae1d7c27e51fd812d5c7b

pytorch3d/io/experimental_gltf_io.py

@@ -740,7 +740,10 @@ class _GLTFWriter:
         json_length = len(json_bytes)
 
         # write header
-        header = struct.pack("<III", _GLTF_MAGIC, 2, json_length + byte_offset)
+        version = 2
+        total_header_length = 28  # (file header = 12) + 2 * (chunk header = 8)
+        file_length = json_length + byte_offset + total_header_length
+        header = struct.pack("<III", _GLTF_MAGIC, version, file_length)
         self.buffer_stream.write(header)
 
         # write json

pytorch3d/io/pluggable.py

@@ -155,6 +155,9 @@ class IO:
             include_textures: If textures are present, whether to try to save
                                 them.
         """
+        if not isinstance(data, Meshes):
+            raise ValueError("Meshes object expected.")
+
         if len(data) != 1:
             raise ValueError("Can only save a single mesh.")
 
@@ -204,6 +207,9 @@ class IO:
             path: file to write
             binary: If there is a choice, whether to save in a binary format.
         """
+        if not isinstance(data, Pointclouds):
+            raise ValueError("Pointclouds object expected.")
+
         if len(data) != 1:
             raise ValueError("Can only save a single point cloud.")