pytorch3d/docs/notes/meshes_io.md

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Meshes and IO

The Meshes object represents a batch of triangulated meshes, and is central to much of the functionality of PyTorch3D. There is no insistence that each mesh in the batch has the same number of vertices or faces. When available, it can store other data which pertains to the mesh, for example face normals, face areas and textures.

Two common file formats for storing single meshes are ".obj" and ".ply" files, and PyTorch3D has functions for reading these.

OBJ

Obj files have a standard way to store extra information about a mesh. Given an obj file, it can be read with

  verts, faces, aux = load_obj(filename)

which sets verts to be a (V,3)-tensor of vertices and faces.verts_idx to be an (F,3)- tensor of the vertex-indices of each of the corners of the faces. Faces which are not triangles will be split into triangles. aux is an object which may contain normals, uv coordinates, material colors and textures if they are present, and faces may additionally contain indices into these normals, textures and materials in its NamedTuple structure. A Meshes object containing a single mesh can be created from just the vertices and faces using

    meshes = Meshes(verts=[verts], faces=[faces.verts_idx])

If there is texture information in the .obj it can be used to initialize a Textures class which is passed into the Meshes constructor. Currently we support loading of texture maps for meshes which have one texture map for the entire mesh e.g.

verts_uvs = aux.verts_uvs[None, ...]  # (1, V, 2)
faces_uvs = faces.textures_idx[None, ...]  # (1, F, 3)
tex_maps = aux.texture_images

# tex_maps is a dictionary of {material name: texture image}.
# Take the first image:
texture_image = list(tex_maps.values())[0]
texture_image = texture_image[None, ...]  # (1, H, W, 3)

# Create a textures object
tex = Textures(verts_uvs=verts_uvs, faces_uvs=faces_uvs, maps=texture_image)

# Initialise the mesh with textures
meshes = Meshes(verts=[verts], faces=[faces.verts_idx], textures=tex)

The load_objs_as_meshes function provides this procedure.

PLY

Ply files are flexible in the way they store additional information. PyTorch3D provides a function just to read the vertices and faces from a ply file. The call

    verts, faces = load_ply(filename)

sets verts to be a (V,3)-tensor of vertices and faces to be an (F,3)- tensor of the vertex-indices of each of the corners of the faces. Faces which are not triangles will be split into triangles. A Meshes object containing a single mesh can be created from this data using

    meshes = Meshes(verts=[verts], faces=[faces])