text: # Copyright (c) 2017 Hiroharu Kato
  # Copyright (c) 2018 Nikos Kolotouros
  # Copyright (c) 2019 Shichen Liu
  
  # Permission is hereby granted, free of charge, to any person obtaining a copy
  # of this software and associated documentation files (the "Software"), to deal
  # in the Software without restriction, including without limitation the rights
  # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  # copies of the Software, and to permit persons to whom the Software is
  # furnished to do so, subject to the following conditions:
  
  # The above copyright notice and this permission notice shall be included in all
  # copies or substantial portions of the Software.
  
  # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  # SOFTWARE.
  
  import torch
  
  def face_vertices(vertices, faces):
      """
      :param vertices: [batch size, number of vertices, 3]
      :param faces: [batch size, number of faces, 3]
      :return: [batch size, number of faces, 3, 3]
      """
      assert (vertices.ndimension() == 3)
      assert (faces.ndimension() == 3)
      assert (vertices.shape[0] == faces.shape[0])
      assert (vertices.shape[2] == 3)
      assert (faces.shape[2] == 3)
  
      bs, nv = vertices.shape[:2]
      bs, nf = faces.shape[:2]
      device = vertices.device
      faces = faces + (torch.arange(bs, dtype=torch.int32, device=device) * nv)[:, None, None]
      vertices = vertices.reshape((bs * nv, 3))
      # pytorch only supports long and byte tensors for indexing
      return vertices[faces.long()]
  