def _encode_gif(images, fps):
  """Encodes numpy images into gif string.

  Args:
    images: A 4-D `uint8` `np.array` (or a list of 3-D images) of shape
      `[time, height, width, channels]` where `channels` is 1 or 3.
    fps: frames per second of the animation

  Returns:
    The encoded gif string.

  Raises:
    IOError: If the ffmpeg command returns an error.
  """
  writer = WholeVideoWriter(fps)
  writer.write_multi(images)
  return writer.finish()