datasets.py 文件源码

def decode_image(contents, channels=None, name=None):
  """Convenience function for `decode_gif`, `decode_jpeg`, and `decode_png`.
  Detects whether an image is a GIF, JPEG, or PNG, and performs the appropriate
  operation to convert the input bytes `string` into a `Tensor` of type `uint8`.

  Note: `decode_gif` returns a 4-D array `[num_frames, height, width, 3]`, as
  opposed to `decode_jpeg` and `decode_png`, which return 3-D arrays
  `[height, width, num_channels]`. Make sure to take this into account when
  constructing your graph if you are intermixing GIF files with JPEG and/or PNG
  files.

  Args:
    contents: 0-D `string`. The encoded image bytes.
    channels: An optional `int`. Defaults to `0`. Number of color channels for
      the decoded image.
    name: A name for the operation (optional)

  Returns:
    `Tensor` with type `uint8` with shape `[height, width, num_channels]` for
      JPEG and PNG images and shape `[num_frames, height, width, 3]` for GIF
      images.
  """
  with ops.name_scope(name, 'decode_image') as scope:
    if channels not in (None, 0, 1, 3):
      raise ValueError('channels must be in (None, 0, 1, 3)')
    substr = tf.substr(contents, 0, 4)

    def _gif():
      # Create assert op to check that bytes are GIF decodable
      is_gif = tf.equal(substr, b'\x47\x49\x46\x38', name='is_gif')
      decode_msg = 'Unable to decode bytes as JPEG, PNG, or GIF'
      assert_decode = control_flow_ops.Assert(is_gif, [decode_msg])
      # Create assert to make sure that channels is not set to 1
      # Already checked above that channels is in (None, 0, 1, 3)
      gif_channels = 0 if channels is None else channels
      good_channels = tf.not_equal(gif_channels, 1, name='check_channels')
      channels_msg = 'Channels must be in (None, 0, 3) when decoding GIF images'
      assert_channels = control_flow_ops.Assert(good_channels, [channels_msg])
      with ops.control_dependencies([assert_decode, assert_channels]):
        return gen_image_ops.decode_gif(contents)

    def _png():
      return gen_image_ops.decode_png(contents, channels)

    def check_png():
      is_png = tf.equal(substr, b'\211PNG', name='is_png')
      return control_flow_ops.cond(is_png, _png, _gif, name='cond_png')

    def _jpeg():
      return gen_image_ops.decode_jpeg(contents, channels)

    is_jpeg = tf.logical_or(tf.equal(substr, b'\xff\xd8\xff\xe0', name='is_jpeg0'),
                           tf.equal(substr, b'\xff\xd8\xff\xe1', name='is_jpeg0'))

    return control_flow_ops.cond(is_jpeg, _jpeg, check_png, name='cond_jpeg')