def extract_data(filename, num_images):
  """
  Extract the images into a 4D tensor [image index, y, x, channels].
  Values are rescaled from [0, 255] down to [-0.5, 0.5].
  """
  print('Extracting', filename)
  with gzip.open(filename) as bytestream:
    bytestream.read(16)
    buf = bytestream.read(IMAGE_SIZE * IMAGE_SIZE * num_images)
    data = np.frombuffer(buf, dtype=np.uint8).astype(np.float32)
    data = (data - (PIXEL_DEPTH / 2.0)) / PIXEL_DEPTH
    data = data.reshape(num_images, IMAGE_SIZE, IMAGE_SIZE, 1)
    return data
Extract the images into a 4D tensor [image index, y, x, channels].
  Values are rescaled from [0, 255] down to [-0.5, 0.5].