import Image, ImageEnhance
def reduce_opacity(im, opacity):
  """Returns an image with reduced opacity."""
  assert opacity >= 0 and opacity <= 1
  if im.mode != 'RGBA':
    im = im.convert('RGBA')
  else:
    im = im.copy()
  alpha = im.split()[3]
  alpha = ImageEnhance.Brightness(alpha).enhance(opacity)
  im.putalpha(alpha)
  return im
def watermark(im, mark, position, opacity=1):
  """Adds a watermark to an image."""
  if opacity < 1:
    mark = reduce_opacity(mark, opacity)
  if im.mode != 'RGBA':
    im = im.convert('RGBA')
  # create a transparent layer the size of the image and draw the
  # watermark in that layer.
  layer = Image.new('RGBA', im.size, (0,0,0,0))
  if position == 'tile':
    for y in range(0, im.size[1], mark.size[1]):
      for x in range(0, im.size[0], mark.size[0]):
        layer.paste(mark, (x, y))
  elif position == 'scale':
    # scale, but preserve the aspect ratio
    ratio = min(
      float(im.size[0]) / mark.size[0], float(im.size[1]) / mark.size[1])
    w = int(mark.size[0] * ratio)
    h = int(mark.size[1] * ratio)
    mark = mark.resize((w, h))
    layer.paste(mark, ((im.size[0] - w) / 2, (im.size[1] - h) / 2))
  else:
    layer.paste(mark, position)
  # composite the watermark with the layer
  return Image.composite(layer, im, layer)
def test():
  im = Image.open('test.png')
  mark = Image.open('overlay.png')
  watermark(im, mark, 'tile', 0.5).show()
  watermark(im, mark, 'scale', 1.0).show()
  watermark(im, mark, (100, 100), 0.5).show()
if __name__ == '__main__':
  test()