invert.py

import numpy as np
from colorsys import rgb_to_hls, hls_to_rgb

def _rgb2hls(im): 
  return np.stack(np.vectorize(rgb_to_hls)(*im.transpose(2,0,1)/256), axis=-1) 

def _hls2rgb(im): 
  return (255*np.stack(np.vectorize(hls_to_rgb)(*im.transpose(2,0,1)), axis=-1)).astype(np.uint8)

def invert(img): 
  """Invert a rgb image via lightness inversion 
  
  Parameters
  ----------
  img: np.ndarray
    The rgb image to be inverted. 
    Expects img.shape=(n,m,3) and img.dtype=np.uint8
  """
  if not img.ndim==3 and img.shape[-1]==3 and img.dtype==np.uint8:
    raise ValueError("input image should be of shape (n,m,3) and type np.uint8!")
  tmp = _rgb2hls(img) 
  tmp[...,1] = 1-tmp[...,1] 
  return _hls2rgb(tmp)



if __name__ == '__main__':
  # test it 
  from skimage.draw import line_aa 
  import matplotlib.pyplot as plt

  # draw random colored lines
  img = 255*np.ones((256,256,3), np.uint8)
  for _ in range(10):
    rr, cc, val = line_aa(*np.random.randint(0,img.shape[0],4))
    img[rr, cc] = np.random.randint(100,255,3)
                          
  plt.imshow(np.concatenate([img,invert(img)], axis=1))