Thank you ;) After your answers, my solution: 1 - convert in.png to RGBA 2 - I did a loop and i change the alpha values. For example: for i, j in zip(range(255+1), list(reversed(range(255+1)))): (i,i,i,j) (255,255,255,0) (254,254,254,1) (253,253,253,2) ... Thank you ;)