I came up with this concept several years ago (I can’t tell exactly when, because I’ve mislaid the original code!) but never posted any of the images until now.
Some of you may like to guess what’s going on before I tell. For more hints, see these patterns’ 254 siblings.
No takers? In a nutshell,
… where
maskis given as the last two characters of the image file’s name.It gets more interesting in color.