eub | a try at that Spanish Castle thing

(Shocking-to-me perceptual trick ganked from here.)

I gave it a try and mine sort of works; the indefinite persistence is there, but it doesn't manage full color saturation. It might just be my choice of source image. I could try oversaturating my inverse-color image, but it would clamp very soon. I simply set the inverse-color image's luminance to 50% everywhere; I didn't check to see if that's what the "Spanish castle" one did.

What I'm really fascinated by is what characteristic of the grayscale information of the image is needed to make the color afterimage persist. It can't possibly work with a flat gray field, right? But it does work with the castle's sky. So maybe it works with a grayscale gradient, or with a flat area alongside something else in the frame. I should try some of those.