I think O(log(M)) where M is the number of remembered pixels. Index them by their (R,G,B,A) quad and do a range lookup sequentially by pixel, since any valid p

I think O(log(M)) where M is the number of remembered pixels. Index them by their (R,G,B,A) quad and do a range lookup sequentially by pixel, since any valid pixels would be within the same range as earlier potential matches.