Matt says: "Inspired by those illusions [you wrote about yesterday], the naughty part of me has decided to see if it's possible to do a similar thing with text — the idea of a t-shirt with something offensive on it that turns into something innocuous if anyone comes to complain tickles my fancy — and I've made some progress on reverse-engineering the technique. It seems to work best if the two images are of a similar shape and have a few common features, which makes text somewhat problematic, but I've come up with a proof-of-concept. It's not perfect, but for half an hour's fiddling I'm quite pleased with the effect.
"The test image can be found here." Link
Reader comment: Mike Cramer says: "This illusion is fairly easy to do in the Gimp (and presumably Photoshop). Take two headshots and crop them so they line up reasonably well. Blur one enough that the detail disappears, but the face is still recognizable. Use an 'Edge Detect' filter on the other. Overlay the two and viola! The edge-detected image will be visible up close, and the blurred image will be visible if you squint."
