Comments on: Illusions

By: Matt McIrvin

Matt McIrvin — Fri, 29 Aug 2003 18:35:56 +0000

Computers can be fooled too, if not so subtly. Some of the people who insist that the squares in the checkershadow illusion are provably not the same color just might be viewing them on computers that actually are giving them different colors, because they’re using some sort of error-diffusion dither that can be led astray by local contrast effects. They’re the same color on my screen, though.

By: Tim Lambert

Tim Lambert — Thu, 28 Aug 2003 16:57:13 +0000

It made a great demo in class the other day. “They are the same colour! Who believes me?” No one, it would seem. Using GIMP’s pencil tool, I fill in the area between the squares with the same colour. 300 students go “Oooh”.

By: Murray Smigel

Murray Smigel — Thu, 28 Aug 2003 15:30:54 +0000

Ok, I cut uniform blocks of the A and B squares into a blank canvas, and yes they are the same, wow! I also did the test with small blocks containing the letters A and B, and there was a little difference between the colors, but nothing as large as the actual shadow illusion. Amazing.

By: bad Jim

bad Jim — Thu, 28 Aug 2003 11:10:18 +0000

Edwin Land? Retinex theory? I thought Oliver Sacks gave it a thought. The eye integrates shading across boundaries, or something. This is the second time I’ve seen this illusion. The first time I envisioned using a hole punch to make two carefully spaced apertures to test it. Both times I wondered why I trusted my monitor to give a calibrated result, and didn’t bother.

By: Elina

Elina — Thu, 28 Aug 2003 08:47:14 +0000

cerebrocrat, I agreed that there was no way the squares were the same colour but copying and pasting into paint and then erasing everything but the two squares I am convinced. Which all just goes to show how great the illusion is.

By: cerebrocrat

cerebrocrat — Thu, 28 Aug 2003 04:46:48 +0000

I believe in this instance, the illusion may be suffering from the limitations of the medium involved. The illusion is certainly for real; I’ve seen it many times before (it’s understandably a favorite in perceptual psychology). But the image at this link does not reproduce the illusion accurately; the two squares, on my lcd screen, are demonstrably not the same.

By: Dell Adams

Dell Adams — Thu, 28 Aug 2003 04:19:46 +0000

Covering the intervening space with my finger – making both squares contrast to the same object, in other words – also convinced me they were the same color. But I now have to suppose that didn’t prove anything, did it? Hmmm.

By: Andrew Edwards

Andrew Edwards — Thu, 28 Aug 2003 01:53:39 +0000

Save the image and load it into Photoshop or Paint or the GIMP. Use the eyedropper to sample the colour from one, and paint it over the other. Try using the paintbrush to slowly expand the squares, and see how much you need to do to get them to appear the same. It took me a while, but the REALLY ARE IDENTICAL.

By: Joshua

Joshua — Wed, 27 Aug 2003 20:11:57 +0000

Try covering up the letters A and B as well—the fact that one is black and the other white is an additional contrast cue. I found that once I did that they did indeed appear to be the same shade of gray (which is what my little web-design color selecter tool insisted despite the evidence of my lying eyes).

By: Brian Weatherson

Brian Weatherson — Wed, 27 Aug 2003 19:56:55 +0000

That’s surprising. Yes, that should eliminate the illusion. It could be because you’re at some level remembering what is under the card. Also there’s going to be a mild contrast effect between the squares and the letters inside them, but it’s hard to imagine that could explain everything. (In other words I don’t know how to explain your result and I’m guessing a bit.) I wasn’t convinced the squares were the same shade until I threw the picture into Photoshop and deleted everything but the squares, which I guess is something like covering the board apart from the squares. They looked the same shade then. (This does seem to undercut what I said last paragraph about the letters causing a contrast effect, doesn’t it?)

By: Murray Smigel

Murray Smigel — Wed, 27 Aug 2003 19:20:47 +0000

I put up a card with holes over all but the A and B parts of the checkshaddow illusion image, and the squares continue to look different in brightness to me. Shouldn’t the masking of the rest of the image abolish the illusion?