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Gray squares  @psychohistorian.org

Gray squares

posted: 4616 days ago, on Saturday, 2006 Sep 30 at 21:03
tags: psychology.

Two squares, A & B, are marked in the illustration. How much darker is the one compared to the other?

You may (or may not) be surprised to hear that they don't differ the two squares are the same shade of gray (their RGB values are 120:120:120, give or take a bit since the image is slightly dithered by the JPG compression).

So what's going on? At least two effects are at work. The first is the visual cue of a chess board, suggesting that A and B are black and white squares, thus setting up the expectation that they are supposed to differ. The second effect is more beautiful: square B seems to lie in the shadow of the green pillar, suggesting that it is darkend, which suggests that in reality, it should actually be a bit brighter than it appears. Your brain obliges, and dutifully modifies the equally-bright (120:120:120) incoming photons, rendering them lighter.

The exploded diagram below should convince you. At step 1, A & B should appear different, but by the third and fourth steps, their similarity should be obvious.

nothing more to see. please move along.


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