Wednesday, July 18, 2007

"It's Called An Illusion, Michael."

Ran across this fascinating tidbit yesterday on NASA's very-good-in-a-kinda-nerdy-way Astronomy Picture of the Day website. Check out the image below:

Would you believe that the two squares labeled "A" and "B" are the exact same shade of grey? Because they are. If you're like me, you probably already looked ahead a little bit and saw this proof:

These images were created by some guy named Edward H. Adelson, and were used by some other guy, apparently for some project at MIT. At least, that's where I got this explanation from:

"The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

"A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible."

So, I guess that means our eyeballs and brain are just screwy, eh? Not so, says the MIT dude:

"As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view."

Call me fascinated. If you'd like further proof that the squares are indeed the same shade of grey, click here.

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