APOD: The Same Color Illusion (2023 Dec 18)

[Tekija]

Sorry but they don't look the same color to me, with or without the bridge. Not even close, one is definitely lighter, one darker.

But that's the whole point of an optic illusion ! - In this case: that they "don't look the same color", but yet they are.
If you measure the color in the two squares, you will find that they both are RGB(120, 120, 120).

And the reason is because of the way our brain compensates for shadows. We interpret the chessboard as having squares with only two colors, uninfluenced by shading.

Indeed. Here's how the creator of the illusion explained it:

"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.

The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

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."

[johnnydeep]

There is no brightness (color) gradient. The number of pixels and the pixel density are not relevant, only the value of the pixels, which is identical for all the pixels in both squares.

Your eyes may be fooling you, or you may have some artifact created by your screen. But the numbers don't lie. Which is why when we analyze images, we look at the numbers.
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greyillusion_wikipedia_960-copy-2.jpg

Hmm. Now that the two squares are isolated, the color of the A is definitely lighter than the color of B (and blurrier)! Though they both consist of many different pixel colors, A has more 0x"5n5n5n" values and B has more 0x"4n4n4n" pixel values. I wonder if that affects our perception of the illusion when the rest of the squares are present.

greyillusion_wikipedia_960_nl.jpg

Thx. So the answer to my last question is "no".

[Chris Peterson]

both the left and right sides of the image are showing grass of the same color

Actually, no. In the image there are darker colors on the left and lighter colors on the right. This can be verified using tools. What we see in the image is a true difference in colors. Very often the difference between an area in a shadow and an area with direct sunlight seen in photographs is dramatic, apparently because our eyes are better at making relevant adjustments but what is relevant here is the image, just the image. I think our eyes can properly interpret the image of grass provided previously.

Instrumentally. Your brain immediately recognizes that the color is the same on both sides. It requires deliberate, conscious reasoning to note that the apparent colors are different.