May 01, 2005: Checker Shadow Illusion

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Checker Shadow

Are the squares marked A and B the same shade of gray?

The original image of the illusion is shown above. The squares marked A and B are the same shade of gray, yet they appear different.

Below is the original image plus two stripes. By joining the squares marked A and B with two vertical stripes of the same shade of gray, it becomes apparent that both squares are the same.

Checker Shadow with stripes.

If you still don't belive it and think that there must be something sneaky going on in those images, please try the following to convince yourself.

1) Try putting the image in Photoshop.
Using the eyedropper tool you can determine
that the RGB values of the grays in both square A and square B are 120-120-120.

2) Cut out a cardboard mask.
By viewing patches of the squares without the surrounding context, you can remove the effect of the illusion. A piece of cardboard with two circles removed will work as a mask for a computerscreen or for a printed piece of paper.

3) Print the image and cut out the sqaures.
This is another way to isolate the patches form their surrounding context. Cut out each square along the edges. Remove them. Hold them side by side. Please note that some printers, especially laser printers, have "enhancement" processing that increases the contrast of edges. This can cause the printed squares to have slightly different values of gray.

4) Get a photometer.
This might be a bit expensive if you don't have one already. If you don't have one of these, try options 1-3

Why does the illusion work?

The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray "paint" that belongs to the surface.

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.

Click here to find out more about Edward H. Adelson

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