Mike Southby

Sometimes when I am browsing the net, I come across something that completely stumps me.  Today, whilst looking at something completely unrelated I came across a site which had a picture of a checker board with a number of blocks, 2 of the blocks had letters in them – Blocks A and B.  Now, what would you say if I told you that both blocks A and B were identical shades of grey?  You’d believe me right.

Are blocks A and B really the same colour?

Bear with me.  Here’s the official explanation from the site I came across [Link]:

The visual system needs to determine the colour 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 neighbouring 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 neighbours. 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 colour 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 objects 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 colour 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.

Convinced yet?

OK, If you have photoshop installed, download the image and use the colour picker to check the values for both boxes, like me, I’m sure you’ll be amazed that they are both r120 g120 b120, or #787878.  Now finally, here’s another picture, the same one in fact but with 2 colour strips to help you see the proof:

Convinced yet?

There are also some other equally clever optical illusions on the site, enjoy!

• Twitter
• Facebook
• LinkedIn
• Myspace
• Digg
• Delicious
• StumbleUpon
• Posterous
• Tumblr
• Reddit
• Google Reader

• Google Reader
• Google Reader
• Google Reader
• Google Reader
• Google Reader
• Google Reader
• Google Reader