Your understanding of the word 'colour' does not match what properties of the world your brain is trying to identify and categorize when it interprets 'colour'. The interesting constant property of objects in the world that makes 'colour' useful to your visual system for purposes of object identification and categorization is really the surface properties that interact with incident lighting. Your brain attempts to ignore effects due to lighting variation and assign a 'colour' label to objects that is more or less an invariant property of the surface under... (read more)

Well said (including your later comment about color constancy). Along the same lines, this is why cameras often show objects in shadows as blacked out -- because that's the actual image it's getting, and the image your own retinas get! It's just that your brain has cleverly subtracted out the impact of the shadow before presenting it to you, so you can still see significant contrast and colors in the shadowed objects.

4mattnewport10yWhen we covered this phenomenon in my psychology degree it was referred to as colour constancy []. I now work as a 3D graphics programmer and so know a lot about the physics of light transport. The illusion does not surprise me any more, in fact it seems a little surprising that I ever could have thought that the RGB colour value of an onscreen pixel was directly related to the property of objects in the real world that we call 'colour'.

Bizarre Illusions

by MrHen 1 min read27th Jan 2010310 comments


grey square illusion
Illusions are cool. They make me think something is happening when it isn't. When offered the classic illusion pictured to the right, I wonder at the color of A and B. How weird, bizarre, and incredible.

Today I looked at the above illusion and thought, "Why do I keep thinking A and B are different colors? Obviously, something is wrong with how I am thinking about colors." I am being stupid when my I look at this illusion and I interpret the data in such a way to determine distinct colors. My expectations of reality and the information being transmitted and received are not lining up. If they were, the illusion wouldn't be an illusion.

The number 2 is prime; the number 6 is not. What about the number 1? Prime is defined as a natural number with exactly two divisors. 1 is an illusionary prime if you use a poor definition such as, "Prime is a number that is only divisible by itself and 1." Building on these bad assumptions could result in all sorts of weird results much like dividing by 0 can make it look like 2 = 1. What a tricky illusion!

An optical illusion is only bizarre if you are making a bad assumption about how your visual system is supposed to be working. It is a flaw in the Map, not the Territory. I should stop thinking that the visual system is reporting RGB style colors. It isn't. And, now that I know this, I am suddenly curious about what it is reporting. I have dropped a bad belief and am looking for a replacement. In this case, my visual system is distinguishing between something else entirely. Now that I have the right answer, this optical illusion should become as uninteresting as questioning whether 1 is prime. It should stop being weird, bizarre, and incredible. It merely highlights an obvious reality.

Addendum: This post was edited to fix a few problems and errors. If you are at all interested in more details behind the illusion presented here, there are a handful of excellent comments below.