Recall that last time we considered the issue how computer vision isn’t fooled by optical illusions. Especially good it was it at defeating illusions based on measurements. There was one toss-up. A (uniformly) gray bar was not recognized as an object because its background varied from lighter than that of the bar to darker.

Now let’s take a look at something similar - the “same color” illusion. These letters have the exact same gray level, but one looks darker - in the second image. A common reaction is “OMG, it’s hard to believe! They look totally different!” The question is, is it a mistake to see them as different?

In my view the fact that these letters have the same gray level is incidental. After all, they aren’t even close to each other. What is much more important is how each object fits into the image. What a person sees first is the relation between the object and the adjacent area (the background). The crucial difference is then that one A is dark on light background and the other is light on dark. It turns out, one is “dark” and the other is “light” – even though they have the same gray level! Why is this distinction so important? Look at it this way – the “dark” is an object while the “light” is a hole (or vice versa).

So what’s the conclusion? The two identical A’s look different because they are different! In fact, computer vision should follow human vision here and should be “fooled” by this “illusion” (analysis with Pixcavator below: dark objects are red, light are green).