The recent colored cube illusion showed us how our brain compensates for a color cast, making hues seem constant, even when they’re really not.
Our visual system does a similar thing with tones, discounting the effect of shadows and grouping tones into meaningful sets.
This checkerboard is an example. I painted it using the exact same gray mixture for the dark square in the light area (1) as I did for the light square in the shadow (2).
Don’t believe it? Here are the exact same squares with everything else made white.
Why do the tones seem so different? The light square is surrounded by darker squares. This makes our visual system automatically conclude that the actual tone is light in value, and we group it with the other light squares.
We interpret the diagonal bars of darker tone as shadows for the following reasons:
1. They have soft edges, and soft edges usually belong to shadows.
2. Those edges are parallel, and cast shadows from the sun on flat surface are parallel.
3. The tones of adjacent colored squares gradate to an equal degree, just the way shadows do.
Our visual system is designed to help us determine the actual color of objects in the world. The fact that it seems to deceive us is not a defect of our vision. It’s central to our survival.
But we have to know about this mechanism of visual perception if we want to paint tones accurately.
A related illusion from Edward Adelson’s website.