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As you can see, my orange dog, Betsy, has a white underside. So do sharks, deer, penguins, snakes, squirrels, alligators, and many other animals that do not seem directly related to one another. Why is that?

It's called countershading, and the principle is exactly the opposite of what we were taught at industrial design school, in our very first rendering class. Think of that first sphere that you had to draw with charcoal or markers. The instructor tells you the light source is always up above, so you've got the highlight up top, and as it falls off the bottom half of the sphere gets darker. This gives the object three-dimensionality, making the flat drawing "pop."

Image source: Zurb.net

Countershading, a concept zoologists are familiar with, is an evolutionary feature that does the exact opposite. A base form of camouflage, the idea is that the sun is always up above, so animals that have darker coloration up top and lighter coloration below have a better chance of appearing "flat," blending into the environment. To not be seen is beneficial for both predators and prey. You can see the effects of countershading in this photo of ibexes. (There are three.)

By Ori~ for Stefanie Gromann - Own work

Military camouflage is often of the pattern-based variety, but every so often you'll see examples of countershading being used. Here's a WWII-era fighter plane, a German Focke-Wulf 190, that has been countershaded to better blend into the sky when it's in flight.

Image source: National Museum

But yeah, you can see why countershading would be a terrible rendering practice.

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