As far as assumptions go, it’s a pretty basic one. When animals are hunting, they look for 3d objects and ignore the background texture, which is flat.

Some animals, like this antelope, have counter-shading. Their skin is patterned to counteract the natural shading of the sun overhead, and to flatten their appearance. Thus they don’t pop out to predators.

Matt Webb, S&W, posted 2006-04-13 (talk on 2006-02-08)