Mind Hacks

Hack #20. Fool Yourself into Seeing 3D

How do you figure out the three-dimensional shape of objects, just by looking? At first glance, it’s using shadows.

Looking at shadows is one of many tricks we use to figure out the shape of objects. As a trick, it’s easy to fool—shading alone is enough for the brain to assume what it’s seeing is a real shadow. This illusion is so powerful and so deeply ingrained, in fact, that we can actually feel depth in a picture despite knowing it’s just a flat image.

In Action

Have a look at the shaded circles in Figure 2-8, following a similar illustration in Kleffner and Ramachandran’s “On the Perception of Shape from Shading.” ¹

I put together this particular diagram myself, and there’s nothing to it: just a collection of circles on a medium gray background. All the circles are gradient-filled black and white, some with white at the top and some with white at the bottom. Despite the simplicity of the image, there’s already a sense of depth.

The shading seems to make the circles with white at the top bend out of the page, as though they’re bumps. The circles with white at the bottom look more like depressions or even holes.

To see just how strong the sense of depth is, compare the shaded circles to the much simpler diagram in Figure 2-9, also following Kleffner and Ramachandran’s paper.

The only difference is that, instead of being shaded, the circles are divided into solid black and white halves. Yet the depth completely disappears.

How It Works

Shadows are ...