Mind Hacks

Hack #21. Objects Move, Lighting Shouldn’t

Moving shadows make us see moving objects rather than assume moving light sources.

Shadows get processed early when trying to make sense of objects, and they’re one of the first things our visual system uses when trying to work out shape. [Hack #20] further showed that our visual system makes the hardwired assumption that light comes from above. Another way shadows are used is to infer movement, and with this, our visual system makes the further assumption that a moving shadow is the result of a moving object, rather than being due to a moving light source. In theory, of course, the movement of a shadow could be due to either cause, but we’ve evolved to ignore one of those possibilities—rapidly moving objects are much more likely than rapidly moving lights, not to mention more dangerous.

In Action

Observe how your brain uses shadows to construct the 3D model of a scene. Watch the ball-in-a-box movie at:

• http://gandalf.psych.umn.edu/~kersten/kersten-lab/images/ball-in-a-box.mov (small version)

• http://gandalf.psych.umn.edu/~kersten/kersten-lab/demos/BallInaBox.mov (large version, 4 MB)

Note

If you’re currently without Internet access, see Figure 2-12 for movie stills.

The movie is a simple piece of animation involving a ball moving back and forth twice across a 3D box. Both times, the ball moves diagonally across the floor plane. The first time, it appears to move along the floor of the box with a drop shadow directly beneath and touching the ...