Shadow & Depth Perception
After Kersten, Mamassian, and Knill (1997)
Shadow moves horizontally → ball floats
Shadow tracks ball → ball rolls on surface
The ball follows the identical diagonal path in both displays and its size never changes.
The only difference is the shadow. When the shadow moves only horizontally, your visual system
interprets the vertical separation between ball and shadow as height above the surface — so the
ball appears to rise and float. When the shadow tracks directly beneath the ball, the ball appears
to travel along the receding floor. A vivid demonstration that cast shadows are a powerful cue to
3-D spatial layout.