Consider the example of Figure 13.1. In Figure 13.1(a), a staircase is lit by a single light source that casts no shadows. A ball is visible in the foreground. Although the ball is not occluded by any other object, it is impossible to determine whether the ball is resting on a step or if it is airborne. We have no clue on its relative position with respect to the staircase, even though we can judge from a priori knowledge that the ball is not too small to be closer to the viewer than to the staircase. Figure 13.1(b) shows three possible position/size combinations that could have produced the same version of the ball raster from the same viewpoint.
Now, if we add shadows, the set of visible constraints that the eye needs to extract the relative distance of the objects is complete. Figure 13.1(c)–(e) show the result of the three different ball positions of Figure 13.1(b) when shadows are applied to the scene.