This illustration depicts two people on a level surface at differing distances. The man in the background, although smaller, looks perfectly normal when compared to the man in the foreground.
In the lower right, you will see that the man in the background has been brought to the same elevation as the man in the foreground. Now you have another size illusion. This illustration is the opposite of the previous Shepard illustration.
In the Shepard illustration, the foreground figure (normally with a larger visual angle) is placed in the background. This causes the background figure to appear larger in comparison with the foreground figure.
In this illustration, the background figure (with the normally smaller visual angle) is moved to the foreground.
Another variable that needs to be considered is whether the object is perceived as resting on a ground or "floating" off the ground. This variable does affect how you perceive the size illusion. Moving the figure off the ground drastically changes your perception of the scene. An object or figure that is floating off the ground is qualitatively different from an object that is resting on a surface.
The perspective background is also extremely important, as it is what suggests a scene in depth. If you remove the perspective background, the scene is perceived as flat, and you no longer perceive the size illusion, or if you do it is very weak.
Changing the figure's "height" without perspective becomes meaningless, and so the size illusion will not work. (Your visual system can, however, infer a horizon, and so you can still get an effect. See the floating boxes illusion.
This demonstrates that your visual system exploits many regularities from the environment when it determines size and distance relationships.
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