Motion Parallax

Relative motion of near and far objects is a cue to distance. Children discover this when looking out the window of a car. Faraway objects appear to hold still while nearby trees, poles, and other objects zip by. (Children may ask, "Why are the trees going by faster than the moon?") This is also called motion parallax.

What is motion parallax?

Motion parallax. When an observer moves, objects at different distances move at varying speeds and in different directions relative to the observer. This serves a depth cue.

Sfumato (haze)

What is sfumato?

An additional depth cue is haze. Distant objects are more likely to be obscured by fog or haze, and their colors are somewhat purplish and washed out. This effect is deliberately employed in the art technique called sfumato (smoke), for example, in the background of the Mona Lisa.

What happens to the depth cue provided by shadows, when you turn a picture upside down?

Shadows provide a depth cue. Our default assumption is that light comes from above. When shadows appear on the top of a circular pattern, this makes it look like a depression. When the picture is inverted (turned upside down) the shadow appears to be on the bottom of the pattern, which makes it look like an extruded shape or bump. In the above picture, the bowl of the plastic teaspoon on the right has shadows along its upper rim, so you can see its depth. On the left is part of the same image (the bowl of the teaspoon) rotated 180 degrees. Now the shadow falls on the underside. This creates the impression of a three-dimensional object such as an egg.

Shadows provide a depth cue that is reversed when the picture is turned upside-down.

Drawings of rectangular shapes often suggest depth by showing angular corners. We are so accustomed to seeing corners and edges of rectangular objects that we readily interpret line drawings as 3-D objects. The impossible triangle from Penrose and Penrose (1958) is a famous example. M. C. Escher was inspired by the Penrose and Penrose article and used the same principle in his spectacular artwork to portray many "impossible" situations.

The impossible triangle

How does the impossible triangle play with our depth perception?

The impossible triangle plays with our unconscious inference that 2-dimensional drawings represent 3-dimensional objects. Each corner appears to be a "legal" three-dimensional shape, but the different corners do not add up to a legal gestalt. The figure is coherent on a local level (each corner by itself) but not on a global level (the figure as a whole).

Write to Dr. Dewey at psywww@gmail.com.

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