A tendency to see objects and patterns as three-dimensional when certain visual cues are present.
People have evolved to see things as three-dimensional whenever possible — even when things are clearly not three-dimensional. The following visual cues are commonly used to encourage the perception of three-dimensional relationships:
When two overlapping objects are presented, the overlapped object is perceived to be farther away than the overlapping object.
When two similar objects of different size are presented together, the smaller object is perceived to be farther away than the larger object. The size of familiar objects can also be used to indicate the size and depth of unfamiliar objects.
When two objects are presented at different vertical locations, the object at the higher elevation is perceived to be farther away.2
Perspective When two vertical lines converge near their top ends, the converging ends of the lines are perceived to be farther away than the diverging ends.
When the texture of a surface varies in density, the areas of greater density are perceived to be farther away than areas of lesser density.
When an object has shading or shadows, the shaded areas are perceived to be the farthest away from the light source and the light areas are interpreted as being closest to the light source.
Perspective When multiple objects are presented together, the objects that are bluer and blurrier are perceived to be farther away than the objects that are less blue and blurry.
Consider these visual cues in the depiction of three-dimensional elements and environments. Strongest depth effects are achieved when the visual cues are used in combination; therefore, use as many of the cues as possible to achieve the strongest effect, making sure that the cues are appropriate for the context.
See also Figure-Ground Relationship and Top-Down Lighting Bias.
Video games make ample use of three-dimensional projection to represent three-dimensional environments on two-dimensional screens. For example, the game Black & White uses three-dimensional projection to create a believable and navigable three-dimensional world. All of the depth cues are demonstrated in these screenshots from the game.