The Effect of Multiple Depth Cues in the Perception of Occlusion

The perception of depth in visual displays containing multiple sources of depth information was investigated. A computer graphics system generated randomly textured visual patterns that were presented on a television screen. One half of each pattern moved continuously toward the remaining stationary half. The texture elements of the moving half of the pattern were deleted from view as they contacted the stationary pattern area. This kinetic occlusion of texture elements tended to be perceived as one surface passing behind another. Two experiments were performed in which the depth cues of brightness, texture density, and relative velocity were systematically added to kinetic occlusion patterns.

The first experiment explored the effect of a single depth cue, brightness or texture density, when combined with kinetic occlusion. The moving half of each pattern consisted of adjacent horizontal sections. A moving section could contrast in brightness or texture density with the other moving sections or with the stationary area. Subjects reported the moving sections as passing behind the stationary area regardless of prevailing brightness or texture density differences. The effect of brightness or texture density was to vary the perceived depth ordering of the moving sections passing behind the stationary area.

In the second experiment, brightness, texture density, and relative velocity were simultaneously combined with kinetic occlusion. The moving horizontal sections of a visual pattern could differ in relative velocity, and both the moving sections and stationary area could differ in brightness and/or texture density. The perceived depth ordering of the moving sections was determined by brightness and texture density; relative velocity did not exert a systematic effect. Subjects agreed in their judgments of the moving sections, but individual differences emerged in the judged depth of the stationary area relative to the moving sections. Occlusion was the most frequently occurring perceptual organization (i.e., all moving sections passing behind the stationary area), but other organizations occurred as well. The stationary area could be perceived as: 1) at a greater depth than all the moving sections; 2) at an intermediate depth between two moving sections; and 3) at the same depth as one of the moving sections. Some subjects used only one organization, whereas others used two or more organizations. These perceptual organizations represented different ways of resolving conflicts between kinetic occlusion and brightness/texture density combinations.