Abstract
When computer generated images are used for real-time display in simulator applications, much of the fine detail available from the natural world, or even from video-film, is not available to an observer. This lack of detail leads to a reduction in the number of sources of depth information (cues to depth) that are available to specify the layout of the displayed scene. Amongst the cues normally available are luminance gradients and luminance contrast gradients, each deriving from luminance differences between components of the displayed scene; however, in computer generated images, these two cues do not always conform to the intended natural world image, and can offer conflicting information. While not referring explicitly to luminance gradients, Ames (1949) demonstrated that the brighter of two otherwise identical objects would appear nearer; his Demonstration 18 offering a negative luminance gradient similar to that arising in the natural world from atmospheric perspective. Similarly, Ross (1967, 1993) and O'Shea, Blackburn and Ono (1994) have shown a similar effect to Ames (1949), but with higher contrast replacing increased brightness, which they liken to the negative luminance contrast gradients that are also available in the natural world due to atmospheric perspective. The luminance gradient, and luminance contrast gradient cues are generally in accord when the scene background is light, but are in conflict where the background is dark. The experiments reported here show that either gradient can function as a cue to depth, and hence to the spatial layout of a depicted scene, and that conflicts between them are resolved in a way that takes into account the amount and type of other depth information available to an observer. Such a form of conflict resolution and cue combination is in accord with the separate items of depth information being processed either partly or wholly in parallel, so that the strength of each cue is determined by reference to the other available cues. When applied to simulators using computer generated images, these results suggest that both users, and scenario designers, require an awareness of the possible effect of a change to any item of depth information, and in particular to depth information that has its origin in luminance differences between objects in the depicted scene.