The effectiveness of depth cues such as occlusion and shading was examined in images defined by color, texture, binocular disparity or motion. Line drawings represented in any of these modalities were able to signal shape and occlusion showing that contour occlusions are analyzed at a high level, following the reintegration of the separate representations of visual attributes such as color and motion. Subjective contours, on the other hand, could be seen only if the figures were defined by luminance differences. Figures whose depth depended on the interpretation of shadows also required luminance differences: shadow regions had to be darker than the surrounding, non-shadow regions. Shadows areas filled with colors or textures that could not occur in natural scenes were perceived as shadows as readily as real shadows. Even when shadow and non-shadow regions had different depths or had textures that moved in different directions, the depth from shading was still seen as long as there was an appropriate brightness difference. These findings indicate a variety of mechanisms analyze cues to 3-dimensional structure. Occlusion cues in line drawings appear to be analyzed by a general purpose mechanism having access to all pathways of the visual system. Subjective contours and shadows appear to depend on special purpose processes accessing only the luminance pathway. Finally, although natural constraints have proved useful in solving many visual problems, they did not play a significant role in the interpretation of the depth cues examined here. © 1987 Academic Press, Inc. All rights reserved.
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