The determinants of visual scale (size and distance) under monocular viewing are still largely unknown. The problem of visual scale under monocular viewing becomes readily apparent when one moves about within a virtual environment. It might be thought that the absolute motion parallax of stationary objects (both in real and virtual environments), under the assumption of the stationarity, would immediately determine their apparent size and distance for an observer who is walking about. We sought to assess the effectiveness of observer- produced motion parallax in scaling apparent size and distance within near space. We had subjects judge the apparent size and distance of real and virtual objects under closely matched conditions Real and virtual targets were 4 spheres seen in darkness at eye level. the targets ranges in diameter from 3.7 cm to 14.8 cm and were viewed monocularly from difference distances, with a subset of the size/distance combinations resulting in projectively equivalent stimuli at the viewing origin. subjects moved laterally plus and minus 1 m to produce large amounts of motion parallax. when angular size was held constant and motion parallax acted as a differential cue to target size and distance, judged size varied by a facto or 1.67 and 1.18 for the real and virtual environments, respectively, well short of the fourfold change in distal size. Similarly, distance judgments varied by factors of only 1.74 and 1.07 respectively. We conclude that absolute motion parallax only weakly determines the visual scale of nearby objects varying over a fourfold range in size.
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