Absolute Motion Parallax Weakly Determines Visual Scale in Real and Virtual Environments

The determinants of visual scale (size a d distance) under monocular viewing are still largely unknoyh. 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 their stationarity, would immediately determine their apparent size and dis@ce for an observer who is walking about. We sought to assess the effectiveness of observer-produced motion parallax irl scaling apparent size and distance within near space. We had subjects judge the apparent size and distance of real and v q objects under closely matched conditions. Real and virtual targets were 4 spheres seen in darkness at eye level. The target$ ranged in diameter fiom 3.7 cm to 14.8 cm and were viewed monocularly fiom different distances, with a subset of the size/bstance combinations resulting in projectively equivalent stimuli at the viewing origin. Subjects moved laterally plus and binus 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 factor of 1.67 and 1.18 for the real and virtual environments, respectively, well short of the four-fold 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 four-fold range in size.