Horizontal and vertical disparity, eye position, and stereoscopic slant perception

The slant of a stereoscopically defined surface cannot be determined solely from horizontal disparities or from derived quantities such as horizontal size ratio (HSR). There are four other signals that, in combination with ho rizontal disparity, could in principle allow an unambiguous estimate of sla nt: the vergence and version of the eyes, the vertical size ratio (VSR), an d the horizontal gradient of VSR. Another useful signal is provided by pers pective slant cues. The determination of perceived slant can be modeled as a weighted combination of three estimates based on those signals: a perspec tive estimate, a stereoscopic estimate based on HSR and VSR, and a stereosc opic estimate based on HSR and sensed eye position. In a series of experime nts, we examined human observers' use of the two stereoscopic means of esti mation. Perspective cues were rendered uninformative. We found that VSR and sensed eye position are both used to interpret the measured horizontal dis parities. When the two are placed in conflict, the visual system usually gi ves more weight to VSR. However, when VSR is made difficult to measure by u sing short stimuli or stimuli composed of vertical lines, the visual system relies on sensed eye position. A model in which the observer's slant estim ate is a weighted average of the slant estimate based on HSR and VSR and th e one based on HSR and eye position accounted well for the data. The weight s varied across viewing conditions because the informativeness of the signa ls they employ vary from one situation to another. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.