Submitted to SDA 2012 (San Francisco, January 2012)

Sidrah Laldin, Laurie Wilcox, Carly Hylton, Rob Allison

In stereoscopic vision, there is non-linear mapping between real space and disparity. In a stereoscopic 3D scene, this non-linear mapping could produce distortions of space when camera geometry differs from natural stereoscopic geometry. When the viewing distance and zero screen parallax setting are held constant and interaxial separation (IA) is varied, there is an asymmetric distortion in the mapping of stereoscopic to real space. If an object traverses this space at constant velocity, one might anticipate distortion of the perceived trajectory. This prediction is based on the premise that when the object traverses compressed space, it should appear to move slower than when it passes through expanded space. In addition, this effect should depend on the saliency of the depth information in the scene.  To determine if the predicted distortions are in fact perceived, we assessed observers’ percepts of acceleration and deceleration using an animation of a ball moving in depth through a simulated environment, viewed stereoscopically.

The method of limits was used to measure transition points between perceived acceleration and deceleration as a function of IA and context (textured vs. non-textured background).Eleven observers with normal binocular vision were tested using four IAs (35, 57.4, 65.7, and 68.21mm). The range of acceleration / deceleration rates presented was selected to bracket the predicted values based on the IA and the viewing geometry. Two environments were used to provide different levels of monocular depth cues, specifically an untextured and a tiled ground plane. For each environment and IA combination, four measures were made of the transition points between perceived acceleration and deceleration. For two of these measures, the series of clips began with an obviously accelerating object and progressed to an obviously decelerating object. The participants’ task was to identify the point at which the percept changed from accelerating to decelerating. In the other two measures, the converse procedure was used to identify the deceleration to acceleration transition.

Based on binocular geometry, we predicted that the transition points would shift toward deceleration for small IA and towards acceleration for large IA. This effect should be modulated by monocular depth cues. However, the average transition values were not influenced by IA or the simulated environment. These data suggest that observers are able to discount distortions of stereoscopic space in interpreting the trajectory of objects moving through simple environments. It remains to be seen if velocity constancy will be similarly maintained in more complex scenes or scenes containing multiple moving objects. These results have important implications for the rendering or capture of effective stereoscopic 3D content.