The power of shadows: shadow stereopsis.

Our ability to localize objects in three-dimensional space relies primarily on the stereoscopic capability of our visual system. It is generally believed that parallax disparities in the retinal images in our two eyes are required for experiencing stereovision. Traditionally, parallax disparities refer to points that are well defined within the objects, such as edges or boundaries. Shadows can create abrupt luminance changes in the scene but are neither edges nor boundaries, and their position varies with the position of the light sources. It is demonstrated that retinal images with no parallax disparity but with different shadows are fused stereoscopically, imparting depth perception to the imaged scene. Shadows are shown to be an important, hitherto undescribed stereoscopic cue for depth perception.