Abstract
The shading cue is supposed to be a major factor in monocular stereopsis. However, the hypothesis is hardly corroborated by available data. For instance, the conventional stimulus used in perception research, which involves a circular disk with monotonic luminance gradient on a uniform surround, is theoretically ‘explained’ by any quadric surface, including spherical caps or cups (the conventional response categories), cylindrical ruts or ridges, and saddle surfaces. Whereas cylindrical ruts or ridges are reported when the outline is changed from circular to square, saddle surfaces are never reported. We introduce a method that allows us to differentiate between such possible responses. We report observations on a number of variations of the conventional stimulus, including variations of shape and quality of the boundary, and contexts that allow the observer to infer illumination direction. We find strong and expected influences of outline shape, but, perhaps surprisingly, we fail to find any influence of context, and only partial influence of outline quality. Moreover, we report appreciable differences within the generic population. We trace some of the idiosyncrasies (as compared to shape from shading algorithms) of the human observer to generic properties of the environment, in particular the fact that many objects are limited in size and elliptically convex over most of their boundaries.
Highlights
T1hIenstrhoadduinctgiocnue (Horn and Brooks 1989; Luckiesh 1916; Metzger 1975; Turhan 1935) is one of the generic pictorial, or monocular, depth cues
Hatched chicks peck at circular disks filled with linear luminance gradients in their visual fields (Hershberger 1970; Hess 1950; Metzger 1975), yielding a heightened probability to aim pecking activity at graminoid seed grains, promoting foraging success (Riedl 1984)
J Wagemans, A J van Doorn, J J Koenderink skies (Minnaert 1993). It is a common understanding in the literature that the ‘light from above assumption’ is a crucial part of the optical interfaces of the majority of genera (Riedl 1984), including man (Brewster 1832; Rittenhouse 1786)
Summary
T1hIenstrhoadduinctgiocnue (Horn and Brooks 1989; Luckiesh 1916; Metzger 1975; Turhan 1935) is one of the generic pictorial, or monocular, depth cues. Hatched chicks peck at circular disks filled with linear luminance gradients in their visual fields (Hershberger 1970; Hess 1950; Metzger 1975), yielding a heightened probability to aim pecking activity at graminoid seed grains, promoting foraging success (Riedl 1984) In both examples the direction of the luminance gradient is important. J Wagemans, A J van Doorn, J J Koenderink skies (the zenith being the brightest patch in the scene) (Minnaert 1993) It is a common understanding in the literature that the ‘light from above assumption’ is a crucial part of the optical interfaces of the majority of genera (Riedl 1984), including man (Brewster 1832; Rittenhouse 1786). The relations are important for the present paper, which is why we start with a more formal discussion, our contribution is mainly of an empirical, investigative nature
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