Abstract
A single experiment is reported that measured the apparent stereoscopic shapes of symmetric and asymmetric objects at different viewing distances. The symmetric stimuli were specifically designed to satisfy the minimal conditions for computing veridical shape from symmetry. That is to say, they depicted complex, bilaterally symmetric, plane-faced polyhedra whose symmetry planes were oriented at an angle of 45° relative to the line of sight. The asymmetric stimuli were distorted versions of the symmetric ones in which the 3D position of each vertex was randomly displaced. Prior theoretical analyses have shown that it is mathematically possible to compute the 3D shapes of symmetric stimuli under these conditions, but those algorithms are useless for asymmetric objects. The results revealed that the apparent shapes of both types of objects were expanded or compressed in depth as a function of viewing distance, in exactly the same way as has been reported in many other studies, and that the presence or absence of symmetry had no detectable effect on performance.
Highlights
A single experiment is reported that measured the apparent stereoscopic shapes of symmetric and asymmetric objects at different viewing distances
Our ability to achieve shape constancy is far from perfect, and measurable distortions of apparent shape have been reported for several different sources of optical information, such as shading, texture, motion, or binocular disparity
One well-documented violation of shape constancy occurs when objects are viewed stereoscopically at different distances, such that more distant objects appear compressed in depth relative to physically identical ones that are closer to the observer (e.g., Baird & Biersdorf, 1967; Campagnoli et al, 2017; Campagnoli & Domini, 2019; Gilinsky, 1951; Glennerster et al, 1996, 1998; Harway, 1963; Hecht et al, 1999; Heine, 1900; Johnston, 1991; Johnston et al, 1994; Norman et al, 1996; Scarfe & Hibbard, 2006; Shaffer et al, 2008; Todd & Norman, 2003; Toye, 1986; Wagner, 1985)
Summary
A single experiment is reported that measured the apparent stereoscopic shapes of symmetric and asymmetric objects at different viewing distances. One well-documented violation of shape constancy occurs when objects are viewed stereoscopically at different distances, such that more distant objects appear compressed in depth relative to physically identical ones that are closer to the observer (e.g., Baird & Biersdorf, 1967; Campagnoli et al, 2017; Campagnoli & Domini, 2019; Gilinsky, 1951; Glennerster et al, 1996, 1998; Harway, 1963; Hecht et al, 1999; Heine, 1900; Johnston, 1991; Johnston et al, 1994; Norman et al, 1996; Scarfe & Hibbard, 2006; Shaffer et al, 2008; Todd & Norman, 2003; Toye, 1986; Wagner, 1985). In order to obtain an accurate estimate of 3D structure from binocular disparity, it is necessary to somehow compensate for variations in viewing distance, and the empirical evidence indicates that human observers are unable to do that accurately
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