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Andrea Li, Qasim Zaidi; Isotropic textures convey distance not 3-D shape. Journal of Vision 2002;2(7):112. doi: https://doi.org/10.1167/2.7.112.
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© ARVO (1962-2015); The Authors (2016-present)
Textures like polka-dots and isotropic filtered noise contain equal energy at all orientations in the spatial frequency spectrum. Isotropic patterns have been used to render convex 3-D shapes because it is thought that systematic deviations from isotropy in the projected image convey the shape of the surface. Li & Zaidi (Vis Res, 2000, 2001) and Zaidi & Li (Vis Res, 2002) showed that signature patterns of orientation modulations in the image are necessary for veridical shape perception of corrugated surfaces. Projected surfaces with isotropic textures do not exhibit these orientation modulations, but show increases in 2-D spatial frequency as distance increases, and increases in 1-D frequency along the direction of slant for increasing slant angles. Using a 5AFC task, we measured percepts of perspectively projected half-cycles of vertical sinusoidal corrugated surfaces overlaid with isotropic patterns. Images of upright concave and convex half-cycles exhibit lower frequencies in the central fronto-parallel section and higher frequencies in the slanted edge sections. Conversely, images of upright rightward and leftward slanted half-cycles contain lower frequencies along the edges and higher frequencies in the center. Observers correctly identified convexities, but consistently misperceived concavities and slants as convex and concave respectively. Spatial frequency modulations were therefore being interpreted exclusively as cues to distance: low frequencies were interpreted as closer and high frequencies as farther from the observer. For patterns like compound vertical gratings, frequency modulations cannot be detected, and corrugated surfaces appear flat. In images of pitched corrugations, isotropic patterns flow along lines of maximum curvature. These lines are nearly parallel across the image, and observers confuse sign of pitch and sign of curvature. Isotropic textures thus convey distance but do not, in general, convey qualitatively veridical 3-D shape.
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