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Tandra Ghose, Stephen E. Palmer; Surface convexity and extremal edges in depth and figure-ground perception. Journal of Vision 2005;5(8):970. doi: https://doi.org/10.1167/5.8.970.
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Edges arising from depth discontinuities are powerful cues to figure-ground organization (FGO) in 2-D images. We studied pychophysically whether surface convexity and extremal edges (EEs) are effective cues to depth and FGO. EEs arise when a curved surface partly occlude itself such that the line of sight lies in the EE's tangent plane. An EE thus normally belongs to the curved surface, which is closer to the observer. It is much less likely that the curved object is occluded by a closer surface whose edge just happens to align with the EE.
If an EE is visible in a bipartite display containing a 2-D rendering of a curved surface and a flat surface, observers should tend to see the edge as belonging to the curved surface which should appear closer and figural. Displays of curved surfaces were rendered in 2D using shading gradients on one side and various flat surfaces on the other side that controlled for complexity, luminance, and other factors. As expected, the convex surface appeared closer and figural. To determine whether these results were solely due to surface convexity, we studied bipartite displays containing two orthogonal cylinders. The convex surface to which the EE belonged appeared figural more often than the other, equally convex side. We also studied bipartite figures in which one region was a surface of revolution with an EE and the other region had classical figural characteristics such as smaller size, edge convexity and/or greater meaningfulness. In most cases EEs appeared to be the stronger cue to FGO. Further experiments examined similar issues using texture gradients to render surface curvature. All preliminary experiments support the idea that surface convexity and extremal edges are powerful cues to FGO. Examples of our stimuli can be seen at http://socrates.berkeley.edu/~plab/projects.htm
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