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A preference for global convexity in local shape perception

A preference for global convexity in local shape perception. Michael S. Langer Heinrich H. B ü lthoff Max-Planck-Institute for Biological Cybernetics T übingen, Germany. Shape from Shading. Depth-reversal ambiguity in shape-from- shading. valley. hill.

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A preference for global convexity in local shape perception

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  1. A preference for global convexity in local shape perception • Michael S. Langer Heinrich H. Bülthoff • Max-Planck-Institute for Biological CyberneticsTübingen, Germany

  2. Shape from Shading

  3. Depth-reversal ambiguity in shape-from- shading valley hill

  4. Hollow Mask Illusion (Luckiesh, 1916)

  5. Hollow mask illusion is the sum of two factors (Johnston et a. ’92, Hill & Bruce ’94) • face • familiarity + global convexity

  6. Global shape discrimination is easy convex concave “face” “mask”

  7. Procedure

  8. Fixation Mark (1 sec.)

  9. Task: hill or valley ?

  10. Three prior assumptions were tested • light source direction (Rittenhouse 1786,…..) • viewpoint direction (Reichel & Todd 1990, Mamassian & Landy 1998) • global shape (Johnston et. al 1992, Hill & Bruce 1994 )

  11. Example in which all three priors assumptions are met 1. light from above 2. viewpoint from above 3. shape is convex

  12. Example in which all three prior assumptions fail shape is concave viewpoint from below light from below

  13. light source direction (collimated source) light from above light from below

  14. viewing direction (Reichel and Todd 1990) view from above view from below

  15. viewing direction (globally concave surface) view from below viewfrom above

  16. Design • three factors : • - light direction • - viewpoint • - global shape • 2 x 2 x 2 within observer • 512 trials (64 per condition)

  17. ANOVA Results (12 naïve observers) • Main effects: • light direction F(1,11) = 6.8, p = .025 • viewpoint F(1,11) = 9.6, p = .01 • global shape F(1,11) = 46.1, p < .001

  18. Linear Regression • percent correct • = 51 + 10 * light source direction • + 11 * viewing direction • + 13 * global shape • (Each factor had value of –1 or 1)

  19. Examples: 87% (best) 15% (worst)

  20. Conclusion • The prior for global convexity is used in local shape from shading. • The global convexity prior had roughly the same strength as the light-from-above and viewpoint-from-above priors.

  21. Open questions • What are the spatial scales over which shading information is analyzed? • How does the prior on global shape depend on visual angle?

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