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Exploring Global Convexity in Local Shape Perception: Insights from Shape-from-Shading

This study investigates how global convexity influences local shape perception through shape-from-shading. By examining the interactions between light direction, viewpoint, and global shape in shape perception, the research tests three prior assumptions regarding visual interpretation. Results from 512 trials with naïve observers reveal significant effects of light direction, viewpoint, and global shape on perceptual accuracy. The findings suggest that the prior for global convexity is comparably strong to established lighting and viewpoint expectations, prompting further exploration into spatial scales and visual angles in shape analysis.

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Exploring Global Convexity in Local Shape Perception: Insights from Shape-from-Shading

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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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