1. Perception of illumination and shadows Lavanya Sharan February 14th, 2011

2. Studied indirectly Not a lot of studies examine illumination or shading directly Role of illumination and shading in perception of 3-D shape, reflectance, object identity and space

3. Outline Shape from shading Illumination estimation Shadows

4. Shape from shading is under- constrained. Fig 9.11, VPfaCGP And yet, we perceive unique and stable shapes.

5. Theoretical cues for shape from shading Reflectance map (Horn, 1977) Isophotes (Koenderink & Van Doorn, 1980) Image orientation (Fleming, Torralba & Adelson, 2004)

6. Theoretical cues for shape from shading Reflectance map (Horn, 1977) Representation of scene brightness as a function of 3-D surface orientation Ignores shadows, inter-reflections, vignetting, translucency etc. Unclear whether this relationship between image intensity & surface orientation is used by visual system Horn & Sjoberg, 1978

7. Theoretical cues for shape from shading Isophotes (Koenderink & Van Doorn, 1980) Curves of constant intensity, depend on illumination and shape Patterns of isophotes can reveal shape (under assumptions of lighting) The visual system could use these as a cue Fig 9.12, VPfaCGP

8. Theoretical cues for shape from shading Image orientation (Fleming, Torralba & Adelson, 2004) Orientation filters have strong responses for strong curvature regions. By measuring these across a surface can get local geometry The visual system could use this relationship between image orientation and surface curvature. Fig 9.13, VPfaCGP

9. Why is shape from shading hard? Lots of ambiguities. Convex vs. concave? Surface orientation change vs. surface reflectance change? Bas-relief ambiguity

10. Ambiguities in shape-from-shading Convex vs. concave? (Ramachandran, 1988)

11. Reflectance vs. orientation change? (Knill & Kersten, 1991) Ambiguities in shape-from-shading

12. Reflectance vs. orientation change? (Knill & Kersten, 1991) Ambiguities in shape-from-shading

13. Bas-relief ambiguity (Belhumeur et al., 1999) Ambiguities in shape-from-shading

14. Gauge figure task to study shape perception (Koenderink et al., 1992)

15. What have we learnt from gauge figure tasks? Subjects are consistent. Their (inferred) shapes are related by affine transforms. (Koenderink et al., 1992) For simple shapes, contours are often enough for estimating shape, shading plays a lesser role. (Mamassian & Kersten., 1996; Koenderink et al., 1996; Cole et al., 2009) Illumination changes causes subtle distortions of perceived shape. (Koenderink et al., 1996; Caniard & Fleming, 2007)

16. Intrinsic image analysis Fig 9.15, VPfaCGP Idea: Visual system separates retinal image into layers that represent distinct physical causes. (Barrow & Tenenbaum, 1978) How? Proposals include Retinex, anchoring theory, etc.

17. Mutual illumination affects reflectance perception Ruppertsberg & Bloj, 2007 We can distinguish black and white rooms seen in isolation based on inter- reflections. (Gilchrist & Jacobsen, 1984) Mutual illumination estimation is not perfect, sometimes perceived as surface color. (Bloj et al., 1999; Doerschner et al., 2004)