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Perception of illumination and shadows

Perception of illumination and shadows. Lavanya Sharan February 14th, 2011. 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. Outline. Shape from shading

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Perception of illumination and shadows

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

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

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

  14. 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)

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

  16. Mutual illumination affects reflectance perception 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) Ruppertsberg & Bloj, 2007

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