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 Illumination estimation Shadows

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

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

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

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

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

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

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

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

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

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

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)

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.

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)

Testing illumination perception directly Koenderink et al., 2003 Subjects can tell direction (upto convex/concave ambiguity. Worse at estimating elevation of light source (related to bas-relief ambiguity) Confirmed by other studies (Ho et al., 2006; Pont & Koenderink 2003)

Testing illumination perception directly Subjects can estimate diffuseness of a light source, errors related to collimated/diffuse. (Pont & Koenderink, 2007) Can account for two collimated light sources (Doerschner et al., 2007) Can to some extent account for changes across space (Snyder et al., 2005) and time (Gerhard & Maloney, 2010)

Light from above prior (Ramachandran, 1988) A is usually seen as convex => light from above Discs in B cannot be both convex or concave => one light source

Light from above and left prior Time to search in display depended on the shading direction of distractors. Should be lowest when shading matches prior. (Sun & Perona, 1998)

Shadows help interpret scenes Kersten et al., 1999 The spatial position of objects is influenced by cast shadows

Shadows help interpret scenes (Let’s watch a movie )

Last word about shadows Do these results hold for real-world images?

Surprisingly hard to see errors (Ostrovsky et al., 2005)

(Lopez-Moreno et al., 2010) Need to understand this better...

Summary ✓ We can estimate illumination, tested indirectly by probing reflectance and shape, and also directly. ✓ We are not perfect, but the problem is also hard (lots of ambiguities). We are consistent in our estimates, e.g., light-from-above(-and-left) prior. ✓ Shading tells us about shape, shadows about scene. ✓ Don’t know a lot about cues and effects in real- world scenes.