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Computer Vision CS 776 Spring 2014 Photometric Stereo and Illumination Cones Prof. Alex Berg (Slide credits to many folks on individual slides)

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Photometric Stereo and Illumination Cones flickr.com/photos/38102718@N05/3800198895/

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Photometric stereo (shape from shading) Can we reconstruct the shape of an object based on shading cues? Luca della Robbia, Cantoria, 1438 Slide by Svetlana Lazebnik

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Photometric stereo Assume: A Lambertian object A local shading model (each point on a surface receives light only from sources visible at that point) A set of known light source directions A set of pictures of an object, obtained in exactly the same camera/object configuration but using different sources Orthographic projection Goal: reconstruct object shape and albedo SnSn ??? S1S1 S2S2 F&P 2 nd ed., sec. 2.2.4

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Surface model: Monge patch F&P 2 nd ed., sec. 2.2.4

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Image model Known: source vectors S j and pixel values I j (x,y) Unknown: normal N(x,y) and albedo ρ(x,y) Assume that the response function of the camera is a linear scaling by a factor of k Lambert’s law: F&P 2 nd ed., sec. 2.2.4

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Least squares problem Obtain least-squares solution for g(x,y) (which we defined as N(x,y) (x,y) ) Since N(x,y) is the unit normal, (x,y) is given by the magnitude of g(x,y) Finally, N(x,y) = g(x,y) / (x,y) (n × 1) known unknown (n × 3)(3 × 1) For each pixel, set up a linear system: F&P 2 nd ed., sec. 2.2.4

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Example Recovered albedo Recovered normal field F&P 2 nd ed., sec. 2.2.4

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Recall the surface is written as This means the normal has the form: Recovering a surface from normals If we write the estimated vector g as Then we obtain values for the partial derivatives of the surface: F&P 2 nd ed., sec. 2.2.4

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Recovering a surface from normals Integrability: for the surface f to exist, the mixed second partial derivatives must be equal: We can now recover the surface height at any point by integration along some path, e.g. (for robustness, can take integrals over many different paths and average the results) (in practice, they should at least be similar) F&P 2 nd ed., sec. 2.2.4

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Surface recovered by integration F&P 2 nd ed., sec. 2.2.4

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More reading & thought problems (cone of images – great “old timey” figures) What is the set of images of an object under all possible lighting conditions? P. Belhumeur & D. Kriegman CVPR 1996 (implementing this one is the extra credit) Recovering High Dynamic Range Radiance Maps from Photographs. Paul E. Debevec and Jitendra Malik. SIGGRAPH 1997 (people can perceive reflectance) Surface reflectance estimation and natural illumination statistics. R.O. Dror, E.H. Adelson, and A.S. Willsky. Workshop on Statistical and Computational Theories of Vision 2001 gelsight.com

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