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A Projective Framework for Radiometric Image Analysis CVPR 2009 Date: 2010/3/9 Reporter : Annie lin

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Image reflectance Image reflectance : diffuse reflectance + specular reflectance

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How to estimate shape from images? 1. shape from shading – add more constrain 2. photometric stereo – take more images -> get “normal field” but GBR (Generalize bas-Relief) ambiguity the same image can be produced by different surface under corresponding lighting direction. - GBR Transformation Shape: “depth scaling” and ‘additive plane’

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Specular reflectance and GBR GBR is resolved by ‘almost any’ additive specular reflectance component. –Only requirements Spatially uniform Isotropic Reciprocal Isotropy + reciprocity: –Local light/view directions equivalent at distinct points –Observed reflectance equal

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Outline Reflectance symmetries on the plane Isotropic and reciprocal Isotropic-reciprocal quadrilateral GBR Transformation –Reconstruction via photometric stereo and show how to resolve shape ambiguities in both “uncalibrated ” and “calibrated ” cases Applications –Uncalibrated photometric stereo –Calibrated photometric stereo Conclusion

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Reflectance symmetries on the plane The real projective plane provides and effective tool for analyzing reflectance symmetries, of which we focus on reciprocity and isotropy in this paper

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Isotropic Pair form an isotropic pair with respect to, if Gauss sphere

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Isotropic Curve and Symmetry Isotropic curve: Union of isotropic pairs Radiance function is symmetric Gauss sphere, top view

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Reciprocal Pair form an reciprocal pair with respect to if equal BRDF value Gauss sphere

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Reciprocal Curve & Symmetry Reciprocal curve Union of reciprocal pairs BRDF symmetry along curve Gauss sphere, top view

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Isotropic – reciprocal quadrilateral

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Transformation Reconstruction via photometric stereo and show how to resolve shape ambiguities in both “uncalibrated ” and “calibrated ” cases Linear transformations of the normal field correspond to projective transformations of the loane, so now disscusses the behavior of our symmetry-induced structure under projective tranformations

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Propositions Proposition 1. –A rotation (about the origin) and a uniform scaling are the only linear transformations that preserve isotropic pairs with respect to two or more lighting directions that are non-coplanar with the view direction. Proposition 2. –If the principal meridian vs is known, a classic bas-relief transformation is the only linear transformation that preserves isotropic pairs with respect to two or more sources that are non- collinear with the view direction Proposition 3. – If the lighting directions are known, the identity transformation is the only linear transformation that preserves isotropic-reciprocal quadrilaterals with respect to –two or more lighting directions that are non-collinear with the view direction.

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Application : uncalibrated photometric stereo GBR transformation affects the normals and source directions A GBR transformation scales and translates the quadrilateral to and moves the source to a different point ¯s on the principal meridian To resolve the GBR ambiguity, we must find the transformation that maps back to its canonical position. Proposition 4. –The GBR ambiguity is resolved by the isotropy and reciprocity constraints in a single image.

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By isotropy and reciprocity, an isotropic-reciprocal quadrilateral (n,m, n′,m′) with respect to s, satisfies: Using (6)(7) A hypothesis (′ v1, λ1) yields hypotheses for the point h × (v × s) and BRDF value at each point. These in turn induce a hypothesis for the reciprocal match ¯m as the intersection of the iso-BRDF curve and the join of n and the hypothesized h×(v×s) provides a measure of inconsistency, and the exhaustive 2D search is used to minimize this inconsistency.

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calibrated photometric stereo Given a set of images I(x, y, t) captured using a cone of known source directions s(t), t ∈ [0, 2) centered about view direction v, this method yields one component of the normal at every image point (x, y). if the surface is differentiable, the surface gradient direction can be recovered at each point, but the gradient magnitude is unknown. This means that one can recover the ‘iso-depth contours’ of the surface, but that these curves cannot be ordered Consider a surface S = {x, y, z(x, y)} that is described by a height field z(x, y) on the image plane. A surface point with gradient zx, zy is mapped via the Gaussian sphere to point n ≃ (zx, zy,−1) in the projective plane, and the ambiguity in gradient magnitude from[1] corresponds to a transformation of normal field ¯n(x, y) ≃ diag(1, 1, (x, y))n(x, y), where the per-pixel scaling λ (x, y) is unknown

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Proposition 5. In the general case, if differentiable height fields z1(x, y) and z2 = h(z1) are related by a differentiable function h and possess equivalent sets of iso-slope contours, the function h is linear If the surface has uniform reflectance (or has a uniform separable component), the match ¯n′ can be located by intersecting this line with the iso-intensity contour passing through ¯n. Such isotropic matches ¯n′(t) under all light directions s(t), t ∈ [0, 2) define the iso-slope contour if the spatially varying BRDF is of the form in Eq. (8), a necessary condition for two points (x1, y1) and (x2, y2) to have normal directions forming an isotropic pair is In(x1, y1, t) = In(x2, y2, t) ∀ t ∈ [0, 2). This is because normalizing the temporal radiance at each pixel to [0, 1] removes the effects of the spatially-varying reflectance terms f1 and f2.

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