Least-Squares Rigid Motion Using SVD

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1 Least-Squares igid Motion Using SVD Olga Sorkine Abstract his note summarizes the steps to computing the rigid transformation that aligns two sets of points Key words: Shape matching, rigid alignment, rotation, SVD Problem statement Let P = {p, p 2,, p n } and Q = {q, q 2,, q n } be two sets of corresponding points in d We wish to find a rigid transformation that optimally aligns the two sets in the least squares sense, ie, we seek a rotation and a translation vector t such that (, t = armgin,t where > 0 are weights for each point pair (p i + t q i 2, ( In the following we will detail the derivation of and t; readers that are interested in the final recipe may skip the proofs and go directly Section 4 2 Computing the translation Assume is fixed and denote F (t = n (p i + t q i 2 We can find the optimal translation by taking the derivative of F wrt t and searching for its roots: 0 = F n t = 2 (p i + t q i = ( n ( n = 2t + 2 p i 2 q i (2 echnical notes 24 February 2009

2 Denote p = n p i n, q = n q i n (3 By rearranging the terms of (2 we get t = q p (4 In other words, the optimal translation t maps the transformed weighted centroid of P to the weighted centroid of Q Let us plug the optimal t into our objective function: (p i + t q i 2 = p i + q p q i 2 = (5 = (p i p (q i q 2 (6 We can thus concentrate on computing the rotation by restating the problem such that the translation would be zero: So we look for the optimal rotation such that x i := p i p, y i := q i q (7 = argmin x i y i 2 (8 3 Computing the rotation Let us simplify the expression we are trying to minimize in (8: x i y i 2 = (x i y i (x i y i = (x i y i (x i y i = = x i x i y i x i x i y i + y i y i = = x i x i y i x i x i y i + y i y i (9 We got the last step by remembering that rotation matrices imply = I (I is the identity matrix Note that x i y i is a scalar: x i has dimension d, is d d and y i is d For any scalar a we trivially have a = a, therefore x i y i = (x i y i = y i x i (0 2

3 herefore we have x i y i 2 = x i x i 2y i x i + y i y i ( Let us look at the minimization and substitute the above expression: argmin = argmin = argmin x i y i 2 = argmin (x i x i 2yi x i + yi y i = ( n x i x i 2 yi x i + yi y i = ( 2 yi x i (2 he last step (removing n x i x i and n y i y i holds because these expressions do not depend on at all, so excluding them would not affect the minimizer he same holds for multiplication of the minimization expression by a scalar, so we have argmin ( 2 y i x i = argmax yi x i (3 We note that yi x i = tr ( W Y X, (4 where W = diag(w,, w n is an n n diagonal matrix with the weight on diagonal entry i; Y is the d n matrix with y i as its columns and X is the d n matrix with x i as its columns We remind the reader that the trace of a square matrix is the sum of the elements on the diagonal: tr(a = n a ii See Figure for an illustration of the algebraic manipulation herefore we are looking for a rotation that maximizes tr ( W Y X Matrix trace has the property tr(ab = tr(ba (5 for any matrices A, B of compatible dimensions herefore tr ( W Y X = tr ( (W Y (X = tr ( XW Y (6 Let us denote the d d covariance matrix S = XW Y ake SVD of S: S = UΣV (7 3

4 w w 2 y w y 2 x x 2 x n = w n y n w y w 2 y 2 x x 2 x n w y x w 2 y 2 x 2 w * = = w n y n * w n y n x n Fig Schematic explanation of n y i x i = tr(w Y X Now substitute the decomposition into the trace we are trying to maximize: tr ( XW Y = tr (S = tr ( UΣV = tr ( ΣV U (8 he last step was achieved using the property of trace (5 Note that V, and U are all orthogonal matrices, so M = V U is also an orthogonal matrix his means that the columns of M are orthonormal vectors, and in particular, m j m j = for each M s column m j herefore all entries m ij of M are smaller than in magnitude: d = m j m j = m 2 ij m ij m ij < (9 So what is the maximum possible value for tr(σm? emember that Σ is a diagonal matrix with non-negative values σ, σ 2,, σ d 0 on the diagonal herefore: σ σ2 tr(σm = σ d m m 2 m d m 2 m 22 m 2d d d = σ i m ii σ i (20 m d m d2 m dd herefore the trace is maximized if m ii = Since M is an orthogonal matrix, this means that M would have to be the identity matrix! I = M = V U V = U = V U (2 4

5 Orientation rectification he process we just described finds the optimal orthogonal matrix, which could potentially contain reflections in addition to rotations Imagine that the point set P is a perfect reflection of Q we will then find that reflection, which aligns the two point sets perfectly and yields zero energy (8 the global minimum in this case However, if we restrict ourselves to rotations only, there might not be a rotation that perfectly aligns the points Checking whether = V U is a rotation is simple: if det(v U = it contains reflection, otherwise det(v U = + Assume det(v U = : this means that the global maximum of tr(σm is generally not attainable by a rotation, and we need to look for the next best thing Let us look for other (local maxima of tr(σm as a function of M s diagonal values m ii : tr(σm = σ m + σ 2 m σ d m dd =: f(m,, m dd (22 If we consider the m ii s as variables, the domain of (m,, m dd is a subset of [, ] d he function f is linear in the m ii s, so it attains its extrema on the boundary of the domain (no extrema on the interior Since our domain is rectilinear, the extrema will be attained at the vertices (±, ±,, ± We had to rule out (,,, since that gave a reflection, therefore the next best shot is (,,,, : tr(σm = σ + σ σ d σ d (23 his is larger than any other combination (except (,,, because σ d is the smallest singular value o summarize, we arrive at the fact that if det(v U =, we need M = V U = = V U (24 We can write a general formula that encompasses both cases, det(v U = and det(v U = : = V det(v U U (25 5

6 4 igid motion computation summary Let us summarize the steps to computing the optimal translation t and rotation that minimize (p i + t q i 2 ( Compute the weighted centroids of both point sets: p = n p i n, q = n q i n (2 Compute the centered vectors x i := p i p, y i := q i q, i =, 2,, n (3 Compute the d d covariance matrix S = XW Y, where X and Y are the d n matrices that have x i and y i as their columns, respectively, and W = diag(w, w 2,, w n (4 Compute the singular value decomposition S = UΣV he rotation we are looking for is then = V (5 Compute the optimal translation as U det(v U t = q p 6

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