# Registration D.A. Forsyth, UIUC

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1 Registration D.A. Forsyth, UIUC

2 Registration Place a geometric model in correspondence with an image could be 2D or 3D model up to some transformations possibly up to deformation Applications very important in medical imaging building mosaics representing shapes form of object recognition

3 Correspondence Registration implies correspondence because once they re in register, correspondence is easy Correspondence yields registrations take correspondences and solve for best registration Interact in a variety of ways in the main algorithms

4 Medical Application Register scan of patient to actual patient To remove only affected tissue To minimize damage by operation planning To reduce number of operations by planning surgery Register viewing device to actual patient virtual reality displays

5 Images courtesy of Eric Grimson

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11 Algorithms Hypothesize and test Iterative closest point Coarse-to-fine search

12 Registration by Hypothesize and Test General idea Hypothesize correspondence Recover pose Render object in camera (widely known as backprojection) Compare to image Issues where do the hypotheses come from? How do we compare to image (verification)? Simplest approach Construct a correspondence for all object features to every correctly sized subset of image points These are the hypotheses Expensive search, which is also redundant.

13 Correspondences yield transformations 2D models to 2D images Translation one model point-image point correspondence yields the translation Rotation, translation one model point-image point correspondence yields the translation one model direction-image direction correspondence yields the rotation Rotation, translation, scale two model point-image point correspondences

14 Correspondences yield transformations 3D models to 3D info Translation one model point-image point correspondence yields the translation Rotation, translation points, directions one model point-image point correspondence yields the translation two model direction-image direction correspondences for rotation Rotation, translation, scale points, directions two model point-image point correspondences and one direction lines two disjoint line correspondences yield rotation, translation, scale Many other correspondences work

15 Correspondences yield transformations 3D models, 2D images, calibrated orthographic camera Translation one model point-image point correspondence yields all that can be known Translation, rotation three model point-image point correspondence yields all that can be known Etc (perspective cameras, and so on)

16 Pose consistency A small number of correspondences yields a camera Strategy: Generate hypotheses using small numbers of correspondences (e.g. triples of points for a calibrated perspective camera, etc., etc.) Backproject and verify Notice that the main issue here is camera calibration Appropriate groups are frame groups

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18 Figure from Huttenlocher+Ullman 1990

19 Voting on Pose Each model leads to many correct sets of correspondences, each of which has the same pose Vote on pose, in an accumulator array This is a hough transform, with all it s issues.

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26 Verification Is the object actually there? Edge based project object model to image, score whether image edges lie close to object edges Orientation based project object model to image, score whether image edges lie close to object edges at the right orientation More sophisticated Opportunity!

27 Figure from Rothwell et al, 1992

28 Iterative closest point For registering 2D-2D or 3D-3D point sets typically under translation, rotation and scale Iterate Find closest point on measurement to each point on model using current pose Minimize sum of distances to closest points as a function of pose Variants model consists of lines, surface patches, etc.

29 Model: triangle set of 8442 triangles Point set Figure from Besl+McKay, 1992

30 Figure from Besl+McKay, 1992 Points registered to triangles

31 Model: Bezier patches Registered to point set Figure from Besl+McKay, 1992

32 Variants Use Levenberg-Marquardt on robust error measure ignore failures of differentiability caused by correspondence Fitzgibbon 2003

33 Initial alignment (Fitzgibbon, 2003, red rabbit to blue rabbit)

34 Solution (Fitzgibbon, 2003, red rabbit to blue rabbit)

35 Coarse to fine search General idea: Advantage many minima may be available for registration problems eg ICP for 2D object to points on image edges search a coarse representation at multiple points take each local minimum, search a refined representation possibly repeat multiple times coarse representation is fast to search so you can look at many poses fine representation gives accurate estimates Figure from Fitzgibbon, 2003

36 Registration and deformation Medical applications often deal with deformable objects Real objects often deform, too equivalently, deformation is an important part of matching e.g. matching one car to another matching one flower to another, etc. Idea: build parametric deformation model into registration process

37 MRI CTI NMI USI

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39 Parametric deformation models Assume we have a set of points (x, y) which should deform to (u, v) Good models u = f 1 (x, y, θ),v = f 2 (x, y, θ) Affine u = a 00 x + a 01 y + a 2,v = a 10 x + a 11 y + a 3 More deformation (here the f s are small ) u = f 1 (x, y, θ)+a 00 x + a 01 y + a 2,v = f 2 (x, y, θ)+a 10 x + a 11 y

40 Radial basis function deformations Choose some special points in x, y space deformation functions become: f l (x, y, θ) = i (x i,y i ) θ i φ(x, y; x i,y i ) where phi depends only on distance: eg φ(x, y; x i,y i )= 1 (x x i )2 +(y y i )2 + ɛ 2

41 Radial basis function deformation j points We must choose theta, a s least squares [ (uj {[ i θ i,1φ(x j,y j ; x i,y i )] + a 00x + a 01 y + a 2 }) 2 + (v j {[ i θ i,2φ(x j,y j ; x i,y i )] + a 00x + a 01 y + a 2 }) 2 ] Solving this gives a linear system! but we might get f s that are too big

42 Radial basis function deformation j points Penalize the least squares [ (uj {[ i θ i,1φ(x j,y j ; x i,y i )] + a ] 00x + a 01 y + a 2 }) 2 + (v j {[ i θ i,2φ(x j,y j ; x i,y i )] + a 00x + a 01 y + a 2 }) 2 + λ(θi,1 2 + θi,2) 2 And we still have a linear system! ICP matching: Iterate Solve for a s, thetas Fix a s, thetas, choose correspondences

43 Deformation is like flow Notice the similarity between estimating deformation estimating flow field Recall I_1(x, y) -> I_2(u(x, y), v(x, y)) I(x, y, t) -> I(x+a(x, y), y+b(x,y), t+1) accurate local estimates of flow are hard (no good local description) options parametric flow model smooth

44 Deformation is like flow Plausible flow model Not much help if the m s are fixed Idea: let the m s vary with space, and penalize derivatives Cost function: I(x, y) I(x + m 1 x + m 2 y + m 3,y+ m 4 x + m 5 y + m 6 ) (I(x, y) I(x + m 1 x + m 2 y + m 3,y+ m 4 x + m 5 y + m 6 )) 2 x,y simplify to first order term in Taylor series x,y ((m 1 x + m 2 y + m 3 ) I x +(m 4x + m 5 y + m 6 ) I y )2

45 Deformation is like flow Overall cost function [ ((m1 x + m 2 y + m 3 ) I x +(m ] 4x + m 5 y + m 6 ) I y )2 + l ( m 2 l x + m l 2 y ) x,y

46 Periaswamy Farid 03

47 What about multiple modes? We could model the change in intensity eg I_1 -> a I_1 + b then bung it in minimizer Use mutual information P(s_1=a, s_2=b) (loosely) geometric registration between images gives a model of sensors maximize the mutual information in this model

48 Periaswamy Farid 03

49 Periaswamy Farid 06

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