# Automatic Image Alignment

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1 Automatic Image Alignment Mike Nese with a lot of slides stolen from Steve Seitz and Rick Szeliski : Computational Photography Alexei Efros, CMU, Fall 2011

2 Live Homography DEMO Check out panoramio.com Look Around feature! Also see OpenPhoto VR:

3 Image Alignment How do we align two images automatically? Two broad approaches: Feature-based alignment Find a few matching features in both images compute alignment Direct (pixel-based) alignment Search for alignment where most pixels agree

4 Direct Alignment The simplest approach is a brute force search (hw1) Need to define image matching function SSD, Normalized Correlation, edge matching, etc. Search over all parameters within a reasonable range: e.g. for translation: for tx=x0:step:x1, for ty=y0:step:y1, compare image1(x,y) to image2(x+tx,y+ty) end; end; Need to pick correct x0,x1 and step What happens if step is too large?

5 Direct Alignment (brute force) What if we want to search for more complicated transformation, e.g. homography? wx' wy' w = a d g c x f y i 1 for a=a0:astep:a1, for b=b0:bstep:b1, for c=c0:cstep:c1, for d=d0:dstep:d1, for e=e0:estep:e1, for f=f0:fstep:f1, for g=g0:gstep:g1, for h=h0:hstep:h1, compare image1 to H(image2) end; end; end; end; end; end; end; end; b e h

6 Problems with brute force Not realistic Search in O(N 8 ) is problematic Not clear how to set starting/stopping value and step What can we do? Use pyramid search to limit starting/stopping/step values For special cases (rotational panoramas), can reduce search slightly to O(N 4 ): H = K 1 R 1 R 2-1 K 2-1 (4 DOF: f and rotation) Alternative: gradient decent on the error function i.e. how do I tweak my current estimate to make the SSD error go down? Can do sub-pixel accuracy BIG assumption? Images are already almost aligned (<2 pixels difference!) Can improve with pyramid Same tool as in motion estimation

7 Image alignment

8 Feature-based alignment 1. Find a few important features (aka Interest Points) 2. Match them across two images 3. Compute image transformation as per Project #4 Part I How do we choose good features? They must prominent in both images Easy to localize Think how you did that by hand in Project #4 Part I Corners!

9 Feature Detection

10 Feature Matching How do we match the features between the images? Need a way to describe a region around each feature e.g. image patch around each feature Use successful matches to estimate homography Issues: Need to do something to get rid of outliers What if the image patches for several interest points look similar? Make patch size bigger What if the image patches for the same feature look different due to scale, rotation, etc. Need an invariant descriptor

11 Invariant Feature Descriptors Schmid & Mohr 1997, Lowe 1999, Baumberg 2000, Tuytelaars & Van Gool 2000, Mikolajczyk & Schmid 2001, Brown & Lowe 2002, Matas et. al. 2002, Schaffalitzky & Zisserman 2002

12 Today s lecture 1 Feature detector scale invariant Harris corners 1 Feature descriptor patches, oriented patches Reading: Multi-image Matching using Multi-scale image patches, CVPR 2005

13 Invariant Local Features Image content is transformed into local feature coordinates that are invariant to translation, rotation, scale, and other imaging parameters Features Descriptors

14 Applications Feature points are used for: Image alignment (homography, fundamental matrix) 3D reconstruction Motion tracking Object recognition Indexing and database retrieval Robot navigation other

15 Harris corner detector C.Harris, M.Stephens. A Combined Corner and Edge Detector. 1988

16 The Basic Idea We should easily recognize the point by looking through a small window Shifting a window in any direction should give a large change in intensity

17 Harris Detector: Basic Idea flat region: no change in all directions edge : no change along the edge direction corner : significant change in all directions

18 Harris Detector: Mathematics Change of intensity for the shift [u,v]: x, y [ ] 2 E( u, v) = w( x, y) I( x + u, y + v) I( x, y) Window function Shifted intensity Intensity Window function w(x,y) = or 1 in window, 0 outside Gaussian

19 Harris Detector: Mathematics For small shifts [u,v] we have a bilinear approximation: u E( u, v) [ u, v] M v where M is a 2 2 matrix computed from image derivatives: M 2 I x I xi y = w( x, y) 2 x, y I xi y I y

20 Harris Detector: Mathematics Classification of image points using eigenvalues of M: λ 2 Edge λ 2 >> λ 1 Corner λ 1 and λ 2 are large, λ 1 ~ λ 2 ; E increases in all directions λ 1 and λ 2 are small; E is almost constant in all directions Flat region Edge λ 1 >> λ 2 λ 1

21 Harris Detector: Mathematics Measure of corner response: R = det M Trace M det M trace M = λ λ 1 2 = λ + λ 1 2

22 Harris Detector The Algorithm: Find points with large corner response function R (R > threshold) Take the points of local maxima of R

23 Harris Detector: Workflow

24 Harris Detector: Workflow Compute corner response R

25 Harris Detector: Workflow Find points with large corner response: R>threshold

26 Harris Detector: Workflow Take only the points of local maxima of R

27 Harris Detector: Workflow

28 Harris Detector: Some Properties Rotation invariance Ellipse rotates but its shape (i.e. eigenvalues) remains the same Corner response R is invariant to image rotation

29 Harris Detector: Some Properties Partial invariance to affine intensity change Only derivatives are used => invariance to intensity shift I I + b Intensity scale: I a I R threshold R x (image coordinate) x (image coordinate)

30 Harris Detector: Some Properties But: non-invariant to image scale! All points will be classified as edges Corner!

31 Scale Invariant Detection Consider regions (e.g. circles) of different sizes around a point Regions of corresponding sizes will look the same in both images

32 Scale Invariant Detection The problem: how do we choose corresponding circles independently in each image? Choose the scale of the best corner

33 Feature selection Distribute points evenly over the image

34 Adaptive Non-maximal Suppression Desired: Fixed # of features per image Want evenly distributed spatially Sort points by non-maximal suppression radius [Brown, Szeliski, Winder, CVPR 05]

35 Feature descriptors We know how to detect points Next question: How to match them?? Point descriptor should be: 1. Invariant 2. Distinctive

36 Descriptors Invariant to Rotation Find local orientation Dominant direction of gradient Extract image patches relative to this orientation

37 Multi-Scale Oriented Patches Interest points Multi-scale Harris corners Orientation from blurred gradient Geometrically invariant to rotation Descriptor vector Bias/gain normalized sampling of local patch (8x8) Photometrically invariant to affine changes in intensity [Brown, Szeliski, Winder, CVPR 2005]

38 Descriptor Vector Orientation = blurred gradient Rotation Invariant Frame Scale-space position (x, y, s) + orientation (θ)

39 Detections at multiple scales

40 MOPS descriptor vector 8x8 oriented patch Sampled at 5 x scale Bias/gain normalisation: I = (I µ)/σ 40 pixels 8 pixels

41 Feature matching?

42 Feature matching Exhaustive search for each feature in one image, look at all the other features in the other image(s) Hashing compute a short descriptor from each feature vector, or hash longer descriptors (randomly) Nearest neighbor techniques kd-trees and their variants

44 Feature-space outlier rejection Let s not match all features, but only these that have similar enough matches? How can we do it? SSD(patch1,patch2) < threshold How to set threshold?

45 Feature-space outlier rejection A better way [Lowe, 1999]: 1-NN: SSD of the closest match 2-NN: SSD of the second-closest match Look at how much better 1-NN is than 2-NN, e.g. 1-NN/2-NN That is, is our best match so much better than the rest?

46 Feature-space outliner rejection Can we now compute H from the blue points? No! Still too many outliers What can we do?

48 RAndom SAmple Consensus Select one match, count inliers

49 RAndom SAmple Consensus Select one match, count inliers

50 Least squares fit Find average translation vector

51 RANSAC for estimating homography RANSAC loop: 1. Select four feature pairs (at random) 2. Compute homography H (exact) 3. Compute inliers where SSD(p i, H p i) < ε 4. Keep largest set of inliers 5. Re-compute least-squares H estimate on all of the inliers

52 RANSAC

53 Example: Recognising Panoramas M. Brown and D. Lowe, University of British Columbia

54 Why Recognising Panoramas?

55 Why Recognising Panoramas? 1D Rotations (θ) Ordering matching images

56 Why Recognising Panoramas? 1D Rotations (θ) Ordering matching images

57 Why Recognising Panoramas? 1D Rotations (θ) Ordering matching images

58 Why Recognising Panoramas? 1D Rotations (θ) Ordering matching images 2D Rotations (θ, φ) Ordering matching images

59 Why Recognising Panoramas? 1D Rotations (θ) Ordering matching images 2D Rotations (θ, φ) Ordering matching images

60 Why Recognising Panoramas? 1D Rotations (θ) Ordering matching images 2D Rotations (θ, φ) Ordering matching images

61 Why Recognising Panoramas?

62 Overview Feature Matching Image Matching Bundle Adjustment Multi-band Blending Results Conclusions

63 RANSAC for Homography

64 RANSAC for Homography

65 RANSAC for Homography

66 Probabilistic model for verification

67 Finding the panoramas

68 Finding the panoramas

69 Finding the panoramas

70 Finding the panoramas

71 Homography for Rotation Parameterise each camera by rotation and focal length This gives pairwise homographies

72 Bundle Adjustment New images initialised with rotation, focal length of best matching image

73 Bundle Adjustment New images initialised with rotation, focal length of best matching image

74 Multi-band Blending Burt & Adelson 1983 Blend frequency bands over range λ

75 Results

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