# Image Stitching. Ali Farhadi CSE 576. Several slides from Rick Szeliski, Steve Seitz, Derek Hoiem, and Ira Kemelmacher

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1 Image Stitching Ali Farhadi CSE 576 Several slides from Rick Szeliski, Steve Seitz, Derek Hoiem, and Ira Kemelmacher

2 Combine two or more overlapping images to make one larger image Add example Slide credit: Vaibhav Vaish

3 How to do it? Basic Procedure 1. Take a sequence of images from the same position 1. Rotate the camera about its optical center 2. Compute transformation between second image and first 3. Shift the second image to overlap with the first 4. Blend the two together to create a mosaic 5. If there are more images, repeat

4 1. Take a sequence of images from the same position Rotate the camera about its optical center

5 2. Compute transformation between images Extract interest points Find Matches Compute transformation?

6 3. Shift the images to overlap

7 4. Blend the two together to create a mosaic

8 5. Repeat for all images

9 How to do it? Basic Procedure 1. Take a sequence of images from the same position 1. Rotate the camera about its optical center 2. Compute transformation between second image and first 3. Shift the second image to overlap with the first 4. Blend the two together to create a mosaic 5. If there are more images, repeat

10 Compute Transformations Extract interest points Find good matches Compute transformation Let s assume we are given a set of good matching interest points

11 Image reprojection mosaic PP The mosaic has a natural interpretation in 3D The images are reprojected onto a common plane The mosaic is formed on this plane

12 Example Camera Center

13 Image reprojection Observation Rather than thinking of this as a 3D reprojection, think of it as a 2D image warp from one image to another

14 Motion models What happens when we take two images with a camera and try to align them? translation? rotation? scale? affine? Perspective?

15 Recall: Projective transformations (aka homographies)

16 Parametric (global) warping Examples of parametric warps: translation rotation aspect affine perspective

17 2D coordinate transformations translation: x = x + t x = (x,y) rotation: x = R x + t similarity: x = s R x + t affine: x = A x + t perspective: x H x x = (x,y,1) (x is a homogeneous coordinate)

18 Image Warping Given a coordinate transform x = h(x) and a source image f(x), how do we compute a transformed image g(x ) = f(h(x))? h(x) x x f(x) g(x )

19 Forward Warping Send each pixel f(x) to its corresponding location x = h(x) in g(x ) What if pixel lands between two pixels? h(x) x x f(x) g(x )

20 Forward Warping Send each pixel f(x) to its corresponding location x = h(x) in g(x ) What if pixel lands between two pixels? Answer: add contribution to several pixels, normalize later (splatting) h(x) x x f(x) g(x )

21 Inverse Warping Get each pixel g(x ) from its corresponding location x = h(x) in f(x) What if pixel comes from between two pixels? h -1 (x) x x f(x) g(x ) Richard Szeliski Image Stitching 21

22 Inverse Warping Get each pixel g(x ) from its corresponding location x = h(x) in f(x) What if pixel comes from between two pixels? Answer: resample color value from interpolated source image h -1 (x) x x f(x) g(x ) Richard Szeliski Image Stitching 22

23 Interpolation Possible interpolation filters: nearest neighbor bilinear bicubic (interpolating)

24 Motion models Translation Affine Perspective 2 unknowns 6 unknowns 8 unknowns

25 Finding the transformation Translation = 2 degrees of freedom Similarity = 4 degrees of freedom Affine = 6 degrees of freedom Homography = 8 degrees of freedom How many corresponding points do we need to solve?

26 Simple case: translations How do we solve for?

27 Simple case: translations Displacement of match i = Mean displacement =

28 Simple case: translations System of linear equations What are the knowns? Unknowns? How many unknowns? How many equations (per match)?

29 Simple case: translations Problem: more equations than unknowns Overdetermined system of equations We will find the least squares solution

30 Least squares formulation For each point we define the residuals as

31 Least squares formulation Goal: minimize sum of squared residuals Least squares solution For translations, is equal to mean displacement

32 Least squares Find t that minimizes To solve, form the normal equations

33 Solving for translations Using least squares 2n x 2 2 x 1 2n x 1

34 Affine transformations How many unknowns? How many equations per match? How many matches do we need?

35 Affine transformations Residuals: Cost function:

36 Affine transformations Matrix form 2n x 6 6 x 1 2n x 1

37 Solving for homographies

38 Solving for homographies

39 Direct Linear Transforms Defines a least squares problem: 2n 9 9 2n Since is only defined up to scale, solve for unit vector Solution: = eigenvector of with smallest eigenvalue Works with 4 or more points

40 Matching features What do we do about the bad matches? Richard Szeliski Image Stitching 40

41 RAndom SAmple Consensus Select one match, count inliers Richard Szeliski Image Stitching 41

42 RAndom SAmple Consensus Select one match, count inliers Richard Szeliski Image Stitching 42

43 Least squares fit Find average translation vector Richard Szeliski Image Stitching 43

44

45 RANSAC for estimating homography RANSAC loop: 1. Select four feature pairs (at random) 2. Compute homography H (exact) 3. Compute inliers where p i, H p i < ε Keep largest set of inliers Re-compute least-squares H estimate using all of the inliers CSE 576, Spring 2008 Structure from Motion 45

46 Simple example: fit a line Rather than homography H (8 numbers) fit y=ax+b (2 numbers a, b) to 2D pairs 47 46

47 Simple example: fit a line Pick 2 points Fit line Count inliers 3 inliers 48 47

48 Simple example: fit a line Pick 2 points Fit line Count inliers 4 inliers 49 48

49 Simple example: fit a line Pick 2 points Fit line Count inliers 9 inliers 50 49

50 Simple example: fit a line Pick 2 points Fit line Count inliers 8 inliers 51 50

51 Simple example: fit a line Use biggest set of inliers Do least-square fit 52 51

52 RANSAC Red: rejected by 2nd nearest neighbor criterion Blue: Ransac outliers Yellow: inliers

53 Computing homography Assume we have four matched points: How do we compute homography H? Normalized DLT 1. Normalize coordinates for each image a) Translate for zero mean b) Scale so that average distance to origin is ~sqrt(2) ~ x = Tx ~ x! = T! x! This makes problem better behaved numerically 2. Compute using DLT in normalized coordinates 3. Unnormalize: H ~ H = T" 1 ~ HT x! i = Hx i

54 HZ Tutorial 99 Computing homography Assume we have matched points with outliers: How do we compute homography H? Automatic Homography Estimation with RANSAC 1. Choose number of samples N 2. Choose 4 random potential matches 3. Compute H using normalized DLT 4. Project points from x to x for each potentially matching pair: x! i = Hx i 5. Count points with projected distance < t E.g., t = 3 pixels 6. Repeat steps 2-5 N times Choose H with most inliers

55 Automatic Image Stitching 1. Compute interest points on each image 2. Find candidate matches 3. Estimate homography H using matched points and RANSAC with normalized DLT 4. Project each image onto the same surface and blend

56 RANSAC for Homography Initial Matched Points

57 RANSAC for Homography Final Matched Points

58 RANSAC for Homography

59 Image Blending

60 Feathering =

61 Effect of window (ramp-width) size 1 left 1 0 right 0

62 Effect of window size

63 Good window size 1 Optimal window: smooth but not ghosted Doesn t always work... 0

64 Pyramid blending Create a Laplacian pyramid, blend each level Burt, P. J. and Adelson, E. H., A multiresolution spline with applications to image mosaics, ACM Transactions on Graphics, 42(4), October 1983,

65 Gaussian Pyramid G 2 G n The Laplacian Pyramid L G i = Gi i+ i = Li + Gi+ expand( G 1 ) expand( 1 ) Laplacian Pyramid L = n G n - = L2 G 1 - = L 1 G 0 L 0 - =

66 Laplacian level 4 Laplacian level 2 Laplacian level 0 left pyramid right pyramid blended pyramid

67 Laplacian image blend 1. Compute Laplacian pyramid 2. Compute Gaussian pyramid on weight image 3. Blend Laplacians using Gaussian blurred weights 4. Reconstruct the final image

68 Multiband Blending with Laplacian Pyramid At low frequencies, blend slowly At high frequencies, blend quickly Left pyramid blend Right pyramid

69 Multiband blending 1.Compute Laplacian pyramid of images and mask 2.Create blended image at each level of pyramid 3.Reconstruct complete image Laplacian pyramids

70 Blending comparison (IJCV 2007)

71 Poisson Image Editing For more info: Perez et al, SIGGRAPH 2003

72 Alpha Blending I 3 p I 1 I 2 Optional: see Blinn (CGA, 1994) for details: isnumber=7531&prod=jnl&arnumber=310740&arst=83&ared= 87&arAuthor=Blinn%2C+J.F. Encoding blend weights: I(x,y) = (αr, αg, αb, α) color at p = Implement this in two steps: 1. accumulate: add up the (α premultiplied) RGBα values at each pixel 2. normalize: divide each pixel s accumulated RGB by its α value

73 Choosing seams Easy method Assign each pixel to image with nearest center im1 im2 x x Image 2 Image 1

74 Choosing seams Easy method Assign each pixel to image with nearest center Create a mask: Smooth boundaries ( feathering ): Composite im1 im2 x x Image 2 Image 1

75 Choosing seams Better method: dynamic program to find seam along well-matched regions Illustration:

76 Gain compensation Simple gain adjustment Compute average RGB intensity of each image in overlapping region Normalize intensities by ratio of averages

77 Blending Comparison

78 Recognizing Panoramas Some of following material from Brown and Lowe 2003 talk Brown and Lowe 2003, 2007

79 Recognizing Panoramas Input: N images 1. Extract SIFT points, descriptors from all images 2. Find K-nearest neighbors for each point (K=4) 3. For each image a) Select M candidate matching images by counting matched keypoints (m=6) b) Solve homography H ij for each matched image

80 Recognizing Panoramas Input: N images 1. Extract SIFT points, descriptors from all images 2. Find K-nearest neighbors for each point (K=4) 3. For each image a) Select M candidate matching images by counting matched keypoints (m=6) b) Solve homography H ij for each matched image c) Decide if match is valid (n i > n f ) # inliers # keypoints in overlapping area

81 Recognizing Panoramas (cont.) (now we have matched pairs of images) 4. Find connected components

82 Finding the panoramas

83 Finding the panoramas

84 Finding the panoramas

85 Recognizing Panoramas (cont.) (now we have matched pairs of images) 4. Find connected components 5. For each connected component a) Solve for rotation and f b) Project to a surface (plane, cylinder, or sphere) c) Render with multiband blending

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