photo stitchingdhoiem.cs.illinois.edu/courses/vision_spring10...photo stitching panoramas from...
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Photo StitchingPanoramas from Multiple Images
Computer Vision
CS 543 / ECE 549
University of Illinois
Derek Hoiem
04/06/10
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So far, we’ve looked at what can be done with one image
• Recover basic geometry using vanishing points
• Find image boundaries and segment objects
• Categorize images
• Find specific objects and detect objects that are part of some category
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What can we get from multiple images?
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What can we get from multiple images?
• Bigger, Better, Brighter, Sharper images– Panoramas
– Increased dynamic range
– Super-resolution
– Reduced noise/blur
Product example: http://www.vreveal.com/
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What can we get from multiple images?
• Bigger, Better, Brighter, Sharper images– Panoramas
– Increased dynamic range
– Super-resolution
– Reduced noise/blur
Product example: http://www.vreveal.com/
today
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What can we get from multiple images?
• Depth and 3D structure– Two-view stereo
– Multi-view stereo
– Shape carving
– Structure from motion
Thursday + Next Tuesday
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What can we get from multiple images?
• Motion– Optical flow
– Tracking
– Action/activity recognition
Optical flow (source: http://www.borisfx.com/avid/bccavx/classic_features.php)
Tracking (from Deva Ramanan)
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What can we get from multiple images?
• Motion– Optical flow
– Tracking
– Action/activity recognition
Optical flow (source: http://www.borisfx.com/avid/bccavx/classic_features.php)
Tracking (from Deva Ramanan)
April 15
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What can we get from multiple images?
• Motion– Optical flow
– Tracking
– Action/activity recognition
Optical flow (source: http://www.borisfx.com/avid/bccavx/classic_features.php)
Tracking (from Deva Ramanan)
April 20
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Today: Image Stitching• Combine two or more overlapping images to
make one larger image
Add example
Slide credit: Vaibhav Vaish
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Example
Camera Center
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Problem basics
• Do on board
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Basic problem
• x = K [R t] X
• x’ = K’ [R’ t’] X’
• t=t’=0
• x‘=Hx where H = K’ R’ R-1 K-1
• Typically only R and f will change (4 parameters), but, in general, H has 8 parameters
f f'
.x
x'
X
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Image Stitching Algorithm Overview
1. Detect keypoints
2. Match keypoints
3. Estimate homography with four matched keypoints (using RANSAC)
4. Combine images
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Computing homography
• Assume we have four matched points: How do we compute homography H?
Direct Linear Transformation (DLT)0HxxHxx =×⇒= ''
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Only these two provide unique constraints
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Computing homography
Direct Linear Transform
• Apply SVD: UDVT = A
• h = Vsmallest (column of V corr. to smallest singular value)
0Ah0h
x0xxx0
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Computing homography• Assume we have four matched points: How do
we compute homography H?
Normalized DLT1. Normalize coordinates for each image
a) Translate for zero meanb) Scale so that average distance to origin is sqrt(2)
– This makes problem better behaved numerically (see HZ p. 107-108)
2. Compute H using DLT in normalized coordinates3. Unnormalize:
Txx =~ xTx ′′=′~
THTH ~1−′=
ii Hxx =′
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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
HZ Tutorial ‘99
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Computing homography
• Assume we have matched points with outliers: How do we compute homography H?
Automatic Homography Estimation with RANSAC1. Choose number of samples N2. Choose 4 random potential matches3. Compute H using normalized DLT4. Project points from x to x’ for each potentially
matching pair:5. Count points with projected distance < t
– t ~= 6 σ ; σ is measurement error (1-3 pixels)6. Repeat steps 2-5 N times
– Choose H with most inliers
HZ Tutorial ‘99
ii Hxx =′
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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. Transform second image and blend the two images
– Matlab: maketform, imtransform
Some details from a class project
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Recognizing Panoramas
Brown and Lowe 2003, 2007Some of following material from Brown and Lowe 2003 talk
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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 imagea) Select M candidate matching images by counting
matched keypoints (m=6)
b) Solve homography Hij for each matched image
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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 imagea) Select M candidate matching images by counting
matched keypoints (m=6)
b) Solve homography Hij for each matched image
c) Decide if match is valid (ni > 8 + 0.3 nf )
# inliers # features in overlapping area
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RANSAC for Homography
Initial Matched Points
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RANSAC for Homography
Final Matched Points
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Verification
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RANSAC for Homography
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Recognizing Panoramas (cont.)
(now we have matched pairs of images)
4. Find connected components
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Finding the panoramas
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Finding the panoramas
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Recognizing Panoramas (cont.)
(now we have matched pairs of images)
4. Find connected components
5. For each connected componenta) Perform bundle adjustment to solve for rotation
(θ1, θ2, θ3) and focal length f of all cameras
b) Project to a surface (plane, cylinder, or sphere)
c) Render with multiband blending
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Bundle adjustment for stitching
• Non-linear minimization of re-projection error
• where H = K’ R’ R-1 K-1
• Solve non-linear least squares (Levenberg-Marquardt algorithm)– See paper for details
)ˆ,(1∑∑∑ ′′=N M
j k
i
disterror xx
Hxx =′ˆ
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Bundle Adjustment
• New images initialised with rotation, focal length of best matching image
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Bundle Adjustment
• New images initialised with rotation, focal length of best matching image
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Straightening
• Rectify images so that “up” is vertical
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Blending
• Gain compensation: minimize intensity difference of overlapping pixels
• Blending– Pixels near center of image get more weight
– Multiband blending to prevent blurring
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Multi-band Blending
• Burt & Adelson 1983– Blend frequency bands over range ∝ λ
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Multiband blending
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Blending comparison (IJCV 2007)
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Blending Comparison
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Further reading
• DLT algorithm: HZ p. 91 (alg 4.2), p. 585
• Normalization: HZ p. 107-109 (alg 4.2)
• RANSAC: HZ Sec 4.7, p. 123, alg 4.6
• Recognising Panoramas: Brown and Lowe, IJCV 2007 (also bundle adjustment)
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Things to remember• Homography relates rotating cameras
• Recover homography using RANSAC and normalized DLT
• Bundle adjustment minimizes reprojectionerror for set of related images
• Details to make it look nice (straightening, blending)