surf: speeded-up robust features computer vision and image understanding ( cviu ) 2008. advisor :...
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SURF: SPEEDED-UP ROBUST FEATURES
Computer Vision and Image Understanding (CVIU) 2008.
Advisor : Sheng-Jyh Wang
Student : 劉彥廷
2011/10/17
OUTLINE
Introduction
Related Works
Speed-Up Robust Features
• Detection• Description
Experiments
Conclusion
2
OUTLINE
Introduction
Related Works
Speed-Up Robust Features
• Detection• Description
Experiments
Conclusion
3
• Why do we care about feature matching?
Object Recognition
Wide baseline matching
Tracking
4
Introduction
Types of variance
• Illumination• Scale• Rotation• Affine• Perspective
We want to find
Repeatability、 Distinctiveness
features
5
Challenges
OUTLINE
Introduction
Related Works
Speed-Up Robust Features
• Detection• Description
Experiments
Conclusion
6
Related Works
• Harris Corner Detector - Harris 1988• Laplacian of Gaussian - Lindeberg 1998• Difference of Gaussian - Lowe 2004
7
Related Works
• Harris Corner Detector - Harris 1988• Laplacian of Gaussian - Lindeberg 1998• Difference of Gaussian - Lowe 2004
flat edge corner
Illumination invariance !!! 8
Related Works
• Harris Corner Detector - Harris 1988• Laplacian of Gaussian - Lindeberg 1998• Difference of Gaussian - Lowe 2004
* =
characteristic scale
9
LoG can detect blob-like structures at locations
“Feature Detection with Automatic Scale Selection”, IJCV ‘98
Related Works
• Harris Corner Detector - Harris 1988• Laplacian of Gaussian - Lindeberg 1998• Difference of Gaussian - Lowe 2004
2 2( , , ) ( , , ) ( 1) ( , , )G x y k G x y k G x y
10
Computational efficiency !
Keep the same keypoint in all scale !
Compare to 26 neighbors
Motivation
• Lindeberg uses Laplacian of Gaussian, one could obtain scale invariant features.
• Lowe uses difference of Gaussian to approximate Laplacian of Gaussian. (SIFT)
• This paper uses Hessian - Laplacian to approximate Laplacian of Gaussian, to improve calculation speed.
11
OUTLINE
Introduction
Related Works
Speed-Up Robust Features
• Detection• Description
Experiments
Conclusion
12
Detection
Hessian-based interest point localization
• Lxx(x,y,σ) is the Laplacian of Gaussian of the image.
• It is the convolution of the Gaussian second order derivative with the image.
• This paper use Dxx to approximate Lxx.
13
DetectionScale analysis with constant image size
Approximated second order derivatives with box filters.
14
(DoG)
Integral Images
Using integral images for major speed upIntegral Image (summed area tables) is an intermediate representation for the image and contains the sum of gray scale pixel values of image.
15They can be evaluated at a very low computational cost using integral images with box filters
Fourier v.s. Wavelet
17
• Fourier Transform (FT) is not a good tool –
gives no direct information about when an oscillation occurred.
• Wavelets can keep track of time and frequency information.
Fourier basis
Haar basis
interest point
scale = s
r = 6s
Haar dx
dy
Description
Orientation Assignment
x response y response
• The Haar wavelet responses are represented as vectors
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• Sum all responses within a sliding orientation window covering an angle of 60 degree
• The longest vector is the dominant orientation
Description
• Split the interest region (20s x 20s) up into 4 x 4 square sub-regions.
• Calculate Haar wavelet response dx and dy and weight the response with a Gaussian kernel.
• Sum the response over each sub-region for dx and dy, then sum the absolute value of resp-onse.
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Matching
20
• Fast indexing through the sign of the Laplacian for the underlying interest point
The sign of trace of the Hessian matrix
Trace = Lxx + Lyy
can do match can do match not match
matching
OUTLINE
Introduction
Related Works
Speed-Up Robust Features
• Detection• Description
Experiments
Conclusion
21
Experiments
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• Test keypoint repeatability for
(Viewpoint Change), (Lighting Change) and(Zoom and
Rotation)
Conclusion
• SURF is faster than SIFT by 3 times, and has recall precision not worse than SIFT.
• SURF is good at handling image with blurring or
rotation.
• SURF is poor at handling image with viewpoint .
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