Advanced Computer Vision

Advanced Computer Vision Lecture 04 Roger S. Gaborski Roger S. Gaborski

http://www.youtube.com/watch?v=NPcMS49V5hg • SIFT • http://www.youtube.com/watch?v=U0wqePj4Mx0 • SIFT Object Recognition using VLfeat • http://www.youtube.com/watch?v=PU6BKIh1y4o • http://www.youtube.com/watch?v=0KLuBlGdFvw Roger S. Gaborski

HW#3 Assignment • Due 12/13 • Choose a relevant paper • Make a 10-15 minute presentation Roger S. Gaborski

Harris Edge Detection:Not Invariant to Scaling Corner All points will be classified as edges Roger S. Gaborski

Blob Detection Roger S. Gaborski

Recall: Edge detectionDerivative of Gaussian Edge f Derivativeof Gaussian Edge = maximumof derivative Roger S. Gaborski * Is the convolution operator (otherwise polarity flipped) Source: S. Seitz

Edge detection – Second Derivative of Gaussian Edge f Second derivativeof Gaussian (Laplacian) Edge = zero crossingof second derivative Roger S. Gaborski Source: S. Seitz

Scale Issue – Laplacianwide pulse edge Note Value of Response – about -0.05 Roger S. Gaborski

Scale Issue – Laplaciannarrower pulse Roger S. Gaborski

Scale Issue – Laplacian Pulse matches Laplacian – Max Response Note Value of Response for Matched Pulse – about -0.10 Roger S. Gaborski

Blob Detection • The magnitude of Laplacian response is maximum when scale of Laplacian is matched to the blob • Convolve the blob with Laplacians at several different scales and look for the maximum • BUT – Laplacian response decays as scale increases  see next slide Roger S. Gaborski

Note Response Values Increasing scale – sigma  Decreasing Response Values Roger S. Gaborski

Scale selection We want to find the characteristic scale of the blob by convolving it with Laplacians at several scales and looking for the maximum response However, Laplacian response decays as scale increases: original signal(radius=8) increasing σ Roger S. Gaborski

Scale normalization The response of a derivative of Gaussian filter to a perfect step edge decreases as σ increases Roger S. Gaborski

Scale normalization The response of a derivative of Gaussian filter to a perfect step edge decreases as σ increases To keep response the same (scale-invariant), must multiply Gaussian derivative by σ Laplacian is the second Gaussian derivative, so it must be multiplied by σ2 Roger S. Gaborski

Effect of scale normalization Scale-normalized Laplacian response maximum Original signal Unnormalized Laplacian response Roger S. Gaborski

Blob detection in 2D Laplacian of Gaussian: Circularly symmetric operator for blob detection in 2D Roger S. Gaborski

Blob detection in 2D Laplacian of Gaussian: Circularly symmetric operator for blob detection in 2D Scale-normalized: Roger S. Gaborski

Scale selection At what scale does the Laplacian achieve a maximum response to a binary circle of radius r? r Roger S. Gaborski image Laplacian

Scale selection At what scale does the Laplacian achieve a maximum response to a binary circle of radius r? To get maximum response, the zeros of the Laplacian have to be aligned with the circle The Laplacian is given by (up to scale): The maximum response occurs at r circle Roger S. Gaborski image Laplacian

Characteristic scale We define the characteristic scale of a blob as the scale that produces peak of Laplacian response in the blob center characteristic scale T. Lindeberg (1998). "Feature detection with automatic scale selection."International Journal of Computer Vision30 (2): pp 77--116. Roger S. Gaborski

Scale-space blob detector Convolve image with scale-normalized Laplacian at several scales Find maxima of squared Laplacian response in scale-space Roger S. Gaborski

Scale-space blob detector: Example Roger S. Gaborski

Approximating the Laplacian with a Difference of Gaussians: Efficient implementation (Laplacian) (Difference of Gaussians) Roger S. Gaborski

Scale Space • Start with original image • Create progressively blurred images • Blur with Gaussian function • Progressive increase blur • One octave: doubling of blur • Resize original image to 50% • Repeat creating progressively blurred images • Repeat … Roger S. Gaborski

DoG implementation David G. Lowe. "Distinctive image features from scale-invariant keypoints.”IJCV 60 (2), pp. 91-110, 2004. Roger S. Gaborski

Gaussian Blurred Images and Difference of Gaussian Images Grouped by octave From Striker Roger S. Gaborski

Scale Invariant Feature Transform • Feature Detector and Feature Descriptor • Detection stages for SIFT features: • Scale-space extrema detection • Keypoint localization • Orientation assignment • Generation of keypoint descriptors. Following slides based on Strider, Univ. Toronto and Lowe Roger S. Gaborski

Keypoints are maxima of Difference of Gaussians (DoG) that occur at multiple scales • Start with Gaussian blurred image: L(x; y; σ ) = G(x; y; σ )* I(x; y) Where: G(x; y; σ ) = 1/(2 pi*σ2) (exp-(x2 +y2 )/σ2 ) Roger S. Gaborski

Difference of Gaussian Filter: G(x; y; kσ ) – G(x; y; σ) • Convolve blurred image with Difference of Gaussian: D(x; y; σ ) = L(x; y; k σ) - L(x; y; σ) Which is simply the difference of Gaussian blurred images at scales σ and k σ Roger S. Gaborski

Blurred Images at Different Scales Computation of DoG REF: Lowe , 2004 Roger S. Gaborski

Scale Space Extrema Detection • Group convolved images by octave Octave  doubling of σ • Keypoints are idenitied as local maxima or minima of the DoG images across scales. • Each pixel in the DoG images is compared to its 8 neighbors at the same scale, plus the 9 corresponding neighbors at neighboring scales. • If the pixel is a local maximum or minimum, it is selected as a candidate keypoint Roger S. Gaborski

Local extrema detection – pixel marked with X compared to 26 neighbors in 3x3x3 space of adjacent DoGimages X is a key point if X is either greater (or less than) than all of the neighbors Roger S. Gaborski

Keypoint Selection • Keypoints with low contrast are deleted • Keypoints on edges are deleted • Each remaining keypoint is assigned an orientation Roger S. Gaborski

Keypoint Orientation • For each remaining keypoint • Compute gradient orientation histogram in the neighborhood of keypoint • Contribution of each neighboring pixel is weighted by the gradient magnitude and a Gaussian window with a σ that is 1.5 the scale of the keypoint • Peaks in histogram correspond to dominate orientations • Separate keypoint for direction corresponding to maximum in histogram and any direction within 80% of the maximum Roger S. Gaborski

16x16 Pixels Around Keypoint Divide 16x16 pixels into 16 , 4x4 sub windows Calculate gradient magnitude and direction in each sub window Store gradient and magnitude in 8 bin histograms Roger S. Gaborski

Gradients with an angle between 0 and 44 degrees goes in the first bin, etc. The magnitude depends on the gradients magnitude and the gradient’s distance from the key point Roger S. Gaborski

Key Point Vector • Each 4x4 sub window has 8 numbers associate with the sub window (the histogram) • 16 sub windows • 16 * 8 = 128 numbers • Normalize the vector (divide vector by the sqrt of the sum of the elements squared) • The resulting normalized vector is the feature vector associated with each key point Roger S. Gaborski

SIFT Descriptor • Histogram contains 8 bins • Each descriptor has 16 histograms around keypoint • 4 x 4 x 8 = 128 long feature vector for each keypoint • Normalize feature for illumination invariance Roger S. Gaborski

Recognition using SIFT • Compute SIFT features for input image • Match features in SIFT database Roger S. Gaborski

Gaussian Blurred Images and Difference of Gaussian Images Grouped by octave From Striker Roger S. Gaborski

Maxima of DoG across scales Keypoints after removing low contrast points Roger S. Gaborski

Removal of Edge Points Roger S. Gaborski

Arrows indicate Scale and Orientation Roger S. Gaborski

VLFeat.org • MSER • SURF (Speeded Up Robust Features) Roger S. Gaborski

Image Features http://videolectures.net/mcvc08_gousseau_cmlf/ • The Future of Image Search author: Jitendra Malik, UC Berkeley, University of California, http://videolectures.net/kdd08_malik_fis/ Roger S. Gaborski

SIFT Feature Representation • Keypoint Feature Descriptor • Set of orientation histograms on 4x4 neighborhoods (Lowe 2004) 8x8 set of samples 2x2 Descriptor Array computer from 8x8 set of samples Roger S. Gaborski

Advanced Computer Vision