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Computer Vision with Python 3 by Saurabh Kapur

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Orientation assignment

In order to add rotation invariance, the orientation of the keypoints must be robust and reproducible. For this, SURF proposes calculating Haar wavelet responses in the x and y directions. The responses are calculated in a circular neighborhood of radius 6s around the keypoint, where s is the scale of the image (that is, the value of σ). To calculate the Haar wavelet responses, SURF proposes using a wavelet size of 4s after obtaining the wavelet responses and weighing them with a Gaussian kernel (σ = 2.5s) centered about the keypoint; the responses are represented as vectors. The vectors are represented as the response strength in the horizontal direction along the abscissa, and the response strength in the vertical ...

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