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POHow to apply box filter on integral image? (SURF)
 

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  1. POHow to apply box filter on integral image? (SURF)
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    <p>Assuming that I have a grayscale (8-bit) image and assume that I have an integral image created from that same image. Image resolution is <code>720x576</code>. According to SURF algorithm, each octave is composed of 4 box filters, which are defined by the number of pixels on their side. <br><br>The first octave uses filters with <code>9x9, 15x15, 21x21 and 27x27</code> pixels.<br> The second octave uses filters with <code>15x15, 27x27, 39x39 and 51x51</code> pixels.<br>The third octave uses filters with <code>27x27, 51x51, 75x75 and 99x99</code> pixels. If the image is sufficiently large and I guess <strong>720x576 is big enough (right??!!)</strong>, a fourth octave is added, <code>51x51, 99x99, 147x147 and 195x195</code>. These octaves partially overlap one another to improve the quality of the interpolated results.</p> <pre><code>// so, we have: // // 9x9 15x15 21x21 27x27 // 15x15 27x27 39x39 51x51 // 27x27 51x51 75x75 99x99 // 51x51 99x99 147x147 195x195 </code></pre> <p>The questions are:<br><strong>What are the values in each of these filters? Should I hardcode these values, or should I calculate them?<br> How exactly (numerically) to apply filters to the integral image?</strong></p> <p>Also, for calculating the Hessian determinant I found two approximations:<br> <code>det(HessianApprox) = DxxDyy − (0.9Dxy)^2</code> and<br><code>det(HessianApprox) = DxxDyy − (0.81Dxy)^2</code><br><br><strong>Which one is correct?</strong> (Dxx, Dyy, and Dxy are Gaussian second order derivatives).<br></p>
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