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POIn OpenCV, what data types does cv2.filter2D() expect?
 

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  1. POIn OpenCV, what data types does cv2.filter2D() expect?
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    <p>I'm teaching myself about edge detectors, and I'm trying to use OpenCV's <code>filter2D</code> to implement my own gradient calculator, similar to <code>cv2.Sobel()</code>. In the Python interface to OpenCV, <code>cv2.filter2D()</code> allows users to convolve an image with a custom filter. In OpenCV nomenclature, this filter is called a "kernel."</p> <p>Using an image (<a href="http://dl.dropbox.com/u/35993/OpenCV_StackOverflow/per00001.png" rel="nofollow">per00001.png</a>) from the <a href="http://cbcl.mit.edu/software-datasets/PedestrianData.html" rel="nofollow">MIT pedestrian dataset</a>, I find that <code>cv2.Sobel()</code> produces a reasonable looking output. (Code is below, <a href="http://dl.dropbox.com/u/35993/OpenCV_StackOverflow/gradientX.png" rel="nofollow">output image is here</a>.)</p> <pre><code>#OpenCV's Sobel code (outputs nice-looking gradient) import cv2, numpy img = cv2.imread("./per00001.png") gradientX = cv2.Sobel(img, -1, 1, 0) compression_params = [cv2.cv.CV_IMWRITE_PNG_COMPRESSION, 9] cv2.imwrite("gradientX.png", gradientX, compression_params) </code></pre> <p><b>↑ GOOD</b></p> <hr> <p><b>↓ BROKEN</b></p> <p>When I try to implement my own <code>Sobel()</code>-like code (below), I get an <a href="http://dl.dropbox.com/u/35993/OpenCV_StackOverflow/gradientX_customSobel.png" rel="nofollow">all-black image</a>. I'm speculating that the problem is with the data type of the kernel parameter (<code>horizontalSobelMtx</code>) that I'm passing into <code>cv2.filter2D()</code>. However, I haven't been able to find any documentation about the kernel data type for <code>cv2.filter2D()</code>.</p> <pre><code>#Custom Sobel code (outputs all-black image) import cv2, numpy img = cv2.imread("./per00001.png") horizontalSobelMtx = [[-1,0,1],[-2,0,2],[-1,0,1]] horizontalSobelMtx = numpy.asanyarray(horizontalSobelMtx) #guessing about appropriate datatype. gradientX_customSobel = cv2.filter2D(img, -1, horizontalSobelMtx) compression_params = [cv2.cv.CV_IMWRITE_PNG_COMPRESSION, 9] cv2.imwrite("gradientX_customSobel.png", gradientX_customSobel, compression_params) </code></pre> <p>So, here are my questions:</p> <p>1) What data type does <code>cv2.filter2D(..., kernel, ...)</code> expect for the <code>kernel</code> parameter?</p> <p>2) If the data type of <code>kernel</code> isn't the problem here, then what's causing my custom Sobel code to output a blank image? </p>
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    1. COI run your code, and get the right result.
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    2. CODid you run the code snippet that has the comment `#Custom Sobel code (outputs all-black image)`?
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    3. COI'm also getting the correct results.
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