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copied!<p>You forgot to say if you are able to see the original image being displayed on the screen.</p> <p>I never get tired of telling people that checking the return of functions is a must!</p> <p>Consider <code>IplImage* img = cvLoadImage("images/00000038.jpg");</code> , how can you tell if this function succeeded or not? As far as I can tell, the error you are having might be from a function failing prior to <code>cvCanny()</code> being called.</p> <p>Anyway, I recently posted a <a href="https://stackoverflow.com/questions/6416117/simple-object-detection-using-opencv-and-machine-learning/6416361#6416361">code that uses cvCanny to improve circle detection</a>. You can check that code and see what you are doing differently.</p> <p><strong>EDIT</strong>:</p> <p>Your problem in this case is that you are passing to cvCanny input and output as a 3 channel image, when it takes only a single channel image. <a href="http://opencv.willowgarage.com/documentation/feature_detection.html#canny" rel="nofollow noreferrer">Check the docs</a>:</p> <blockquote> <p>void cvCanny(const CvArr* image, CvArr* edges, double threshold1, double threshold2, int aperture_size=3)</p> </blockquote> <pre><code>Implements the Canny algorithm for edge detection. Parameters: * image – Single-channel input image * edges – Single-channel image to store the edges found by the function * threshold1 – The first threshold * threshold2 – The second threshold * aperture_size – Aperture parameter for the Sobel operator (see Sobel) </code></pre> <p>So, change your code to:</p> <pre><code>// Create an image to hold our modified input image IplImage* out = cvCreateImage(cvGetSize(img), IPL_DEPTH_8U, 1); // Do some smoothing //cvSmooth(img, out, CV_GAUSSIAN, 3, 3); IplImage* gray = cvCreateImage(cvGetSize(img), IPL_DEPTH_8U, 1); cvCvtColor(img, gray, CV_BGR2GRAY); // Do some Edge detection cvCanny(gray, out, 10, 20, 3); </code></pre>

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