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POCalculating convexityDefects using OpenCV 2.4 in c++
 

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  1. POCalculating convexityDefects using OpenCV 2.4 in c++
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    <p>I'm using OpenCV 2.4 to calculate the convex hull of a image. </p> <p>I'm also doing some processing to remove some noise from the image, which is not really relevant to the question.</p> <p>The code to calculate the convexHull is the following:</p> <pre><code>... cv::Mat sourceImage; // assume something is already here please cv::vector&lt;cv::Vec4i&gt; hierarchy; std::vector&lt;std::vector&lt;cv::Point&gt; &gt; contours; cv::findContours( sourceImage, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE,cv::Point(0, 0)); // Find the convex hull object for each contour vector&lt;cv::vector&lt;cv::Point&gt; &gt;hull( contours.size() ); for (int i = 0; i &lt; contours.size(); i++) { convexHull( contours[i], hull[i], false ); } ... </code></pre> <p>Having both the convexHull and the contours I now want to calculate the convexityDefects of the hull(s), which by looking at opencv documentation I thought it would be like this:</p> <pre><code>cv::Vec4i defects; convexityDefects(cv::Mat(contours), hull, defects); </code></pre> <p>Doing this I get this error:</p> <pre><code>OpenCV Error: Assertion failed (ptnum &gt; 3) in convexityDefects, file ./opencv/opencv/modules/imgproc/src/contours.cpp, line 1969 </code></pre> <p>Any ideas on what I'm doing wrong when using convexityDefects? </p> <p><a href="http://opencv.itseez.com/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=convexitydefects#cv.ConvexityDefects" rel="nofollow" title="Opencv 2.4 ConvexityDefects documentation">Opencv convexityDefects documentation</a></p> <p>Thanks in advance.</p> <p><strong>UPDATE</strong></p> <p>Thanks to Innuendo answer I updated the main loop code to:</p> <pre><code>std::vector&lt;Vec4i&gt; defects; vector&lt;cv::vector&lt;cv::Point&gt; &gt;hull( contours.size() ); for (int i = 0; i &lt; contours.size(); i++) { convexHull( contours[i], hull[i], false ); convexityDefects(contours[i], hull[i], defects[i]); } </code></pre> <p>Using this, the error that I now get is:</p> <pre><code>OpenCV Error: Assertion failed (hull.checkVector(1, CV_32S) &gt; 2) in convexityDefects </code></pre>
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