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    <p>First to select the polygon use <a href="http://docs.opencv.org/modules/highgui/doc/user_interface.html#setmousecallback" rel="nofollow">setMouseCallback</a> to get a callback every time the user clicks on the image. Add each clicked point to a list of your polygon-points. To visualize the points so far you can use the <a href="http://docs.opencv.org/doc/tutorials/core/basic_geometric_drawing/basic_geometric_drawing.html" rel="nofollow">drawing functions in OpenCV</a>. There is also some sample code on how to use setMouseCallback <a href="http://code.opencv.org/projects/opencv/repository/revisions/master/entry/samples/cpp/ffilldemo.cpp" rel="nofollow">here</a>. You can for example let the user pick points until he presses a button or uses right click for the last point to finish picking.</p> <p>To use the resulting polygon look at the tutorial <a href="http://www.pieter-jan.com/node/5" rel="nofollow">here</a>. Additionally after this you could also use <a href="http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=boundingrect#boundingrect" rel="nofollow">boundingRect</a> to crop the image around the polygon.</p> <p>Edit: Just realized that it is probably smarter to use <a href="http://docs.opencv.org/modules/core/doc/drawing_functions.html#fillpoly" rel="nofollow">fillPoly</a> to create the mask.</p> <pre><code>vector&lt;vector&lt;cv::Point&gt; &gt; polygons; // draw function takes list of polygons cv::Mat mask = cv::Mat::zeros(height, width, CV_8UC3); cv::fillPoly(mask, polygons, cv::Scalar(255, 255, 255)); </code></pre> <p>It seems the above code is not documented (however it works at least in an older version of Opencv 2.3.1). Another way of doing it is this:</p> <pre><code>cv::Mat mask = cv::Mat::zeros(height, width, CV_8UC3); // Image vector&lt;vector&lt;cv::Point&gt; &gt; polygons; // draw function takes list of polygons vector&lt;int&gt; polygonSizes; vector&lt;cv::Point*&gt; polygonPointers; for (int i=0; i&lt;polygons.size(); i++) { polygonSizes.push_back(polygons[i].size()); polygonPointers.push_back(&amp;polygons[i][0]); } cv::fillPoly(mask, (const cv::Point**) &amp;polygonPointers[0], &amp;polygonSizes[0], (int)polygons.size(), cv::Scalar(255, 255, 255)); </code></pre>
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    1. COThanks a lot, that's exactly how I did it finally ;) I first tried fillConvexPoly but that was not appropriate so I switched to fillPoly. Thanks for your answer, I really appreciate it!
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