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    <p>If I understood you're question correctly, I assume that you want the keypoint matches in <code>std::vector&lt;cv::DMatch&gt;</code> for the purpose of drawing them with the OpenCV <code>cv::drawMatches</code> or usage with some similar OpenCV function. Since I was also doing matching "by hand" recently, here's my code that draws up arbitrary matches contained originally in a <code>std::vector&lt;std::pair &lt;int, int&gt; &gt; aMatches</code> and displays them in a window:</p> <pre><code>const cv::Mat&amp; pic1 = img_1_var; const cv::Mat&amp; pic2 = img_2_var; const std::vector &lt;cv::KeyPoint&gt; &amp;feats1 = img_1_feats; const std::vector &lt;cv::KeyPoint&gt; &amp;feats2 = img_2_feats; // you of course can work directly with original objects // but for drawing you only need const references to // images &amp; their corresponding extracted feats std::vector &lt;std::pair &lt;int, int&gt; &gt; aMatches; // fill aMatches manually - one entry is a pair consisting of // (index_in_img_1_feats, index_in_img_2_feats) // the next code draws the matches: std::vector &lt;cv::DMatch&gt; matches; matches.reserve((int)aMatches.size()); for (int i=0; i &lt; (int)aMatches.size(); ++i) matches.push_back(cv::DMatch(aMatches[i].first, aMatches[i].second, std::numeric_limits&lt;float&gt;::max())); cv::Mat output; cv::drawMatches(pic1, feats1, pic2, feats2, matches, output); cv::namedWindow("Match", 0); cv::setWindowProperty("Match", CV_WINDOW_FULLSCREEN, 1); cv::imshow("Match", output); cv::waitKey(); cv::destroyWindow("Match"); </code></pre> <p>Alternatively, if you need fuller information about the matches for purposes more complicated than drawing then you might also want to set the distance between matches to a proper value. E.g. if you want to calculate distances using <em>L2</em> distance, you should replace the following line:</p> <pre><code>for (int i=0; i &lt; (int)aMatches.size(); ++i) matches.push_back(cv::DMatch(aMatches[i].first, aMatches[i].second, std::numeric_limits&lt;float&gt;::max())); </code></pre> <p>with this (note, for this a reference to feature descriptor vectors is also needed):</p> <pre><code>cv::L2&lt;float&gt; cmp; const std::vector &lt;std::vector &lt;float&gt; &gt; &amp;desc1 = img_1_feats_descriptors; const std::vector &lt;std::vector &lt;float&gt; &gt; &amp;desc2 = img_2_feats_descriptors; for (int i=0; i &lt; (int)aMatches.size(); ++i){ float *firstFeat = &amp;desc1[aMatches[i].first]; float *secondFeat = &amp;desc2[aMatches[i].second]; float distance = cmp(firstFeat, secondFeat, firstFeat-&gt;size()); matches.push_back(cv::DMatch(aMatches[i].first, aMatches[i].second, distance)); } </code></pre> <p>Note that in the last snippet, <code>descX[i]</code> is a descriptor for <code>featsX[i]</code>, each element of the inner vector being one component of the descriptor vector. Also, note that all descriptor vectors should have the same dimensionality.</p>
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    1. COMy descriptors are of type cv::Mat, how do I convert them into vector<vector<float>> ? or can it be converted naturally ?
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