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POOpenCV different output results using a BruteForceMatcher over two different runtimes with same input
 

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  1. POOpenCV different output results using a BruteForceMatcher over two different runtimes with same input
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    <p><strong>Intro :</strong> First and as an introduction, i am quite "proud" to ask my first question on StackOverflow. I hope I'll be able to help other people as much as they help me.</p> <p><strong>Context :</strong> I am developping an application that searches for features in an image using a SURF algorithm. I compute the keypoints and I extract the descriptors with SURF. Then, I use a bruteforce matcher based on euclidian distance to match descriptors of image 1 with image 2. Here is the problem, i don't get the same results with 2 different runs of the program (using the same image, should I precise :p).</p> <p><strong>Outputs :</strong> Here are the outputs, </p> <p>1st runtime on the 20 first matches out of 3620 </p> <pre><code>0: 0 89 0.292352 1: 1 997 0.186256 2: 2 1531 0.25669 3: 3 2761 0.24148 4: 4 2116 0.286187 5: 5 2996 0.201048 6: 6 3109 0.266272 7: 7 2537 0.17112 8: 8 2743 0.211974 9: 9 2880 0.208735 10: 10 2167 0.269716 11: 11 2431 0.164508 12: 12 1474 0.281442 13: 13 1867 0.161129 14: 14 329 0.18388 15: 15 1580 0.229825 16: 16 1275 0.254946 17: 17 1749 0.203006 18: 18 305 0.221724 19: 19 1501 0.224663 20: 20 917 0.20708 </code></pre> <p>2nd runtime on the 20 first matches out of 3620</p> <pre><code>0: 0 1455 0.25669 1: 1 963 0.186256 2: 2 3008 0.150252 3: 3 2936 0.24148 4: 4 2172 0.286187 5: 5 2708 0.211974 6: 6 730 0.185199 7: 7 3128 0.266272 8: 8 750 0.181001 9: 9 2272 0.17112 10: 10 2842 0.208735 11: 11 55 0.229677 12: 12 2430 0.269716 13: 13 2360 0.164508 14: 14 1497 0.229825 15: 15 2443 0.254148 16: 16 1784 0.161129 17: 17 1209 0.254946 18: 18 311 0.18388 19: 19 944 0.228939 20: 20 533 0.221724 </code></pre> <p><strong>Code :</strong> Here is a part of the code i use</p> <pre><code>SurfFeatureDetector detector(400); vector&lt;KeyPoint&gt; keypoints1, keypoints2; detector.detect(img1, keypoints1); detector.detect(img2, keypoints2); SurfDescriptorExtractor extractor; Mat descriptors1, descriptors2; extractor.compute(img1, keypoints1, descriptors1); extractor.compute(img2, keypoints2, descriptors2); vector&lt;DMatch&gt; filteredMatches; matching(descriptors1,descriptors2,filteredMatches,1); </code></pre> <p>Here is the matching function</p> <pre><code>void crossCheckMatching( const Mat&amp; descriptors1, const Mat&amp; descriptors2, vector&lt;DMatch&gt;&amp; filteredMatches12, int knn=1 ) { BruteForceMatcher&lt;L2&lt;float&gt;&gt; matcher; filteredMatches12.clear(); vector&lt;vector&lt;DMatch&gt; &gt; matches12, matches21; matcher.knnMatch( descriptors1, descriptors2, matches12, knn ); matcher.knnMatch( descriptors2, descriptors1, matches21, knn ); debug_writeMatches("D:/jmartel/exec2-log.txt",matches12); ... } </code></pre> <p><strong>Conclusions :</strong> The SURF algorithm gives "realistic" outputs, it is partly proved by the same number of detected keypoints over 2 runtimes with the same 2 images. The BruteForceMatcher gives me really strange output, it is proved by the log file, and with the image i can output which clearly shows it does not match in the same way over two runtimes.</p> <p><em>I also implemented all this code on GPU and my observations are similar. However, SURF provides more point on GPU (with the same parameters).</em></p> <p><strong>If we look carefully to the points, some distances are exactly similar, which can be possible but strange (even if the descriptors between the 2 set of points can be equal...). It's the example of the couple</strong></p> <pre><code> (runtime 1) 1: 1 997 0.186256 / (runtime 2) 1:1 963 0.186256 </code></pre> <p>or even stranger </p> <pre><code> (runtime 1) 14: 14 329 0.18388 / (runtime 2) 18: 18 311 0.18388 </code></pre> <p>The OpenCV2.0 Doc does not say anything particularly interesting from what i read. <a href="http://opencv.willowgarage.com/documentation/cpp/features2d_common_interfaces_of_descriptor_matchers.html?highlight=bruteforcematcher#BruteForceMatcher" rel="nofollow">See BruteForceMatcher C++ Documentation for OpenCV2.1 here</a></p> <p><strong>If you have any explanation, or anything i could change in the code, I'll be glad. Thanks for your help.</strong></p> <p>Julien,</p>
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