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    <p>This is a really huge topic, with answers from 3 lines of code to entire research magazines. </p> <p>I will outline the most common such techniques and their results.</p> <h2> Comparing histograms</h2> <p>One of the simplest &amp; fastest methods. Proposed decades ago as a means to find picture simmilarities. The idea is that a forest will have a lot of green, and a human face a lot of pink, or whatever. So, if you compare two pictures with forests, you'll get some simmilarity between histograms, because you have a lot of green in both. </p> <blockquote> <p>Downside: it is too simplistic. A banana and a beach will look the same, as both are yellow. </p> <p>OpenCV method: compareHist()</p> </blockquote> <h2>Template matching</h2> <p>A good example here <a href="https://stackoverflow.com/questions/8520882/matchtemplate-finding-good-match">matchTemplate finding good match</a>. It convolves the search image with the one being search into. It is usually used to find smaller image parts in a bigger one. </p> <blockquote> <p>Downsides: It only returns good results with identical images, same size &amp; orientation.<br> OpenCV method: matchTemplate()</p> </blockquote> <h2>Feature matching</h2> <p>Considered one of the most efficient ways to do image search. A number of features are extracted from an image, in a way that guarantees the same features will be recognized again even it is rotated/scaled/skewed. The features extracted this way can be matched against other image feature sets. Another image that has a high proportion of the features in the first one is most probably depicting the same object/scene. It can be used to find the relative difference in shooting angle between pics, or the amount of overlapping.</p> <blockquote> <p>There are a number of OpenCV tutorials/samples on this, and a nice video <a href="http://www.youtube.com/watch?v=FirtmYcC0Vc" rel="noreferrer">here</a>. A whole OpenCV module (features2d) is dedicated to it.<br> Downsides: It may be slow. It is not perfect.</p> </blockquote> <ul> <li>You can find more detailed info and other similar techniques here: <a href="http://answers.opencv.org/question/877/how-to-match-2-hog-for-object-detection/#882" rel="noreferrer">http://answers.opencv.org/question/877/how-to-match-2-hog-for-object-detection/#882</a></li> </ul>
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