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Object tracking is often reduced to recording the trajectory of the center of mass, and there is a simple reason for this: one only needs to compute the mean values of the horizontal and vertical indexes of the pixels, which is easy and fast. But what if one wants to track the object's orientation ? To do this, my favorite method is a simple and elegant approach involving image moments.
In general, one can define an infinity of moments for any solid object in any dimension, which captures interesting features of the object. One can think of them as a generalization of the center of mass, which coordinates are the first-order moments. In images, the second-order moments can be used to determine the equivalent ellipse of an object, from which one can extract the orientation of the major and/or minor axes. That's what we are going to put into practice and discuss in this post.