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    <p><strong>First question :</strong></p> <p>It is ok to do so since you have single object in image. If more objects, and want to find moments for each, use contours. Visit : <a href="http://opencvpython.blogspot.in/2012/06/contours-3-extraction.html" rel="noreferrer">http://opencvpython.blogspot.in/2012/06/contours-3-extraction.html</a></p> <p><strong>Second Question :</strong></p> <p>(I am too thinking about it after seeing your questions)</p> <p>HuMoments were brought to light by Mr.Ming Kuei Hu in his paper " <a href="http://www.sci.utah.edu/~gerig/CS7960-S2010/handouts/Hu.pdf" rel="noreferrer">Visual Pattern Recognition by Moment Invariants</a> " in 1962.</p> <p>In that paper, he explains them ( with help of high mathematics content, of course). You can check it if you need.</p> <p><strong>He states :</strong></p> <blockquote> <p>Theorem: The central moments are invariants under translation.</p> </blockquote> <p>When you find the moments using moments() function, it returns three types of moments, <code>spatial moments (Mji), Central Moments (MUji) and Central Normalized Moments ( NUji)</code>. Check out the docs for <a href="http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=moments#moments" rel="noreferrer">moments()</a></p> <p>At the end of section 4-C, Hu states that, out of seven relations ( <a href="http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=moments#humoments" rel="noreferrer">Visit here to see them</a>), first six are invariant to translation, size and rotation, while <code>seventh one is skew invariant, which helps to distinguish mirror images.</code></p> <p>I searched a lot about what Hu Moments values means. But all papers says the same dialog : <code>" Seven Hu Moments are calculated from Central Moments which are invariant to size, position and orientation ".</code> They don't say what individual values mean.</p> <p>And the classical book of OpenCV "Learning OpenCV" by Mr.Gary Bradsky has given some insights telling us to explore more. <a href="http://books.google.co.in/books?id=seAgiOfu2EIC&amp;lpg=PA255&amp;ots=hSH1cieHOg&amp;dq=simple%20explanation%20of%20humoments&amp;pg=PA254#v=onepage&amp;q=simple%20explanation%20of%20humoments&amp;f=false" rel="noreferrer">(Refer page 254)</a></p>
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