title: Ways to improve average image filtering speed
body: <p>Average image filtering is a very common image pre-processing function, and a short introduction of average filtering can be found <a href="http://homepages.inf.ed.ac.uk/rbf/HIPR2/mean.htm" rel="nofollow">here</a>. In Opencv this is done by <a href="http://docs.opencv.org/modules/imgproc/doc/filtering.html?highlight=blur#blur" rel="nofollow">cv::blur</a>.</p>
  
  <p>When the size of the average filter increase, the time taken by this processing will increase as well.Common ways of improving the speed of average filtering can be divided into two categories:</p>
  
  <ul>
  <li>Improve speed in the spatial domain such as <a href="http://www.isical.ac.in/~ash/Rakshit-pr-07.pdf" rel="nofollow">Fast mean ﬁltering technique</a></li>
  <li>Improve speed in the frequency domain using 2D-FFT</li>
  </ul>
  
  <p>So here my question is: what method is often used to improve the speed of average image filtering? If someone has worked on this topic, what is the advantage and dis-advantage of each method? Has someone compared the speed of cv::blur with the improved average image filtering? Thanks. </p>