title: Calculate the Gaussian filter's sigma using the kernel's size
body: <p>I find on the <a href="http://docs.opencv.org/modules/imgproc/doc/filtering.html#getgaussiankernel" rel="noreferrer">OpenCV documentation for cvSmooth</a> that sigma can be calculated from the kernel size as follows:
  sigma = 0.3(n/2 - 1) + 0.8</p>
  
  <p>I would like to know the theoretical background of this equation.</p>
  
  <p>Thank you.</p>