title: Adobe Photoshop-style posterization and OpenCV
body: <p>It seems Adobe Photoshop does posterization by quantizing each color channel separately, based on the number of levels specified.  So for example, if you specify 2 levels, then it will take the R value, and set it to 0 if your R value is less than 128 or 255 if your value is >= 128.  It will do the same for G and B.</p>
  
  <p>Is there an efficient way to do this in python with OpenCV besides iterating through each pixel and making that comparison and setting the value separately?  Since an image in OpenCV 2.4 is a NumPy ndarray, is there perhaps an efficient way to do this calculation strictly through NumPy?</p>