title: The convert between YCbCr and RGB on python
body: <p>I want to write some image processing code on python.</p>
  
  <pre><code>A = imread('Example.bmp');
  B  = ycbcr2rgb(rgb2ycbcr(A));
  psnr(A,B) 
  %= 52.7283
  </code></pre>
  
  <p>However , when I try to do the same thing on python 
  the psnr is lower then the result of matlab.</p>
  
  <pre><code>from PIL import Image
  import numpy
  from skimage.measure import compare_psnr
  
  A = Image.open('Example.bmp')
  B = A.convert('YCbCr').convert('RGB')
  compare_psnr(numpy.asarray(A),numpy.asarray(B))
  # = 44.7441927214
  </code></pre>
  
  <p>There is no any problem of the psnr function on both python/matlab , I am sure that they are the same, beacuse i have tried to output the image and measure them on both python/matlab. </p>
  
  <p>The problem is , why a got a lower psnr when i do convert on Python ? </p>