title: Subtract images by using opencv
body: <p>I want to subtract two images. My problem ist that the cvSub()-function saturates. What I want to do is:</p>
  
  <p>1) Convert the original images to grayscale.</p>
  
  <p>2) Take the grayscale-images (values from 0-255).</p>
  
  <p>3) Subtract the images (values from -255 to 255) -> problem of rescaling using cvSub().</p>
  
  <p>4) Rescale by multiplying with 0.5 and adding 128.</p>
  
  <p>I thought of changing the gray-scale images from 8bit to 16bit, but then everything got worse and it became many lines of code and in the end it didn't work out.</p>