title: how does ImageFilter in PIL normalize the pixel values between 0 and 255 after filtering with Kernel or mask
body: <p>how does ImageFilter in PIL normalize the pixel values(not the kernel) between 0 and 255 after filtering with Kernel or mask?(Specially zero-summing kernel like:( -1,-1,-1,0,0,0,1,1,1 ))</p>
  
  <pre>
  my code was like:
  import Image
  import ImageFilter
  Horiz = ImageFilter.Kernel((3, 3), (-1,-2,-1,0,0,0,1,2,1), scale=None, offset=0) #sobel mask
  im_fltd = myimage.filter(Horiz)
  </pre>