Signed Pixels : advantages
  Signed pixels seem at first to be counter-intuitive but they
have numerous advantages.
n A value of 0 or mid grey can be viewed as the ‘most likely’ value that
a pixel takes on in the absence of other information.
n If you do arithmetic on images, in particular subtract one image from
another, then negative values of pixels naturally arise.
n Signed pixels allow straightforward implementation of contrast
adjustments. For instance multiplying an image by 0.5 halves the
contrast in the image.