Occasionally, you may want to reverse the contrast of a black and white (gray scale) image. This could be necessary, if, for example, the scanned hardcopy was a photographic negative and you wanted to use the corresponding positive. Subtle details located in the low DN of an image histogram can often be rendered visible more quickly by reversing the image. Two models are included for these applications: Reverse is a linear operation (see Reverse); Inverse is a non-linear approach