pnmnorm
Normalize the contrast in a PNM image.
TLDR
Force the brightest pixels to be white, the darkest pixels to be black and spread out the ones in between linearly
$ pnmnorm [path/to/image.pnm] > [path/to/output.pnm]
Force the brightest pixels to be white, the darkest pixels to be black and spread out the ones in between quadratically such that pixels with a brightness of n become 50 % bright
$ pnmnorm -midvalue [n] [path/to/image.pnm] > [path/to/output.pnm]
Keep the pixels' hue, only modify the brightness
$ pnmnorm -keephues [path/to/image.pnm] > [path/to/output.pnm]
Specify a method to calculate a pixel's brightness
$ pnmnorm -[luminosity|colorvalue|saturation] [path/to/image.pnm] > [path/to/output.pnm]
