This program is part of Netpbm.
pnminvert reads a PNM image as input, inverts it black for white, and produces a PNM image as output.
If the image is grayscale, pnminvert replaces a pixel with one of complementary brightness, i.e. if the original pixel has gamma-adjusted gray value G, the output pixel has gray value maxval - G.
If the image is color, pnminvert inverts each individual RGB component the same as for a grayscale image.