paminvert

produces a negative of a PAM or PNM image by inverting all pixel values

TLDR

Invert image colors

$ paminvert [input.pam] > [output.pam]

Invert from pipe

$ cat [image.ppm] | paminvert > [inverted.ppm]

paminvert produces a negative of a PAM or PNM image by inverting all pixel values. Each sample value is replaced with maxval minus the sample value.For grayscale, black becomes white and vice versa. For color, each channel is inverted independently.

EXAMPLE

$ # Create negative
paminvert photo.ppm > negative.ppm

# Invert and convert to PNG
paminvert photo.ppm | pnmtopng > negative.png

FORMULA

$ output = maxval - input

For 8-bit (maxval=255):
Black (0) -> White (255)
White (255) -> Black (0)
Gray (128) -> Gray (127)

CAVEATS

Simple inversion; not perceptual negative. Alpha channel also inverted if present.

HISTORY

paminvert is part of Netpbm, providing simple image inversion as a fundamental operation.