pnmhisteq
Equalize histogram of PNM images
TLDR
Equalize histogram
$ pnmhisteq [input.pnm] > [output.pnm]
Verbose output$ pnmhisteq -verbose [input.pnm] > [output.pnm]
Equalize grayscale$ ppmtopgm [input.ppm] | pnmhisteq > [output.pgm]
SYNOPSIS
pnmhisteq [options] [pnmfile]
DESCRIPTION
pnmhisteq performs histogram equalization on a PNM image. This process spreads out the most frequent intensity values, improving contrast in images with clustered brightness levels.
Part of Netpbm toolkit for image enhancement.
PARAMETERS
-gray
Equalize on gray value.-verbose
Print information.
EXAMPLES
$ # Basic histogram equalization
pnmhisteq photo.pgm > enhanced.pgm
# Color image via grayscale
jpegtopnm photo.jpg | pnmhisteq > enhanced.ppm
# Chain with conversion
pnmhisteq dark.pgm | pnmtopng > enhanced.png
pnmhisteq photo.pgm > enhanced.pgm
# Color image via grayscale
jpegtopnm photo.jpg | pnmhisteq > enhanced.ppm
# Chain with conversion
pnmhisteq dark.pgm | pnmtopng > enhanced.png
ALGORITHM
Histogram equalization maps input intensities to output intensities so that the output histogram is approximately uniform. This maximizes image contrast.
CAVEATS
May increase noise in uniform areas. Results depend on image content. Better for grayscale than color images.
HISTORY
pnmhisteq is part of Netpbm by Jef Poskanzer and contributors, implementing classic histogram equalization.
SEE ALSO
pgmhist(1), pnmnorm(1), pgmenhance(1), netpbm(1)