pbmreduce man page

pbmreduce — read a PBM image and reduce it N times


pbmreduce [-floyd|-fs|-threshold] [-value val] N [-randomseed=integer] [pbmfile]

You can abbreviate any option to its shortest unique prefix.


This program is part of Netpbm(1).

pbmreduce reads a PBM image as input and reduces it by a factor of N, producing a PBM image as output.

pbmreduce duplicates a lot of the functionality of pamditherbw; you could do something like pamscale | pamditherbw, but pbmreduce is a lot faster.

You can use pbmreduce to "re-halftone" an image. Let's say you have a scanner that only produces black&white, not grayscale, and it does a terrible job of halftoning (most b&w scanners fit this description).  One way to fix the halftoning is to scan at the highest possible resolution, say 300 dpi, and then reduce by a factor of three or so using pbmreduce.  You can even correct the brightness of an image, by using the -value option.



By default, pbmreduce does the halftoning after the reduction via boustrophedonic Floyd-Steinberg error diffusion; however, you can use this option to specify simple thresholding.  This gives better results when reducing line drawings.


This option alters the thresholding value for all quantizations.  It should be a real number between 0 and 1.  Above 0.5 means darker images; below 0.5 means lighter.


This is the seed for the random number generator that controls the halftoning.

Use this to ensure you get the same image on separate invocations.

This option was new in Netpbm 10.75 (June 2016).

See Also

pamenlarge(1), pamscale(1), pamditherbw(1), pbm(1)


Copyright (C) 1988 by Jef Poskanzer.

Document Source

This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The master documentation is at


Referenced By

pamditherbw(1), pamenlarge(1), pamscale(1), pbmtoescp2(1).

13 April 2016 netpbm documentation