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.
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.
Use this to ensure you get the same image on separate invocations.
This option was new in Netpbm 10.75 (June 2016).