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This program is part of Netpbm.
pnmcrop reads a PBM, PGM, or PPM image as input, removes borders that are the background color, and produces the same type of image as output.
If you don't specify otherwise, pnmcrop assumes the background color is whatever color the top left and right corners of the image are and if they are different colors, something midway between them. You can specify that the background is white or black with the -white and -black options or make pnmcrop base its guess on all four corners instead of just two with -sides.
By default, pnmcrop chops off any stripe of background color it finds, on all four sides. You can tell pnmcrop to remove only specific borders with the -left, -right, -top, and -bottom options.
If you want to leave some border, use the -margin option. It will not only spare some of the border from cropping, but will fill in (with what pnmcrop considers the background color) if necessary to get up to that size.
If you want to chop a specific amount off the side of an image, use pamcut.
If you want to add different borders after removing the existing ones, use pnmcat or pamcomp.
If at least three of the four corners are the same color, pnmcrop takes that as the background color. If not, pnmcrop looks for two corners of the same color in the following order, taking the first found as the background color: top, left, right, bottom. If all four corners are different colors, pnmcrop assumes an average of the four colors as the background color.
The -sides option slows pnmcrop down, as it reads the entire image to determine the background color in addition to the up to three times that it would read it without -sides.
This option was new in Netpbm 10.29 (August 2005).
Without this option, pnmcrop examines the input image and figures out what part of the image is border and what part is foreground (not border). With this option, pnmcrop finds the borders in one image, then uses the those four border sizes (left, right, top, bottom) in cropping a different image.
The point of this is that you may want to help pnmcrop to come to a different conclusion as to where the border are by preprocessing the input image. For example, consider an image that has speckles of noise in its borders. pnmcrop isn't smart enough to recognize these as noise; it sees them as foreground image. So pnmcrop considers most of your borders to be foreground and does not crop them off as you want. To fix this, run the image through a despeckler such as pbmclean and tell pnmcrop to use the despeckled version of the image as the -borderfile image, but the original speckled version as the input image. That way, you crop the borders, but retain the true foreground image, speckles and all.
This option was new in Netpbm 10.29 (August 2005).