pbmmask

Create a mask bitmap from a regular bitmap. See also: pambackground. More information: https://netpbm.sourceforge.net/doc/pbmmask.html.

• Create a mask bitmap separating background from foreground:

pbmmask path/to/image.pbm > path/to/output.pbm

• Expand the generated mask by one pixel:

pbmmask -expand path/to/image.pbm > path/to/output.pbm