pbmmask - Man Page

create a mask bitmap from a regular bitmap

Examples (TL;DR)

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

Description

This program is part of Netpbm(1).

pbmmask reads a PBM image as input and generates a corresponding mask of the foreground areas as another PBM image.

This is probably obsoleted by pambackground.

The color to be interpreted as "background" is determined automatically. Regardless of which color is background, the mask will be white where the background is and black where the figure is.

This lets you do a masked paste like this, for objects with a black background:

    pbmmask obj > objmask
    pnmpaste < dest -and objmask <x> <y> | pnmpaste -or obj <x> <y>

For objects with a white background, you can either invert them or add a step:

    pbmmask obj > objmask
    pnminvert objmask | pnmpaste -and obj 0 0 > blackback
    pnmpaste < dest -and objmask <x> <y> | pnmpaste -or blackback <x> <y>

Note that this three-step version works for objects with black backgrounds too, if you don't care about the wasted time.

You can also use masks with grayscale and color images, using the pnmarith tool. For instance:

    ppmtopgm obj.ppm | pamditherbw -threshold | pbmmask > objmask.pbm
    pnmarith -multiply dest.ppm objmask.pbm > t1.ppm
    pnminvert objmask.pbm | pnmarith -multiply obj.ppm - > t2.ppm
    pnmarith -add t1.ppm t2.ppm

An interesting variation on this is to pipe the mask through pnmsmooth before using it. This makes the boundary between the two images less sharp.

Options

In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options ), pbmmask recognizes the following command line option:

-expand: Expands the mask by one pixel out from the image. This is useful if you want a little white border around your image. (A better solution might be to turn the pbmlife program into a general cellular automaton tool...)

pbmmask is one of the oldest programs in Netpbm. In September 2021,
the date on this manual was August 8, 1989 (being the date of the last
substantial update). We updated the page then just to add this historical
information and recommend pambackground.

It is likely that when Bryan wrote pambackground in 2006, he was
unaware pbmmask existed. Otherwise, he would presumably have
replaced pbmmask with a wrapper around pambackground.