The bitmap program is a rudimentary tool for creating or editing
rectangular images made up of 1's and 0's.  Bitmaps are used in X for
defining clipping regions, cursor shapes, icon shapes, and tile and
stipple patterns.

The bmtoa and atobm filters convert bitmap files to and from ASCII
strings.  They are most commonly used to  quickly print out bitmaps and
to generate versions for including in text.

This is from the modular X.org X11 project.