NAME

       pbmmask - create a mask bitmap from a regular bitmap

SYNOPSIS

       pbmmask [-expand] [pbmfile]

DESCRIPTION

       Reads a portable bitmap as input.  Creates a corresponding mask bitmap and writes it out.

       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 graymaps and pixmaps, using the pnmarith tool.  For instance:
           ppmtopgm obj.ppm | pgmtopbm -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 the pnmsmooth  script  before
       using it.  This makes the boundary between the two images less sharp.

OPTIONS

       -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 tool into a general cellular automaton tool...)

SEE ALSO

       ppmcolormask(1), pnmpaste(1), pnminvert(1), pbm(5), pnmarith(1), pnmsmooth(1)

AUTHOR

       Copyright (C) 1988 by Jef Poskanzer.

                                          08 August 1989                               pbmmask(1)