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 automati-
    cally. 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  back-
    grounds 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 use-
    ful 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.