Man page 1 : pbmmask

pbmmask - create a mask bitmap from a regular bitmap

pbmmask [ -expand ] [ pbmfile ]

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 pnmarithtool. 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 pnmsmoothscript before using it. This makes the boundary between the two images less sharp.

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

R ppmcolormask (1),

R pnmpaste (1),

R pnminvert (1),

R pbm (5),

R pnmarith (1),

R pnmsmooth (1)

Copyright (C) 1988 by Jef Poskanzer.