Man1 - pbmmask.1

pbmmask - create a mask bitmap from a regular bitmap

pbmmask [*-expand*] [/pbmfile/]

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.

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.

-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…)

*ppmcolormask*(1) , *pnmpaste*(1) , *pnminvert*(1) , *pnmarith*(1) , *pnmsmooth*(1)

*pbm*(5) ,

Copyright (C) 1988 by Jef Poskanzer.