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.ppmAn 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.