In other words, this is a convolution. pbmtopgm is similar to a special case of pnmconvol.
You may need a ppmsmooth step after pbmtopgm.
pbmtopgm has the effect of anti-aliasing bitmaps which contain distinct line features.
pbmtopgm works best with odd sample width and heights.
You don't need pbmtopgm just to use a PGM program on a PBM image. Any PGM program (assuming it uses the Netpbm libraries to read the PGM input) takes PBM input as if it were PGM, with only the mininum and maximum gray levels. So unless your convolution rectangle is bigger than one pixel, you're not gaining anything with a pbmtopgm step.
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