-PPM files that are useful for
various things.

<P>For example, if you feed a true PPM to <B>pnmgamma -cieramp
-ungamma</B>, you get as output a file which is PPM in every respect
except that the sample values are directly proportional to the light
intensities in the image.  If you feed such a file to <B>pnmgamma
-cieramp</B>, you get out a true PPM.

<P>The situation for PGM images is analogous.  And <B>pnmgamma</B>
treats PBM images as PGM images.

<P>When you feed a linear PPM image to a display program 