derlying_file</I>
beneath it.

<P>If you add the <B>-alpha</B> option, then <B>pamcomp</B> uses the
image in file <I>alpha-pgmfile</I> as an alpha mask, which means it
determines the level of transparency of each point in the overlay
image.  The alpha mask must have the same dimensions as the overlay
image.  In places where the alpha mask defines the overlay image to be
opaque, the composite output contains only the contents of the overlay
image; the underlying image is totally blocked out.  In places where