
the alpha mask defines the overlay image to be transparent, the
composite output contains none of the overlay image; the underlying
image shows through completely.  In places where the alpha mask shows
a value in between opaque and transparent (translucence), the
composite image contains a mixture of the overlay image and the
underlying image and the level of translucence determines how much of
each.

<P>The alpha mask is a PGM file in which a white pixel represents
opaqueness and a black pixel transparenc