 opaque the overlay image is to be, i.e. how much
of the composite image should be from the overlay image, as opposed to
the underlying image.  <i>opacity</i> is a floating point number, with
1.0 meaning the overlay image is totally opaque and 0.0 meaning it is
totally transparent.  The default is 1.0.

<p>If you specify an alpha mask (the <b>-alpha</b> option),
<b>pamcomp</b> uses the product of the opacity indicated by the alpha
mask (as modified by the <b>-invert</b> option, as a fraction, and
this opaci