n the position you specify, need not fit entirely
within the underlying image.  <b>pamcomp</b> uses only the parts of the
overlay image that appear above the underlying image.  It is possible to
specify positioning such that <em>none</em> of the overlay image is 
over the underlying image -- i.e. the overlay is out of frame.  If you
do that, <b>pamcomp</b> issues a warning.

<P> The overlay and underlying images may be of different formats
(e.g.  overlaying a PBM text image over a full color PPM image) and
