<p><b>ppmshadow</b> sees your image as a foreground on a background.
The background color is whatever color the top left pixel of your image is.
The background is all the pixels that are that color and the foreground
is everything else.  The shadow that <b>ppmshadow</b> generates is a
shadow of the foreground, cast on the background.

<p>The shadow is the same size as the foreground, plus some fringes
as determined by the <b>-b</b> option.  It is truncated to fit in your
image.  The output image is the same