corners
of the input image.  <B>pnmcrop</B> removes borders which are of the
background color.

<P>If at least three of the four corners are the same color,
<B>pnmcrop </B> takes that as the background color.  If not,
<B>pnmcrop</B> looks for two corners of the same color in the
following order, taking the first found as the background color: top,
left, right, bottom.  If all four corners are different colors,
<B>pnmcrop</B> assumes an average of the four colors as the background
color.

<P>The <B>-sides</B