This module computes the values at the saddle points 
\index{saddle point} in a bilinear interpolated image. 
\index{interpolation!bilinear}
The resulting image has one column and one line missing
compared to the original image. The value at $(i,j)$ is the one at the saddle
point in the dual pixel of corners $(i,j)$, $(i+1,j)$, $(i,j+1)$ and
$(i+1,j+1)$ if there is one, and \texttt{FLT\_MAX} (defined in \texttt{math.h})
if there is no saddle point inside the dual pixel. A more appropriate value
would have been \texttt{NAN} (Not A Number), but this value is not always
defined in \texttt{math.h}.

As the resulting image contains values (or rather non-values) not easily
representable, it is not appropriate for viewing via the module \texttt{fview}
for example.

This module is mainly called by the module \texttt{flst\_bilinear}.
