This modules thresholds a Fimage $u$ at a value $t$ (default 128.0), and
produces a Cimage $v$ such that 
$$v(x,y) = \left\{ \begin{array}{l}
255 \mbox{ if } u(x,y) > t,\\
0 \mbox{ if } u(x,y) \leq t.
\end{array}\right.$$
If the \verb+-i+ option is selected, the video inverse 
$v\mapsto 255-v$ is then applied.
