e normal representation.
The result is a vector represented in so called normalized device coordinates
(NDC), in which a visible vector ranges within -1 to 1
in each of x,y, and z dimensions.
For the simulation of real cameras in a robot world,
the perspective projection is used more often than the parallel-projection.
Perspective-projection defines a few more parameters.
<TT>Screenx</TT> and <TT>screeny</TT> are the sizes of the window on the
viewing plane on which observed objects are projected,
and with 