[pstricks] pst-solides3d

[Juergen Gilg]

Dear Philippe,
> I tried to get "smooth" level curves
try this one:

\documentclass{article}
\usepackage{pstricks}
\usepackage{pst-solides3d}

\begin{document}
\psset[pst-solides3d]{%
viewpoint=50 30 20 rtp2xyz,
Decran=40,
lightsrc=viewpoint}

\begin{pspicture}(-7,-8)(7,8)

\psSurface[%
ngrid=.25 .25,
linewidth=0.5\pslinewidth,
axesboxed,
algebraic,
     intersectionplan={[0 0 1 3]},
     intersectioncolor=(bleu),
     intersectionlinewidth=1,
     intersectiontype=0,
tablez=-4 1 -3 {} for,
zcolor=(Red) (Yellow)]
(-4,-4)(4,4){%
((y^2)-(x^2))/4 }
\end{pspicture}
\end{document}

Hope this is the smoothness you were talking about. However i guess you 
will color some regions of z-height in different colors with the lines 
of equivivalent height...

Maybe this t inspires you to find a solution.

Regards,

Juergen