%% 
%%  Ein Beispiel der DANTE-Edition
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%%  7. Auflage
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%%  Beispiel 22-06-73 auf Seite 444.
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%%  Copyright (C) 2016 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% 
%% ==== 
% Show page(s) 1
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%% 
\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{352.81416pt}
\setlength\parindent{0pt}
\def\NormalSin#1#2#3{\psset{linecolor=#3}\pstVerb{
 /xP #1 def /yP #2 def /zP 2 #1 Sin mul #2 Sin mul def
 /normaleX 2 #1 Cos mul #2 Sin mul neg def /normaleY 2 #2 Cos mul #1 Sin mul neg def
 /Norme normaleX dup mul normaleY dup mul add 1 add sqrt def
 /Nx normaleX Norme div def /Ny normaleY Norme div def /Nz 1 Norme div def}
\psSolid[object=vecteur,args=Nx Ny Nz](xP,yP,zP)}
%StartShownPreambleCommands
\usepackage{pst-solides3d}
%StopShownPreambleCommands
\begin{document}
\begin{pspicture}[viewpoint=50 20 20 rtp2xyz,Decran=50](-3,-3)(3,4)
\psSurface[algebraic,ngrid=0.2 0.2,incolor=yellow!20,axesboxed,showAxes=false,
  Zmin=-2,Zmax=2,fillcolor=blue!20](-3,-3)(3,3){2*sin(x)*sin(y)}%
\NormalSin{1.57}{0.75}{red}\NormalSin{1.57}{1.57}{blue}%
\NormalSin{2}{1.57}{green}\NormalSin{-1.57}{-1.57}{magenta}
\end{pspicture}
\end{document}
