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+%%
+%% Ein Beispiel der DANTE-Edition
+%%
+%% 1. Auflage
+%%
+%% Beispiel 15-02-29 auf Seite 749.
+%%
+%% Copyright (C) 2011 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
+%%
+defaultfont:="ptmr8r";
+warningcheck:=0;
+% --- start of displayed preamble in the book ---
+
+% --- end of displayed preamble in the book ---
+% %
+beginfig(1)
+u=8mm;
+vardef project(expr x,y,z)=x*(-1,-1)+y*(1,0)+z*(0,1) enddef;
+vardef f(expr x,y)=sind((x/u)**2 *180 +(y/u)**2 *180)*u/2 enddef;
+numeric m,M,inc,farbe;
+m:=-2u; M:=2u; inc:=.1u;
+for i=m step inc until M:
+ for j=m step inc until M:
+ path p;
+ p=project(i,j,f(i,j))--project(i,j+inc,f(i,j+inc))--
+ project(i+inc,j+inc,f(i+inc,j+inc))--project(i+inc,j,f(i+inc,j))--cycle;
+ dfdx := (f(i,j) - f(i+inc,j))/inc; dfdy := (f(i,j) - f(i,j+inc))/inc;
+ farbe := 1/sqrt( dfdx**2 + dfdy**2 + 1); fill p withcolor farbe*(red+green);
+ endfor;
+endfor;
+endfig;
+end;