Petite illusion d'optique

Les lignes horizontales dans la figure ci-dessous sont-elles parallèles? En cliquant sur le bouton en bas à gauche vous pourrez observer le glissement d'une ligne sur deux et ainsi apprécier l'illusion…

%@AUTEUR: Maxime Chupin
%@DATE: 3 mars 2007
 
verbatimtex
%&latex
\documentclass{article} 
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage[mdugm]{mathdesign}
\begin{document}
etex
 
u = 1cm;
path carre,droite;
carre := (1,1)--(-1,1)--(-1,-1)--(1,-1)--cycle;
droite := (-20u,0)--(20u,0);
numeric max;
max := 9;
 
for i:=0 upto 20:
 beginfig(i+1);
 
 for y:=0 upto max :
  draw droite shifted (10u,y*u) withpen pencircle scaled 1;
 endfor;
 
 for w:=0 upto 4:
  for x:=0 upto max:
   fill carre scaled (0.5*u) shifted (0.5u+2*x*u,0.5*u+2w*u);
  endfor;
 endfor;
 
 for w:=0 upto 3:
  for x:=(-1) upto (max+1):
   fill carre scaled (0.5*u) shifted ((0.5+i/20)*u+2*x*u,0.5*u+2w*u+u);
  endfor;
 endfor;
 
 label.urt(btex \begin{LARGE}Petite illusion d'optique\end{LARGE} etex, 
    (7.5u,-u));
 
 clip currentpicture to 
	(-0.5u,-u)--(20u,-u)--(20u,max*u)--(-0.5u,max*u)--cycle;
 
 
 endfig;
endfor;
 
for i:=0 upto 40:
 beginfig(22+i);
 
 for y:=0 upto 8 :
  draw droite shifted (10u,y*u) withpen pencircle scaled 1;
 endfor;
 
 for w:=0 upto 4:
  for x:=0 upto 9:
   fill carre scaled (0.5*u) shifted (0.5u+2*x*u,0.5*u+2w*u);
  endfor;
 endfor;
 
 for w:=0 upto 3:
  for x:=(-1) upto (max+1):
   fill carre scaled (0.5*u) shifted ((1.5-i/20)*u+2*x*u,0.5*u+2w*u+u);
  endfor;
 endfor;
 
label.urt(btex \begin{LARGE}Petite illusion d'optique\end{LARGE} etex, (7.5u,-u));
 
clip currentpicture to 
	(-0.5u,-u)--(20u,-u)--(20u,max*u)--(-0.5u,max*u)--cycle;
 
 endfig;
endfor;
 
for i:=0 upto 20:
 beginfig(63+i);
 
 for y:=0 upto 8 :
  draw droite shifted (10u,y*u) withpen pencircle scaled 1;
 endfor;
 
 for w:=0 upto 4:
  for x:=0 upto 9:
   fill carre scaled (0.5*u) shifted (0.5u+2*x*u,0.5*u+2w*u);
  endfor;
 endfor;
 
 for w:=0 upto 3:
  for x:=(-1) upto (max+1):
   fill carre scaled (0.5*u) shifted ((-0.5+i/20)*u+2*x*u,0.5*u+2w*u+u);
  endfor;
 endfor;
 
label.urt(btex \begin{LARGE}Petite illusion d'optique\end{LARGE} etex, (7.5u,-u));
 
clip currentpicture to 
	(-0.5u,-u)--(20u,-u)--(20u,max*u)--(-0.5u,max*u)--cycle;
 
 endfig;
endfor;
 
 
 
end