jpegmode
%quadrillage marks
400 setheight

-10 8 setxrange
-5.5 14.5 setyrange

%% le plan de base
/P1 {-9 -3} def
/P2 {4 -5} def
/P3 {8.5 0} def

/a {-6 4.5 0} def    
/b {6 4.5 0} def    
/c {6 -4.5 0} def    
/d {-6 -4.5 0} def    
/s {0 0 11.4} def
/s' {0 0 0} def

/alpha 30 def
/beta 165 def
/vect_I {alpha cos alpha sin .5 mulv} def
/vect_J {beta cos beta sin .9 mulv} def
/vect_K {0 1} def

/xyz2xy {
3 dict begin
   /z exch def
   /y exch def
   /x exch def
   vect_I x mulv
   vect_J y mulv
   vect_K z mulv
   addv addv
end
} def

[/A /B /C /D /S /S'] 
[a b c d s s'] {xyz2xy} capply
mapnp


B C milieu /I defpoint
I D A C interdroite /M defpoint
S S' M paral C S interdroite /M' defpoint

/fillstyle {.9 setgray fill} def
[D I M'] polygone*

gsave
   orange
   .8 setlinewidth
   mixte
   [A C] ligne
grestore

/dotscale {2 dup} def
[S'] {times} plot

1.2 setlinewidth
[A D C S] ligne
[A S D] ligne

gsave
   .8 setlinewidth
   pointilles
   [A B C] ligne
   [B S S'] ligne
   [M M'] ligne
grestore

2 setlinewidth
[-12 P2 P1 xdpoint P2 P3] ligne


12 setfontsize
setTimesItalic
   (A) A dltext   
   (B) B urtext	  
   (C) C drtext	  
   (D) D dltext	  
   (S) S urtext   
   (s) S' brtext   
   (H) P2 (0 7) ultext
   (I) I urtext