autocrop
20 setxunit
-18 13 setxrange
-6 12 setyrange
%quadrillage marks

/q {6 4 0} def    
/r {6 -4 0} def    
/p {-5 0 0} def    
/s {0 0 8} def

/vect_I {-10 cos -10 sin} def
/vect_J {40 cos 40 sin .8 mulv} def
/vect_K {0 1} def

%% [O vect_I] (->) ligne
%% [O vect_J] (->) ligne
%% [O vect_K] (->) ligne

/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

[/P /Q /R /S] 
[p q r s] {xyz2xy} capply
mapnp

/I {3 3} def

%% le plan de base
/P1 {2 -5} def
/P2 {-7 -2} def
/P3 {10 -2} def

gsave
   2 setlinewidth
   [P2 P1 P3] ligne
grestore

%% corrige
S I R Q interdroite /A defpoint
S P I paral 
A P interdroite /J defpoint
I J P1 P2 interdroite /K defpoint
gsave
   .6 setlinewidth
   pointilles
   orange
   [S A P] ligne
   .8 setlinewidth
   noir 
   [I K] ligne
   1.2 setlinewidth
   continu
   [I 8 I J ydpoint] ligne
   [K -5 I J ydpoint] ligne
grestore
J times2

%%%%%%%%%%%

gsave
   .8 setlinewidth
   pointilles
   [P Q] ligne
grestore

/dotscale {2 dup} def
[I] {times} plot

1.2 setlinewidth
[S P R] ligne
[S Q R] ligne
[S R] ligne

12 setfontsize
setTimesItalic
   (I) I ultext   
   (J) J drtext   
   (P) P dltext   
   (Q) Q drtext	  
   (R) R drtext	  
   (S) S urtext   
   (d) 7 I J ydpoint ultext