%!

(ps3d.inc) run

/page-begin {
gsave
72 dup scale
1 72 div setlinewidth
4 5 translate
} def

/page-end {
grestore
showpage
} def

[0 0 8 1] set-eye 
/light [-1 0 1 0 ] normalized def

/A 0 def
/dA 10 def

% lists the vertices of the square
/P [
[ -0.5 -0.5 0]
[ 0.5 -0.5 0]
[ 0.5 0.5 0]
[ -0.5 0.5 0]
] def

% holds the faces of the square
% a face is points + visibility function
/square [
  % one face
  [ [ P 0 get P 1 get P 2 get P 3 get ] [0 0 1 0]]
  % one face
  [ [ P 3 get P 2 get P 1 get P 0 get ] [0 0 -1 0]]
] def

{

page-begin
gsave3d

gsave
1 0 0 setrgbcolor
newpath
0 -1 0 moveto3d
0 2 0 lineto3d
stroke
grestore

0.5 0 0 translate3d
[0 1 0] A rotate3d

/Tinv ctm3d 1 get def

/E get-eye def

/O E Tinv transform3d def
/L light Tinv transform3d def

gsave
0 1 0 setrgbcolor
newpath
0 -1 0 moveto3d
0 2 0 lineto3d
stroke
grestore

/colour [ [1 0 0] [0 1 0] ] def

/S [ 0 1 3 div 2 3 div 1 ] def

0 1 square length 1 sub {
  /i exch def
  /f square i get def
  % f = [ point-array visibility ]
  /v f 1 get def
  v O dot-product 0 ge {
     /p f 0 get def
     /n p length def  
     newpath
     p n 1 sub get aload pop moveto3d
     p {
       aload pop lineto3d
     } forall
     gsave
	 /s S L v dot-product shade def
	   colour i get 0 get s mul
	   colour i get 1 get s mul
	   colour i get 2 get s mul
	 setrgbcolor
     fill
     grestore
     stroke
  } if
} for

page-end
grestore3d

/A A dA add def

} loop