%!

/intersection { 1 dict begin
/L exch def
/A L 0 get def
/B L 1 get def
/C L 2 get def
/f00 A -1 mul B -1 mul add C add def
/f10 A  1 mul B -1 mul add C add def
/f11 A  1 mul B  1 mul add C add def
/f01 A -1 mul B  1 mul add C add def

% f01 f11
% f00 f10

[
  f00 f10 mul 0 lt {
    /t f00 f00 f10 sub div def
    % (1-t)(00) + t(10)
    [-1 -1] [1 -1] t interpolate
  } if
  f10 0 eq {
    [1 -1]
  } if
  f10 f11 mul 0 lt {
    /t f10 f10 f11 sub div def
    [ 1 -1] [1 1] t interpolate
  } if
  f11 0 eq {
    [1 1] 
  } if
  f11 f01 mul 0 lt {
    /t f11 f11 f01 sub div def
    [1 1] [-1 1] t interpolate
  } if
  f01 0 eq {
    [-1 1] 
  } if
  f01 f00 mul 0 lt {
    /t f01 f01 f00 sub div def
    [-1 1] [-1 -1] t interpolate
  } if
  f00 0 eq {
    [-1 -1] 
  } if
]
end } def

% --- utilities ------------------------------------------

% P Q -> (1-t)P + tQ
/interpolate { 1 dict begin
  /t exch def
  /Q exch def
  /P exch def
  [ 
    1 t sub P 0 get mul Q 0 get t mul add
    1 t sub P 1 get mul Q 1 get t mul add
  ]
end } def

% --- sample usage ---------------------------------------

/S 72 def
S dup scale
1 S div setlinewidth

4.25 5.5 translate

newpath
-1 -1 moveto
1 -1 lineto
1 1 lineto
-1 1 lineto
closepath
stroke

/I [3 -1 2] intersection def

I length 2 eq {
  newpath
    I 0 get 0 get
    I 0 get 1 get
  moveto
    I 1 get 0 get
    I 1 get 1 get
  lineto
  stroke
} if