%!
false echo
/Times-Roman findfont 10 scalefont setfont

/box {				% --- => ---
  newpath
  0 0 moveto
  1 0 lineto
  1 1 lineto
  0 1 lineto
  closepath
} def

/unbox {			% --- => ---
  newpath
  0 0 moveto
  0 1 lineto
  1 1 lineto
  1 0 lineto
  closepath
} def

/plane {			% x y width height path unpath gray => dict
  7 dict
  begin
    /gray exch def
    /unpath def
    /path def
    /height exch def
    /width exch def
    /y exch def
    /x exch def
    currentdict
  end
} def

/showit {			% thing => ---
 40 string cvs show
} def

/showplane {			% plane => ---
  gsave
  begin
    x y 15 sub move  0 setgray
    (X pos ) show  x showit
    (, Y pos ) show  y showit
    (, width ) show  width showit
    (, height ) show  height showit
    (, path ) show  path showit
    (, unpath ) show  unpath showit
    (, gray ) show  gray showit
    (.) show
  end
  grestore
} def

/planepath {			% plane => ---
  begin
    x y translate
    width height scale
    path cvx exec
    gray setgray
  end    
} def

/planeunpath {			% plane => ---
  begin
    x y translate
    width height scale
    unpath cvx exec
    gray setgray
  end    
} def

/drawplane {			% plane => ---
  gsave
  planepath gsave fill grestore 0 setgrey stroke
  grestore
} def

/castshadow {			% --- => ---
  0 1 i 1 sub {			% j
    i 1 index sub		% j dist
    planes 2 pick get		% j dist clipplane
    curmatrix setmatrix
    dup /gray get		% j dist clipplane clipgray
    .2 sub			% j dist clipplane shadowgray
    /shadowgray exch def	% j dist clipplane
    planepath			% j dist
% clip others here
    clip
    dup lightx mul		% j dist xoffset
    exch lighty mul		% j xoffset yoffset
    curmatrix setmatrix
    translate			% j
    plane planepath
    shadowgray setgray
    fill
    pop				%
  } for
  curmatrix setmatrix
} def

/drawplanes {			% planes => ---
  5 dict begin
  /planes exch def
  /curmatrix matrix currentmatrix def
  0  1  planes length 1 sub {	% i
    /i exch def
    /plane planes i get def
    i 1 ne { castshadow } if
    plane drawplane
  } for
} def

/planes
[ 30  30  300 300 /box /unbox .2 plane
  60  190 150 50  /box /unbox .4 plane
  100 120 40  130 /box /unbox .4 plane
  70  150 180 80  /box /unbox .4 plane ] def

gsave

5 setlinewidth

true echo
% planes drawplanes

% showpage
% grestore