% draws a line graphics based on a spiral structure
draw(Size) :-
   % Generate a new display name
   gensym(w,Display),
   free('@'Display),
   new('@'Display,picture('Wolfgang Menzel 7',size(Size,Size))),
   send('@'Display,open),
   % define size of the picture
   SizeNew is Size/2.5,
   send('@'Display,background,colour(black)),
   % draw the object on the display
   draw_object('@'Display,1,SizeNew),
   % if desired save the display as .jpg
   write_ln('Save the graphics (y/n): '),
   get_single_char(A),
   put_code(A),nl,
   A=:=121 ->
     (write_ln('enter file name: '),
      read_line_to_codes(user,X),
      atom_codes(File,X),
      atom_concat(File,'.jpg',FileName),
      get('@'Display,image,Image),
      send(Image,save,FileName,jpeg) ).

% draw_object(Display,Angle,Radius)
% draws a line graphics on a spiral structure
% with initial radius Radius onto Display
% Radius is decreased recursively from its initial value to 0
draw_object(_,_,X) :- X<5.
draw_object(Name,Angle,Radius) :-
  % decrement radius and increment angle
  RadiusNew is Radius - 2/sqrt(Radius),
  AngleNew is Angle + 0.2,
  % determine the end points of the current line
  Xfrom is RadiusNew * 1.5 * cos(Angle)/1.5 + 200,
  Yfrom is RadiusNew * 1.5 * sin(Angle)/1.5 + 200,
  Xto is RadiusNew * 0.8 * cos(Angle+0.2)/1.5 + 200,
  Yto is RadiusNew * 1.5 * sin(Angle+0.2)/1.5 + 200,
  % draw the line
  send(Name,display,new(L,line(Xfrom,Yfrom,Xto,Yto))),
  % create a new colour object and send it to the current line
  RColour is 65535,
  GColour is 65535-Radius*250,
  BColour is 0,
  gensym(c,CName),
  new(CObj,colour(CName,RColour,GColour,BColour,rgb)),
  send(L,colour(CObj)),
  % call draw_object recursively
  draw_object(Name,AngleNew,RadiusNew).

:- draw(400).