# Main Command: Go
# Henon_Curves by Guy Walker
# www.logoarts.co.uk

to new
  # set default screen, pen and turtle 
  ResetAll SetScreenSize [400 400] HideTurtle 
  SetSC Black SetPC Green SetPS 1 PenUp 
end

to init
  Make "X 0 Make "Y Minus 0.2
  Make "XScale 128 Make "YScale 440
  Make "Alpha 1.4 Make "Beta 0.3 
end

to angcol :theta
  Make "Red Abs 255 *Cos (:Theta)
  Make "Gre Abs 255 *Cos (:Theta + 120)
  Make "Blu Abs 255 *Cos (:Theta + 240)
  Output (List :Red :Gre :Blu)
end

to hénon
  Make "Xnew 1 - :Alpha*(Power :X 2) + :Y
  Make "Y :Beta * :X
  Make "X :Xnew 
  Dot List (:X*:XScale) (:Y*:YScale)
end

to go
  New Init2
  Repeat 4000 [
    # SetPC AngCol Integer RepCount/10    # comment in for color
    Hénon]
end

# for a second type of curve

to init2
  Make "X 0 Make "Y 0
  Make "XScale 40 Make "YScale 40
  Make "Alpha 0.2 Make "Beta 0.9991
end