Spiral

- optical illusion

A series of concentric circles appear as a spiral with the added black and white markers.

To New
  # set default screen, pen and turtle values
  ResetAll SetScreenSize [400 400] HideTurtle
  SetSC Black SetPC Green SetPS 1 PenUp
End

To Go
  New SetSC LightGray
  For [X 5 175 15] [
    Make "NewRadius 10+90*(1+Cos :X)
    SetPC White SetPW 1 Circle :NewRadius FillZone
    SetPC Black Right 6*PenWidth SetPW (:NewRadius/7)-2
      Repeat 18 [
      PenDown Forward :NewRadius-(PenWidth/2)
      PenUp Back :NewRadius-(PenWidth/2) Left 20]
    SetPC Gray SetPW 1 Circle :NewRadius ]
End

Spiral
Spiral

Procedures blue
Variables pink
Comments green
Library gray