# Main Command: Go 8
# Bulge Illusion by Guy Walker
# www.logoarts.co.uk

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

To GridSq :Order :Side
  # draw tile at each column x row position
  LocalMake "Offset (1+:Order)/2
  For (List "Col 1 :Order) [
    For (List "Row 1 :Order) [
      SetXY :Side*(:Col-:Offset) :Side*(:Row-:Offset)
      Tile :Col :Row :Side] ]
End

To Tile :Col :Row :Side
  # use global value of :Tile to draw selected tile	
  Run (Word "Tile :Tile) :Col :Row :Side
End

To TileBg :Col :Row :Side
  If Even (:Col+:Row) [SetPC [0 102 153]] [SetPC [153 204 255]]
  Square :Side-1 Fill Wait 2
End

To TileDot :Col :Row :Side
  If X>0 [Make "Col :Col+1]
  If Y>0 [Make "Row :Row+1]
  If Even (:Col+:Row) [SetPC Black] [SetPC White]
  SetPW :Side/2 Dot Pos Wait 3
End

To TileSq :Col :Row :Side
  SetPC Gray SetPW :Side/30 Square :Side
End

To Square :Side
  # square (side x side) drawn from centre (tp)
  Back :Side/2 Right 90 Back :Side/2 PenDown
  Repeat 4 [
    Forward :Side Left 90]
  PenUp Forward :Side/2 Left 90 Forward :Side/2     # r2c
End

To Even :Num
  # return 'true' if Num even, else return 'false'
  If (Mod :Num 2)=0 [Output "True] [Output "False]
End

To Go :Order
  New
  Make "Side Int (400/(:Order+1))
  GlobalMake "Tile "Bg GridSq :Order+1 :Side Wait 20
  GlobalMake "Tile "Dot GridSq :Order :Side Wait 20
  GlobalMake "Tile "Sq GridSq :Order-1 :Side
End