Optical illusion

[Richard Russell]

It might not look like it, but the dark blue stripes are horizontal and parallel (the strength of the illusion may depend on your eyesight). The program will run, without modification, in Acorn's ARM BASIC V, Matrix Brandy BASIC, BBC BASIC for Windows and BBC BASIC for SDL 2.0.

Code: Select all

   10 MODE 8
   20 COLOUR 1,144,208,255
   30 COLOUR 2, 64, 64,255
   40 COLOUR 3,255,255,255
   50
   60 GCOL 1
   70 FOR X% = 46 TO 1126 STEP 216
   80   RECTANGLE FILL X%,0,106,1022
   90 NEXT
  100
  110 GCOL 2
  120 FOR Y% = 134 TO 782 STEP 216
  130   RECTANGLE FILL 0,Y%,1278,106
  140 NEXT
  150
  160 GCOL 0
  170 FOR X% = 46 TO 1234 STEP 108
  180   FOR Y% = 134 TO 890 STEP 108
  190     MOVE X%-20,Y%:MOVE BY 20,20:PLOT 113,20,-20
  200   NEXT
  210 NEXT X%
  220
  230 GCOL 3
  240 FOR R% = 0 TO 7 : Y% = 134 + 108 * R%
  250   FOR C% = 0 TO 11 : X% = 46 + 108 * C%
  260     IF (C% EOR R% EOR R% DIV 2) AND 1 THEN
  270       MOVE X%-20,Y%   :MOVE BY 10,10:PLOT 113,10,-10
  280       MOVE X%,Y%      :MOVE BY 10,10:PLOT 113,10,-10
  290     ELSE
  300       MOVE X%-10,Y%+10:MOVE BY 10,10:PLOT 113,10,-10
  310       MOVE X%-10,Y%-10:MOVE BY 10,10:PLOT 113,10,-10
  320     ENDIF
  330   NEXT
  340 NEXT R%

illusion.png