Friday Illusion: Mind-bending chessboard seems to tilt

Caitlin Stier, video intern

 

You may want to get your rulers out: although this oversized chessboard seems to slant when animated, its rows and columns are always perfectly parallel.

The animation, developed by Sinji Nonaka, tricks your brain when alternating rows are shifted horizontally or vertically, skewing the grid pattern. The unusual effect was discovered by a member of vision researcher Richard Gregory's team as he looked at the brick tiling of a Bristol café in the 1970s. Gregory was inspired to recreate the design for a party organised by the BBC TV programme Tomorrow's World. But during the process he made a key discovery: the illusion was dependent on the shade of the mortar between the rows.

To test the effect, Gregory worked with Priscilla Heard, now at the University of the West of England, to develop a customisable version of the wall using reflective surfaces, adjustable lights and moving tiles. By varying the brightness of the mortar, they found that it had to fall in between the contrast of the black and white tiles for the effect to occur. A thinner lining also produced steeper slopes.  

But in addition to the brightness of gaps between rows, the offset of the chessboard is also responsible for the effect. The shift causes like-shaded squares to overlap which affects the perceived brightness of the tiny space in between. The gap appears to be lighter for two dark squares and darker where white ones meet, causing a striation that's processed by our brain as a single line indicating slope. 

If you enjoyed this illusion, check out another trippy chessboard or watch shifting lines create a phantom spin.

http://www.newscientist.com/