What to see
The blue horizontal stripes above are oblique – are they? In my eyes this is an amazingly strong effect.
What to do
- Deselect the checkbox “Check marks” – immediately the stripes become completely parallel.
- Deselect the checkbox “Odd reverse” – if unchecked, all the small diamond-checkerboards have the same orientation, and the effect vanishes.
- Exchanging (⇵) the colours between the horizontal & vertical grating also kills the effect.
A number of further parameters can be adjusted (within limits) to modulate illusion strength. The “Decorations” aren’t doing anything for me. In Victoria’s version there are even more decorations.
Comments
This effect became popular in July 2017, originally designed by Victoria Skye, which I simplified a little for easier programming. While it is interpreted as a version of the Café Wall illusion, I do not think this is correct – the mortar lines are missing here (and they are necessary for Café Wall). The angle illusion here is caused by the small check marks, and that effect was first demonstrated (I think) by Akiyoshi Kiatoka (my animated version →here). In his paper (2007) Kitaoka shows the necessary stimulus arrangement and puts the effect into the context of related ones. Yet I still do not understqnd the mechanism causing this very strong angle illusion. See here for a version where the angle is adjustable.
Sources
Kitaoka A (2007) Tilt illusions after Oyama (1960): A review. Japanese Psychological Research 49:7–19