[pdurrant]

Originally Posted by pdurrant

What's the optical illusion demonstrated by this image? I'm not looking for a technical name, just a description of what you can see in this image that isn't true.

What I expect you see in the image are quite broad green and blue spirals going clockwise out from the centre on a background of very thin pink/purple spirals going counter-clockwise out from the centre.

So what's the illusion?

Spoiler:

The 'green' and the 'blue' bits of the broad clockwise spirals are exactly the same colour. There are only three colours present in the image. Well, the first image has anti-aliasing between the colours, so there are more than three colours at the boundaries, but the second image has no anti-aliasing, and has pixels of just three different values. The thin spirals are 0xFF00FF and 0xFF9900, and the 'green' and 'blue' spirals are both #00FF99.

I find this illusion to be very robust. Even zooming in on the spirals in an image editing program, the 'green' and 'blue' remain for me. (See attached detail image.)

If I try to get a solid colours equivalent to the apparent colours, I get (roughly speaking) 0x99FF4F (green) and 0x5FCFFF (blue). I've added patches of these solid colours just for comparison purposes. They're not quite right, but they're close. I suspect that for different people they might be closer or further from the apparent colours.