I just discovered an optical illusion. Never had that happen before; I've seen optical illusions and heard explanations of them, but this is the first time I've experienced an illusion I'd never seen before and had to work out why it happened for myself.

The background image on my Linux desktop is an indistinct fuzzy cloudy sort of pattern, because I rather like it and I find it very easy to visually distinguish sharply defined objects in front of it. Over this indistinct image I brought up a window for one of my more recent puzzle games, and played it in the normal way. When I'd finished, I leaned back in my swivel chair; and when I brought the chair forward again, I noticed a strange thing. The fuzzy cloudy background around the Dominosa window looked as if it was expanding! That is, not just expanding in the normal way you expect things to expand when your eyes move closer to them, but beyond that – it looked as if it was expanding relative to the screen and to the puzzle window.

I leaned back and moved forward again: same result. Of course the background didn't really expand, but it looked as if it was doing so for just the moment when my head was in motion towards the monitor.

My tentative explanation for the phenomenon is: the cloudy background I use doesn't look like a foreground object. It's more of a background thing, something that you expect to see at a very long distance (like, in fact, clouds). Now the apparent size of an object is inversely proportional to its distance from your eye[1]; so if you move your head twice as close to something it appears twice as big. So suppose I were looking at a foreground object in front of a distant background, and I moved my head sharply to put it twice as close to the foreground object. The apparent size of the foreground object would double, yet the apparent size of the background would barely change (since the relative change in my distance from it would be a tiny fraction of a per cent). So the foreground object would expand relative to the background, in my vision, and in particular a larger amount of background would be hidden from my view behind the foreground object. Thus, this is what my eye expected to see when I brought my head sharply towards a well defined foreground-looking object against an indistinct and cloudy-looking background; and when that didn't happen (because in fact both were displayed on the same monitor and their apparent sizes expanded at the same rate), my visual cortex must have initially assumed it to be because the background was physically expanding – as if the clouds themselves were changing in shape and size as I moved my head.

[1] (That is, in conventional flat-plane perspective; it gets fiddly and trigonometric if you start thinking in the more correct terms of angles subtended at the eye, but the flat plane approximation is adequate for this purpose.)

It's stopped happening now, annoyingly; I think my visual cortex must have figured out it was being had and adapted. Perhaps it was listening in on my conscious mind while I was working out what had happened.

I don't know if this illusion was specific to me or will happen for anyone else; so, in a spirit of experimentation, here's a snapshot of the desktop around the window in question: http://www.tartarus.org/~simon/20050723-illusion.png. To (hopefully) activate the illusion, arrange to move your head rapidly towards the image from a distance of (say) about 1m to (say) about 1/2 m. I found a good way to achieve this was to lean back in my swivel chair and then rapidly let it return to the upright position, but your mileage may vary. (This diary does not endorse dangerous leaning-back-on-chairs and accepts no liability for any injury incurred as a result. If in doubt, get a responsible adult to help you.)