@mouser: It is very interesting, and also a coincidence - I was playing around with 180 degree screen rotation the other day, showing my wife the same thing. I also demonstrated using the mouse (which moved "upside down" too). It nearly did my head in.
This is a relatively well-known perceptual illusion and is attributable to how the brain interprets what the eyes see. Our eyes actually "see" everything upside-down anyway (the image of what the eye receives is upside-down on the retina at the back of the eye), and the brain learns at an early age to translate the image to the right way up, using light, colour, shadows and lines as reference points and cues to make it all make sense. So what we THINK we see is already a perceptual illusion conjured up by our brain's image-processing system.
What does the trick in the case you show (and you can see it just by turning the screen upside-down) is mainly the shadows around the objects, which give a false sense of 3D right-way-up (it's still all 2D), and the brain adroitly reverses it when it's upside-down, rationally using the same reference points and cues.
You can get a similar illusion by:
- Staring at 3D line drawings - they seem to "pop" in and out in your perception.
- Staring at that animated image of the silhouette of a slowly spinning woman. Your perception can make her seem to rotate left or right, but not everyone can deliberately change the perceived rotation. She's not rotating in any direction at all, but she looks as though she is.
Some people might say that animated graphics of Angelina Jolie seem to animate more smoothly and look more attractive when she is pictured naked as opposed to clothed, but I couldn't possibly comment.
