If you’re like many regulars to ScienceBlogs you probably found the cool Purple Nurple optical illusion over at Omni Brain. I don’t really understand why a static object appears as though it’s pulsing, but I do enjoy the effect. Did you ever wonder how much an optical illusion can be distorted and still maintain the illusion? Mighty Optical Illusions has a bunch of items similar in effect to Purple Nurple. I grabbed the one below (it reminds me of a bunch of almonds). It has a very cool wavy effect.

(much more fun below the fold)

The fun begins when we start to manipulate it. For this little adventure I used the very useful freeware viewer/editor XNView. First, is the color what gives it the effect? Here’s a negative version:

OK, how about if we equalize it?

Perhaps the orientation has something to do with it. We can flip it and rotate it:

Still it waves (I kind of like the flipped version even more). How about swapping some colors? In the first version, RGB are swapped to BRG, and in the second to GBR. Both appear to reduce the illusion, the second swap more so than the first.

So much for the simple stuff. How about wholesale alteration of the shapes? First, let’s look at Slice. Here the image has been chopped up but the effect is still apparent.

Next comes Shear:

Lots of sharp jaggies so it’s not the simple, smooth shapes that give it the effect. Next comes Swhirl:

This whirlpool-like effect produces heavy-duty spatial distortion but the illusion remains. Tile is kind of like Shear but there are obvious black voids in the new image. The illusion seems to be less intense so perhaps there’s something about the contiguous nature of the pattern that is creating the illusion:

Our next edit is Waves:

This also produces considerable distortion of the image but yet again, the illusion continues. At this point I decided to try adding Gaussian Noise (at 50%):

The illusion is also reduced. Further increases in noise reduce the illusion even more. Finally, I cropped the image so that only a small segment was left:

Illusion gone.