Even then, it's still not 3d - it's based on lighting and a lack of contrast - it's a projection based illusion that requires distance so that the variations be

Even then, it's still not 3d - it's based on lighting and a lack of contrast - it's a projection based illusion that requires distance so that the variations between the two two-dimensional projections are not enough that your brain re-comprehends what it's looking at. I'm not saying it isn't cool - there's something quite neat about the fact that the brain misinterprets what it's seeing as the nearest known 2d icon. But it's not 3-dimensional.