The brain works out our 3D experience of the world from the 2D light patterns that fall onto our retina at the back of the eye.
This process takes advantage of many of our implicit assumptions of the world, such as the fact that textures will fade as they go farther away, parallel lines will tend to converge in the distance and that objects will seem larger the closer they are.
Julian Beever’s art uses a knowledge of these processes, so when seen from a certain angle, the pictures fool the visual system’s inbuilt processes to produce a false sense of depth.
When seen from an alternative angle, the illusion breaks-down, and it’s possible to see how the artwork was created.
There’s plenty more examples of this amazing effect on Julian’s pages that are well worth checking out.