How to draw illusions?

How do artists know how to draw anamorphic illusions?

• A lot of prep work involved?  Do artists simply see / understand how to sketch to create such illusions?

• Answer:

  Having never actually drawn one on large scale I can't speak for how intuitive a process it is for those adept at it, or how much preparation is involved in the large scale public works we see examples of, but I shall attempt to take a stab at the "How" part. It is basically a cartesian projection trick. If you take a picture of a grid in perspective form that displays depth, overlay the graphic you want to draw in it's final form, and un-distort the perspective grid to a regular square grid, you will distort the overlay image to approximately the form that it needs to take when transfered square by square to a regular life-size grid (such as a sidewalk)... throw in a couple of light, contrast, and fuzziness effects to account for perception of depth and you could end up with something rather believable. The reason sidewalks are such a popular canvas is that they provide a regular grid to follow. It's the same grid that gives us our lines of perspective and thus our depth perception when we look down the sidewalk. All images are 2D projections of a 3D world, giving us the illusion of depth based on relative sizes and positions. If something is smaller and higher up on the canvas, it is further away. If something larger overlaps something smaller, then the larger object is closer and the smaller object is further away, though in reality it's all on the same surface of paper that your image is printed on. So if you turn these expectations on their head, you can create an illusion of different dimensionality. As an illustrative example, if you draw a really tall man directly onto the sidewalk as normal, and look at him from his feet end, his head appears really small since it's further away from you, and feet comparatively larger, which satisfies your brain that this man is lying down on the sidewalk. But if you drew him so his head was disproportionately large, with larger than usual shoulders but not compared to the head, a normal waist, drastically narrowing legs, and really really tiny feet, then what you've done is play with our brain's expectations of a familiar form, and the illusion this might create (if done skillfully) is one of a man standing up in front of us instead of flat along a receding sidewalk. Highly realistic artwork helps the illusion further of course and this is where the real skill, and patience, of the artist comes in.