For both depth drawing and understanding our visual world, perspective drawing and overlapping is a pretty important concept. In fact it's actually difficult to avoid, since objects in parts of objects naturally block our site of something all the time.
So how does it work? Let's begin by taking a look at the drawing below, in which the two parrots are on the same plane.
This birdseye view of where they are sitting shows their physical relationship to you, the observer, here to learn how to draw.
In the drawing below, however, are the parents on different planes, with one further away? We would think so, because we understand from the concept of relative size that bigger objects are nearer and smaller objects are further away.
But no. If we do another birdseye view of the situation, that you can see below, you will understand that the two parrots are in fact still standing on the same plane in relationship to you, still the observer. In other words, we have a big parent and a small parrot on the same plane. So in fact, overlapping takes perspective a step beyond relative size.
So what then are the golden rules for getting overlapping to work properly in realistic drawing:?
1. The two objects or units need to be seen as separate from each other.
2. The two objects or units must be seen as belonging to different planes.
3. The best way to do that is for one of the objects or units to be incomplete, in other words, blocked by another.
This all starts to make sense when you think about all those tourists in Pisa. You know how they like to have their picture taken looking as though they are holding up the Leaning Tower? In fact, the trick is to do it carefully enough so that there is no overlapping to spoil the fun. But if either the tourists pushing or the Tower leaning are incomplete, the optical illusion is a failure
I really do have to make it to Pisa so I can try this out for myself.