In one-point and two-point perspective, horizontal lines converge toward vanishing points on the horizon, but vertical lines are considered to be parallel. That makes sense, because those lines are usually an equal (or nearly equal) distance from your eyes along their whole length.
But imagine for a moment that you’re in an airplane, looking down at a tall building. The sides of the building are no longer an equal distance from you along their entire length. Instead, they’re receding toward a vanishing point (called the nadir) somewhere down below, as in the diagram of Escher’s Tower of Babel at the left. This viewpoint is sometimes called bird’s eye view.
Vertical lines can also converge at a point above the viewer, called the zenith. Imagine yourself lying on the sidewalk looking straight up at a tall building. From that point of view, the side of the building take on the same position relative to you as, say, a road would if you were standing on it – it recedes away toward a vanishing point at your current eye level. Lying on a sidewalk to study perspective is a bit extreme, but standing and looking up creates a similar effect. This is what happens in the photo of the Canadian mint at the right. This viewpoint is sometimes called worm’s eye view.
Three-point perspective is more common in comics, where it’s often used to an exaggerated degree, than it is in landscape art. However, I think it’s useful to study it anyway because it helps in understanding the concept of multiple vanishing points which are not necessarily on the “normal” horizon. And that understanding will be very useful in the next perspective topic – inclines!
