For demonstration of the vanishing point, we are going to take one of my favorite artists in the world for an example, the one who, to my mind, first started seriously using linear perspective, thereby changing Western art and bringing it out of the Middle Ages and into the Renaissance. I’m talking about Giotto.
In this beautiful painting, he has depicted a room that is basically a box, with us as spectators looking into the room as if the fourth wall were virtually not there. Indeed, that transparent fourth wall corresponds to the picture plane.
Let’s see if we can figure out first where the horizon line falls.
We do that by extending the diagonal lines of the picture to see where they meet, as you can see in the picture above. These lines that converge are called "orthogonals", or, um, converging lines. From everywhere in the picture, they seem to meet in one place – this place is called the “vanishing point”.
Now the important thing to know is that these converging lines always meet on the horizon line. Think about a classic drawing of a road going to a horizon, or railroad tracks going to the horizon. Just like with our orthogonals in the example above, they extend off into the distance to meet right on the horizon line, where they vanish. So now we have located that place where they vanish, we need only draw a horizontal line to mark the horizon, which Giotto places conveniently at the top of the garden wall; although to be a little more serious, he did that to greatly strengthen the composition of his painting.
Giotto's composition, note he also had the lines converge at their vanishing
point on the saint's face, to powerfully make Francis the central focus.
And now we have this information, can you tell whether Giotto was
standing, sitting, lying down or on a ladder when he did the picture? Do you need to take another look at Principle 2: The Horizon Line and Viewpoint to be sure?
I would say he was standing on a ladder to have his eyes on the same level as that of the saint's, which as we have said, fall right on the horizon line!