A Silent Conscience.

e1n:

Alternate link in case the pictures won’t enlarge: 1 / 2 / 3 / 4

PERSPECTIVE TIP: How to draw correct perspective lines without using vanishing points (or when the points are located off the page)

I hope the handwriting is legible.  In the future when I have time I’ll make a neater version of this with more example.  The gist of it is to divide the right and left side of the page into points of equal spacing, but give one side more points than the other.

If you know a bit of math, read on: this is actually just a bit of a ratio problem using the idea of similar triangles.  Since any two points on a single perspective line form similar triangles with the vanishing point as its vertex, then the ratio of the distance of the two points, measured from the base of the triangle must be constant.

In the picture above, PRQ and SRT are similar triangles.  R is where the vanishing point is located, S is the point on the right vertical frame, and P is the point on the left vertical frame.  Because they’re similar triangles, when changing the length of ST, you must also change the length of PQ, keeping the ratio of PQ:ST constant.