Bill DeWeese thinks this is interesting:

By default, m34 has a value of zero. We can apply perspective to our scene by setting the m34 property of our transform to -1.0 / d, where d is the distance between the imaginary camera and the screen, measured in points. How do we calculate what this distance should be? We don’t have to; we can just make something up.

Because the camera doesn’t really exist, we are free to decide where it is positioned based on what looks good in our scene. A value between 500 and 1000 usually works fairly well, but you may find that smaller or larger values look better for a given layer arrangement. Decreasing the distance value increases t...