For example, consider one
line segment from (x,0,z) to (x,y,z), and another from (x,0,2z) to (x,y,2z). These are the same length.
These project in the image to
a line from (fx/z,0) to (fx/z, fy/z) and from (fx/z,0)
to (fx/2z, fy/2z), where
we can rewrite the last point as: (1/2)(fx/z,fy/z). The second line is half as long as the
first.