This was a nice link by Myriad:
http://en.wikipedia.org/wiki/Perspective_projection#Perspective_projection
First, this is a basic projection where all viewing lines are parallel. After this, non-parallel lines of sight will be considered.
It is important to remenber that some applications of this projection will be of movement covering only a few feet. The ability to use parallel viewing lines depends on what is being measured.
(This will replace the vector projection relations until I fix the graphics. The results should be identical with some terms fixed)
The graphic posted earlier:
(Actually, the lines of sight in this graphic are not perfectly straight. They are drawn to diverge from the Sauret viewpoint)
[qimg]http://i53.tinypic.com/2aeo45k.png[/qimg]
The parallel case first:
[qimg]http://www.sharpprintinginc.com/911/images/photoalbum/13/1294543052_sauret_geometry1.png[/qimg]
As the top tilts theta, the positions of a(s), b(s) and r(s) change as a function of theta. We only care about theta 0 to 3 degrees.
[qimg]http://www.sharpprintinginc.com/911/images/photoalbum/13/a_b_r_relations.png[/qimg]
4 important features of these curves:
1) Shape of curves: Sinusoidal
2) Value ranges as theta goes from 0 to 3 degrees:
[qimg]http://www.sharpprintinginc.com/911/images/photoalbum/13/a_b_r_values.png[/qimg]
3) How quickly each point drops vertically as theta changes
[qimg]http://www.sharpprintinginc.com/911/images/photoalbum/13/a_b_r_slope.png[/qimg]
4) Comparing the relative rates of vertical drop as theta changes
[qimg]http://www.sharpprintinginc.com/911/images/photoalbum/13/a_b_r_ratios.png[/qimg]
These are basically ratios of how quickly one point will descend vertically relative to another.
The cotangent term is interesting. Obviously, as phi goes to zero, the ratio of vertical descent of the points a to r blows up to infinity. a drops much, much faster than r.
From the Sauret viewpoint, phi is about 13.8 degrees. As mentioned, we can expect a to drop about 3x that of r during the earliest movement, a very detectable range.