Still need to scan convert in screen space... so we need a mapping from t
values to s values.
We know that the all points on the 3-space edge project onto our screen-space
line. Thus we can set up the follow equality:
and solve for s in terms of t giving:
Unfortunately, at this point in the pipeline (after projection) we no longer
have z1 and z2 lingering around. However,
we do have w1 = 1/z1 and w2 = 1/z2
(homogenous coordinates).
