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 (Why?). However, we do have w₁ = 1/z₁ and w₂ = 1/z₂.