What this needs from the film
This is a DRR — the CT of the case in the other tab, integrated along the
patient's true bicoxofemoral axis, so the projection is the one the angles are
actually defined in. Constructions come from ostk's own 2-D projection
(ostk.drr + ostk.project2d), and the values are copied
verbatim from the CT bundle — the two tabs report the same numbers by construction.
On a real film, LL and SS need only the L1 and S1 endplates, but
PI and PT need the femoral head centres — absent from a coned
lumbar lateral, in which case ostk returns them as
n/a rather than estimating.