All distortions are screen-space error in rad (by the 2Atn[r/d] term).
If you omit this term you have the geometric distortion.
No corrections for lifetime here.
Sometimes one or more variables have no effect.
They are nevertheless passed as vars, to get consistent calls and avoid confusion
n=number of polygons
r=radius of virtual object
d=distance to object
k=complexity (see report)
θ=distance between optimal and actual viewpoint