pub fn arc_span(step: f64) -> (Scalar, Scalar)Expand description
The middle control point of an arc through step degrees, at most a
half turn either way: (weight, outwards). It lies where the tangents
at the arc’s ends meet, outwards = 1 / (1 + cos step) along the sum of
the unit directions to the ends, weighted cos(step / 2) —
sqrt2_over_2 for a quarter turn. Revolves and helices both build
their spans with it, so a helix’s quarter lies on the very circle a
revolved quarter does.