Factory interpolator: built ONCE at tween start with (from, to), the returned
function is called with t ∈ [0, 1] each frame. Use for interpolators with
expensive setup (color-space conversion, path-string parsing) — d3-interpolate's
shape exactly. For cheap interpolations the per-tick Interpolate<T> form is
fine; this is the escape hatch when setup-per-tick is wasteful.
Factory interpolator: built ONCE at tween start with (from, to), the returned function is called with
t ∈ [0, 1]each frame. Use for interpolators with expensive setup (color-space conversion, path-string parsing) — d3-interpolate's shape exactly. For cheap interpolations the per-tickInterpolate<T>form is fine; this is the escape hatch when setup-per-tick is wasteful.