Mix two non-opaque colors with “hue” blend mode

Question

I want to implement color blending as described in the W3C compositing and blending spec. (I\'m doing this in JavaScript but the language shouldn\'t really matter for solving my

Answer 1

Starting from here

Hue blending creates a color with the hue of the source color and the saturation and luminosity of the backdrop color.

I can come up with some formulas, but they might be rubbish, although they completely reproduce the original numbers posted:

h: hSource + deltaH * (1 - aSrouce) * aBackdrop * 0.41666666 = 50; 63

s: sBackdrop * 0.9 + deltaS * (1 - aBackdrop) * aSource * 0.20833333 = 45; 47.5

l: lBackdrop * 0.957142857 + deltaL * (1 - aBackdrop) * aSource * 0.77 = 67; 63.3

a: 1 - (1 - aSource)^2 matches always