Fixed an error where I had k=1-s-a for some reason. Anyway, now the formulas are (and yes, i'll keep going until i get the same answer twice!):
which if sigma->infinity (thanks!) becomes:
Which doesn't have a d term anymore, however doesn't have a straight 1/cos(theta_in) either which bugs me. Will keep trying
I'm happy to stick with HG - I thought the user-selectable functions was your idea?
k and a can be texture inputs. I think perhaps a texture R term would be useful - could consider a scattering event as absorption/re-emission with a different spectrum?
Did you want a BxDF to wrap the HG function - or shall I just plug the HG formula in directly?