Do we need Fresnel blend on transmap hair



  • Controversial I know, but I created a hair shader and added Fresnel blending between diffuse and glossy. On blonde hair this seemed to work, however when using darker hair, even ginger, I ended up with artefacts at the edges of individual ribbons. I had corrected for backfacing mirror effect, effectively swapping the normal direction, but these edge effects still appeared. Removing the Fresnel and just going for a defined mix, roughly 35% glossy resulted in much better renders. I know real hair will use Fresnel but we don't have thousands of individual strands, we have flattish ribbons.



  • Those ribbons don't have any thickness, so fresnel really isn't the best option.



  • @shvrdavid That's my take as well, I'm just putting the question out there for people who know more about the physics of the mat room than I do :)



  • My take on it is that conforming hair is not physically accurate (not strands) so why would I expect a hair shader to be physically accurate to what hair does in the real world. Just make something that looks "right" and renders fast.



  • If one had a bump map tangent space normal map (though that requires tools external to Poser to create) which smoothly defines (i.e. rounds off) the edges of the hair "ribbons", or even a transverse curl (though I've seen plenty of hair props made with flattened cylinders instead of flat ribbons, specifically to give this effect), then Fresnel use might avoid such normal vector discontinuity artifacts.


  • Poser Ambassadors

    This is tricky. If I literally answer your topic title question, "Do we need Fresnel blend on transmap hair" the answer is yes.

    Did you use it the way it needs to be used, the answer is no.


  • Poser Ambassadors

    I know you'll next ask how to do it. I don't know.

    I believe the right answer to a hair shader is to use the Marschner hair technique, fully fleshed out, like the Pixar Marschner hair.

    https://www.fxguide.com/featured/pixars-renderman-marschner-hair/

    Only way I know to make that would be if they let me program a new node.

    The Fresnel equation is in the Marschner equation, but buried far deeper than where we usually put it at the end. And it's not just Fresnel blending reflect/diffuse - there are 4 terms in it, not 2.


  • Poser Ambassadors

    @bagginsbill Only way I know to make that would be if they let me program a new node.

    I would support this .



  • @bagginsbill said in Do we need Fresnel blend on transmap hair:

    This is tricky. If I literally answer your topic title question, "Do we need Fresnel blend on transmap hair" the answer is yes.

    Did you use it the way it needs to be used, the answer is no.

    Like it :) Yes I guessed it was the implementation, but if it's got you stumped I don't feel too bad. Did you implement Fresnel in your uber hair shader? Because so far that is the best shader for transmapped hair we have, only downside is the speed, it's fine at small res but doing closeups is a crawl even on my dual GPU rig, so I was trying to get somewhere between your realism and @ghostship's speed.