Render hangs when using sss skin and hair in same scene

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  06 June 2013
Question Render hangs when using sss skin and hair in same scene

When I use either sss skin or hair it works fine, but whenever I have a scene with both of these, even if they don't interact with each other, the render locks up for a few minutes then resumes rendering. It happens after precalculations are done, as soon as it switches to "Current Task: Rendering..." it hangs, then starts up after a few minutes. I know there are some precalculations with sss skin, but this only happens when sss skin and hair are both in the scene. Does anybody know why, and if there is a way to reduce/remove this? It's doubling or tripling my render times.
  06 June 2013
It seems to work here. 3dsmax2014+mrPrim+unified sampling
And it was pretty fast too.

On my test scene:
without hair: 1:18
with hair: 2:29

You don`t grow the hair directly on the character mesh, don`t you?
  06 June 2013
What type of lights are you using and are you rendering the hair to the z-buffer or as geometry?
The Z-Axis
  06 June 2013
I am using an infinite light with mr shadow maps and a skylight, and I am using mr prim hair. It's rendering in mental ray.
  09 September 2013
Sorry to bump this, but it's really giving my trouble now. with sss skin it hangs for a few seconds, with hair it doesn't hang at all, but with both it hangs for about five minutes. There is no way this is working as intended. I must have a setting off somewhere, or there is a glitch, or something. I would really REALLY like find out what the problem is.

EDIT: Increasing mr voxel resolution seems to speed it up somewhat.

Last edited by Carltheshivan : 09 September 2013 at 08:10 AM.
  09 September 2013
It's not uncommon in the past to have to render hair as a separate pass. (GI on hair sort of pointless in many cases.) Maybe try duplicating the faces on the mesh used for the hair out as a separate object and putting a regular material on it. (So your hair is a separate mesh.)

Make sure to uncheck "visible to camera" in the object properties. Also turn off reflections and refractions, and shadow casting.) Basically a shell of the hair geo that's invisible and doesn't affect the scene. (Pretty sure unchcking "Renderable" messes with the hair but it's been a long time since I used MR for it. Might not anymore.)

Chances are it'll work. One caveat is occasionally using this method you'll get a flicker at the scalp line, but putting a push modifier on the hair geo model and upping it ever so slightly solves it. Doesn't always happen.
  09 September 2013
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