rendering with geometry will give you very long render times
this is a copy of a discussion between Guy rabiller and gordon cameron from softimage on the xsi user list :
Jasper - I'd be very interested in what you were trying todo over night and
what was particularly causing you problems, just in case that is not covered
by the fixes/changes we've made, and if we can indeed get you good results
in the meantime without having to wait for an update....
Guy - after speaking with one of the devs, I've collated some responses to
your issues below. Note that the volume shader absolutely should be the way
to go, especially when we get the most recent fixes to you. I've preceded
the responses with the tag [SOFT].
There are 2 shaders available to render the Hair:
- Hair_Renderer ( Volumic and Geometry )
- sib_hair ( Geometry )
By default, the Hair_Renderer is affected.
However, it doesn't work as espected.
So I would advise to not use it, here why:
This shader actualy doesn't 'Shade', curiously.
For the diffuse, Hair are rendered exactly the same wherever are the lights.
ie, doesn't take in account the direction of the light.
[SOFT] This is a feature, not a bug. To give nice uniform lighting on the
hair without experiencing the severe aliasing artifacts you get when you use
the tubular shape of the hair as normal, this is pretty much the only way to
go. It does actually take into account the light direction. The surface
normal is just calculated so as to always aim approximately towards the
light, so that the light contribution work like an average contribution over
the surface of the hair, not at the exact intersection point. We have a
tracking item logged to have a setting to mix between the surface normal of
the hair and the fake normal (the one pointing towards the light), and we'll
give you a call on whether we can get this in as soon as poss.
The specular gives a false impression, and yet, it doesn't take in account
the tubular shape of hairs, so we just got a flat lightning.
[SOFT] The specular was wrong, but has been fixed now internally to
incorporate the same model as used in Monsters, Inc. and Stuart Little. We
will find a way to make this available as soon as we can, as per other mail
thread(s).
There is a workaround, but, unfortunatly, it's bugged:
If you disconnect the Hair_Renderer from the 'shadow' node of the
'Material', you can add for exemple a 'Lambert' node at the color input of
the root color/tipsA/B etc..
[SOFT] You can do this still, but there is currently no guarantee that the
ray-state information needed is satisfactory to the subsequent material
shaders. So effectively you're getting the normal of the intersection point
on the bounding box. This would get fixed with the normal blending option,
mentioned above.
In this case, it works, shading occurs.
But, and this is too bad, it is shaded as if the light comes always from the
camera 
[SOFT] This is most probably fixed in the build we are trying to get to you
- the mr state is now left in the original condition when the shader
returns.
Our last chance is to use the sib_hair shader that we have to connect
manualy in the rendertree.
Surprisingly, it works pretty well, but only works in Geometry mode.
You get 'real' shading, raytraced shadows, etc..
Few tips with this shader:
.Put the 'Texture' parameter to '100', even if you don't have texture.
[SOFT] This doesn't have anything to do with texturing. It's a specular
decay control. Sorry about the confusion. This information wasn't in the
Hairy guide, but is now.
.If you activate the 'Compute Transparency Fallof' set the fallof to 0, else
artefacts.
[SOFT] We will look into this.
.Subsurface Scattering is like fake lighting for backside normally not
lighted ( at the opposite side of the lighted one ).
Set a low value here, else you loose all the benefit of a working shading.
For the Rendering Options:
.Min/Max -1/2 or 3 would be enough but set a low luminance threshold ( ie
0.05 ).
.Mitchell Filter 4/4 is working well.
But, NEVER activate the Jitter option !
