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View Full Version : Shave & Haircut and Mental ray Subdiv approx

mike0006
03-30-2007, 09:10 PM
I'm currently working on a character and trying to add hair to his head using Shave and a Haircut. I detailed him in Zbrush and am using a displacement map on a low poly version of the character with Mental Ray's approximations. (subdivision and displacement) The hair is generated from duplicated polys from the scalp area. I have the displacement map also plugged into the Displacement attribue of the hairshape node. But, when I render there is a gap between where the hair starts and the scalp. If I get rid of the subdivision approximation, the hair renders on the surface and if I smooth the model in Maya the hair also follows the surface. This gap only occurs when I have a subdivision approximation on the model. Even with the Subd approx set to parametric, and the divisions set to 0, the surface still moves just a bit.

Here are some images demonstrating what's happening. This is with no Subd approximation.
http://img118.imageshack.us/img118/1913/subd1oz7.jpg (http://imageshack.us)

Here it is with the subdivision approximation applied but set with 0 divisions.
http://img410.imageshack.us/img410/1600/subd2dk5.jpg (http://imageshack.us)

I would just render without the subdivision approxiamtion applied but my render time jumps from 2:14 to 3:53.

Does anybody have any ideas?
mike0006
04-04-2007, 06:58 PM
Anybody have any ideas? Is there some way I could take the curves that I have styled in S & H and use something else to render them? Mabey paintfx or maya hair?
fahr
04-04-2007, 08:25 PM
This is a very obvious and crippling problem with Maya. It happens with normal Maya hair and fur as well. It completely baffles me why this hasn't been addressed. How on earth does any studio work around this problem??
mike0006
04-05-2007, 03:30 AM
Really? I would've thought this would be a problem with S & H's render buffer. I figured it calculated where the geometry was before the approximation was applied and that was why it was not showing hair on the surface but if this happens with Maya hair aswell, perhaps there is just some option box I haven't checked?
fahr
04-05-2007, 06:15 AM
This is also a problem with Maya hair/fur rendering with Renderman for Maya. Maya does not send the approximaiton information to the hair. So the hair continues to conform to the non subdivided mesh. Its a Maya API issue, I believe. Someone please correct me if Im wrong here. ;). What astounds me is that no one really ever mentions this, yet I see Maya being used all the time to output fur in films. Apparently, there are some solid, reliable workarounds for this issue...? Anyone care to share?
Hezza
04-05-2007, 12:06 PM
I don't know of any solid workrounds, but this problem is meant to have been addressed in renderman for maya 2 (currently in beta)

It would solve so many issues we've had on this production
MaNdRaK18
04-05-2007, 08:53 PM
If I remember correctly, workaround should be to apply some slight displacement to surface, and the same displacement map as a fur/hair offset.
If You don't want any displacement, You might try with uniform displacement map?
mike0006
04-10-2007, 05:57 AM
I have applied a displacement map to the surface and to the hair aswell. The hair reacts by pushing out/in the appropriate amount but the render still has that gap.
MaNdRaK18
04-10-2007, 04:16 PM
Did You apply subdivisional or/and displacement aproximatimation as well?
Everyone say that these 2 don't work in conjunction, meaning subD's excludes displacement, but I made few tests and it doesn't seem so.
Try with just one of them, and then with both, and see what suits You.
mike0006
04-10-2007, 08:17 PM
Yes, i run into this problem when I have subdivision approximation applied. I know they are supposed to be able to work together, because all of the tutorials and official 'how-tos' from Pixologic recommend setting a discplacement and subdivision approximation. If I turn off the subdivision approximation and only use the displacement approximation, that little gap between the hair and surface does not occur. But when I do this my render time jumps from 2:14 to 3:53 per frame.
Dogway
12-11-2007, 03:47 PM
I also have that kind of problem, and cant imagine how much people had to face this with Zbrush so implemented into a 3D pipeline as it is nowdays.
Anyways are you sure that turning off Subd approx "fixes" the issue? Does the model keep the volume?

I tried converting displ to poly by sections, and then wraping the mesh of a previous piece to that converted section, but it crashes UVs.
marcotronic
12-12-2007, 12:08 PM
I also have that kind of problem, and cant imagine how much people had to face this with Zbrush so implemented into a 3D pipeline as it is nowdays.
Anyways are you sure that turning off Subd approx "fixes" the issue? Does the model keep the volume?

I tried converting displ to poly by sections, and then wraping the mesh of a previous piece to that converted section, but it crashes UVs.

Hi,

I donīt know if I have understood you right but if you have a problem with ZBrush and Maya 2008 (especially seams in the displacements caused by the new mental ray subdiv. approx. node), I have found a solution for that recently:

http://www.zbrushcentral.com/zbc/showthread.php?t=54042

Maybe there are some other issues with the way the new subdiv. approx. works which can be solved by using the methods I describe in the above mentioned thread...

Marco
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