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Rendering Fur with GI


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#1 Mikhey

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Posted 12 September 2010 - 08:18 AM

Hi everyone! I wonder what are the ways to take GI to fur, is it possible to bake?

#2 oslo

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Posted 22 September 2010 - 01:41 PM

sorry but its link on topic on russian community of renderman

My link

Edited by oslo, 22 September 2010 - 01:44 PM.

Vimeo Channel -

#3 Serg

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Posted 23 September 2010 - 04:39 AM

I'm experimenting with a technique right now that is looking promising, It's directed at long hair but may work with fur too. The aim being to render hair quickly and without having to use hair specific deep shadow map light rigs n all that junk, because the hair must sit right with area lights etc, and to never trace against the hairs themselves coz it murders render time.

Basically how it works is by converting the fur geo preview into a fog density volume with isooffset (you may need to re-sample the curves to get enough points for accurate enough representation using the point cloud option in the isooffset), then I set this volume to Phantom, and exclude fur from any shadow casting lights, leaving the volume to cast shadows onto the fur and other stuff... And you're basically done! :D

Receiving raytraced shadows from a volume is surprisingly very quick rendering and you get the subsurface lighting for free as the light penetrates into the volume (the volume shader need only be the default Volume Cloud).

It casts shadows onto say a characters head quite nicely but obviously limited by the resolution of the fur to volume conversion, so you wont see individual hair strand shadows, but maybe tracing against a relatively sparse fur distribution (say by tracing against the actual preview fur) in addition to the volume will cheat this.

Bellow are two renders, rendered in MP mode with 10x10 pixel samples, the top is lit by two area lights. The second render is lit by an environment light with an HDRI map. They both took aprox 5min20s on 3 out 4 cores in this i7 940. For some reason, regardless of lighting technique the render takes about 4 mins before is actually starts, then goes very quickly.

Hairy_Teapot_AreaLit.jpg
Hairy_Teapot_HDRI_Environment_Light.jpg
Make_Hair_Volume.jpg

Cheers

S

#4 lisux

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Posted 23 September 2010 - 05:01 AM

Looking interesting Serg, nice technique.
Best Regards
Pablo Gimenez

#5 pclaes

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Posted 23 September 2010 - 05:37 AM

Nice indeed. I believe I saw a Pixar paper where the were using a volume to shade hairs as well.
http://graphics.pixa.../Hair/paper.pdf

#6 Serg

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Posted 23 September 2010 - 08:03 AM

Nice indeed. I believe I saw a Pixar paper where the were using a volume to shade hairs as well.
http://graphics.pixa.../Hair/paper.pdf


That is a very cool paper! I wonder if we could also simulate hair with fluids in Houdini...I think u should have a crack at that one Peter! (if you haven't already) DOPs gimme a headache ;)

Their rendering technique is very different than what I'm doing as it is still centers around deep shadow map tech, i.e. it makes hair look nice but doesn't attempt to solve the raytracing problem.

Transferring the inherent softness of the volume normals to the hairs is very cool. We could possibly do this in Houdini too by making volume normals and transferring the result to the guide curve points.

I would love to be able to run shaders on a volume and transfer to a surface (in this case the hair) at render time without jumping through hoops, kind off like a voxel based point cloud shader. I suppose I could convert the volume into a point cloud then run it as a pcloud shader but that is jumping through hoops.

I now basically need to generate a occlusion mask (by somehow getting the hair density volume transfered onto the hair) which I would use to shadow raytraced indirect diffuse bounces. In other words, ray leaves point on hair, all other hairs are invisible to it, ray hits scene geo to get diffuse bounce, result gets somehow multiplied/shadowed by the hair density field, done. Or maybe enabling opacity in the occlusion vop and tracing the volume like the shadows would work quick enough, hmmm...

cheers
S

#7 roughsporty

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Posted 23 September 2010 - 08:47 AM

Looks promissing

can you provide us with a test file?

Thanks

#8 Serg

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Posted 24 September 2010 - 06:58 AM

Hi, here's a heavily commented sample scene (sorry no mustache or ponytail as that was made with in-house long hair modelling tools ;))

And a picture post :)
Picture_Post.jpg
Attached File  FurShading.rar   96.38KB   465 downloads

#9 lisux

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Posted 24 September 2010 - 10:09 AM

Really nice Serg, quite intersting
Best Regards
Pablo Gimenez

#10 roughsporty

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Posted 25 September 2010 - 03:34 AM

thanks serg. a great approach and very good commented

#11 exu

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Posted 25 September 2010 - 10:21 AM

Hi Serg...
Thanks for sharing!!!
it'a really cool hack!
i just took the scene and do some test using a one env light with HDR and it's seems very promising... :D

cheers!

Cassio Homa
a duck in a swanland

 


#12 Mzigaib

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Posted 28 September 2010 - 04:54 PM

Hi Serg...
Thanks for sharing!!!
it'a really cool hack!
i just took the scene and do some test using a one env light with HDR and it's seems very promising... :D

cheers!


Very cool solution and thanks for sharing the file.

Can you please explain exactly what did you modify on the hair diffuse VOP?

Thanks.

Edited by Mzigaib, 28 September 2010 - 04:54 PM.

Michel M. Zigaib
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VFX TD




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