How to render hair efficiently

[AlexanderLeeCG]

I'm doing some tests rendering VRayMtlHair3 and it's slower than I would like for animation. I'm using Brute Force / Light Cache GI.

I read that you can render the hair in a separate pass using Photon Map GI to speed it up, but when I try that it just disappears completely. Also, if I rendered in a separate pass how would I get the shadows cast from the hair onto the head object?

I have also read that some people use Mental Ray for just the hair pass, but I don't know how that would work either with the shadows. I probably just don't know enough about compositing.

Are there any other options?

Thanks!

[bigbossfr]

What is your render settings ?

[AlexanderLeeCG]

Adaptive image sampler, min 1 max 12, threshold 0.010; DMC sampler defaults; GI - Brute Force subdivs 20, Light Cache default; two area lights in scene, both with 16 subdivs. There's still quite a bit of noise but they're just test settings.

[vlado]

I wouldn't use the photon map for anything. If your hairs are transparent, there are some options in the hair material to omit the transparency for GI and shadow rays, which helps to speed things up. Using Embree for hair also helps though I'm not sure if this went into the last official builds.

Best regards,
Vlado

[beenyweenies]

This is just my own workflow and personal experience speaking here (as in, might be totally wrong) but I think you should crank the subdivs on your lights and materials before dialing up Brute Force subdivs and dropping your threshold as low as .010.

My understanding is that the higher the subdivs on lights and materials, the less work the sampler has to do so you generally get faster, better quality renders with lower render settings. I typically set my lights and really glossy materials to around 50 subdivs, and for final renders crank the subdiv multiplier up to 3 (meaning 150 subdivs). My renders are cleaner and much faster than when I relied on threshold and Brute Force subdivs to clean things up.

I also tend to crank my Max Adaptive Subdivs value to something like 60. I'm no expert on this setting, but I do seem to get much faster renders without any visual downside.

[vlado]

That's true except that for hair, foliage or other cases of fine detail, where you need a lot of AA samples anyways. In those cases, since you are going to need a lot of image samples, it makes more sense to keep the lights/GI/material subdivs lower.

Best regards,
Vlado

[AlexanderLeeCG]

That's true except that for hair, foliage or other cases of fine detail, where you need a lot of AA samples anyways. In those cases, since you are going to need a lot of image samples, it makes more sense to keep the lights/GI/material subdivs lower.

Best regards,
Vlado

Yes, that's basically the approach I was taking. I usually have image sampler Max Subdivs at about 6, but I had to increase it quite a bit to get the hair detail.

Embree did speed up the render a bit, but Opaque GI / shadows didn't because I was using tapering, not transparency. When I turned off tapering and used transparency with Opaque GI / shadows, I got a comparable result in slightly less time. Still possibly longer than I can afford though.

[vlado]

Well, the other option is not to use GI and instead use the "Light multiplier" in the Advanced section of the hair material to brighten up the hair.

Best regards,
Vlado

[AlexanderLeeCG]

Well, the other option is not to use GI and instead use the "Light multiplier" in the Advanced section of the hair material to brighten up the hair.

Best regards,
Vlado

I think that could be really helpful if I turn off GI for the hair only, and then composite it with the rest of the scene rendered with standard settings. Thanks!

I know it's supposed to be possible to composite a separate hair pass cleanly, but anyone know where I can find more information about how to do this?

[vlado]

I think that could be really helpful if I turn off GI for the hair only, and then composite it with the rest of the scene rendered with standard settings. Thanks!

Actually, it should be possible to add V-Ray object properties to the hair node and turn off the "Receive GI" and "Generate GI" options and keep everything in one scene.

For the compositing, there nothing different compared to how you would normally do it - i.e. in one pass you would have the character with the hair set to invisible to camera; you might still need to turn off generate/receive GI for the hair. And in another pass you will have the hair with the character set as matte object. Then you layer the hair over the character in post and that should be all.

Best regards,
Vlado

[AlexanderLeeCG]

Actually, it should be possible to add V-Ray object properties to the hair node and turn off the "Receive GI" and "Generate GI" options and keep everything in one scene.

For the compositing, there nothing different compared to how you would normally do it - i.e. in one pass you would have the character with the hair set to invisible to camera; you might still need to turn off generate/receive GI for the hair. And in another pass you will have the hair with the character set as matte object. Then you layer the hair over the character in post and that should be all.

Best regards,
Vlado

Thanks again! that helps a lot.