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how to render grass properly?

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how to render grass properly?

Unread postby kirkr22 » Sat Apr 13, 2019 6:11 pm

I have either too noisy grass or with increased AA beyond 6 spp an insane render time or with Nvidia denoiser too smeared result.
Based on AOVs most noisy one is diffuse inderect illumination which came from environment light HDR with a super bright pixel casting shadows, no direct light.
My grass blades are flat things with standard mat set to be double sided and backlight diffuse color on. Nothing to reflective or translucent except this backlight thing.

I tired to tweak light and material samples but looks like only AA is what's really matter with so much thin geometry stripes (grass blades) so I wonder if some magic trick could exist to do grass better.
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Re: how to render grass properly?

Unread postby dboude » Mon Apr 15, 2019 12:49 pm

Hi,

6 spp for anti-aliasing is very low. To test if the noise comes from the geometry you should set the integrator model of the raytracer to utility and the output to simple shading and render a couple of frames. If flicking appears you have to increase AA samples.

integrator.png
integrator.png (24.49 KiB) Viewed 528 times


A trick would be to use texture and clip maps with mip mapping to simulate grass clumps on planes. But you will lose some details. You can use render to picture for that as well.

Cheers ;)
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Re: how to render grass properly?

Unread postby kirkr22 » Mon Apr 15, 2019 3:54 pm

Thanks dboude, It's not that bad with those test settings you suggested. Looks like most of the noise is coming from indirect sky illumination . "pbr_diffuse_reflection_i_indirect" AOV is super noisy in the grass. What kind of sampling should I increase to fix that ?

Should I rather keep direct light as a sun and some trick like ambient for indirect? That way it seems looks less real although
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Re: how to render grass properly?

Unread postby dboude » Mon Apr 15, 2019 4:35 pm

Indirect channels are material sampling. And yes you'll have less noise (in direct channels) if you use a gradient or constant color in an env' light + distant.

Cheers ;)
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Re: how to render grass properly?

Unread postby kirkr22 » Mon Apr 15, 2019 5:49 pm

It's looking a little bit too simple that way. Could we force nvidia denoiser to work more on indirect part of image somehow?
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Re: how to render grass properly?

Unread postby vandam » Mon Apr 15, 2019 6:07 pm

Do not be afraid of raising material samples to some higher values, this is not vray ;)
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Re: how to render grass properly?

Unread postby kirkr22 » Mon Apr 15, 2019 6:54 pm

Thanks, I'll try. The render time will be forever I am afraid. Somehow I managed to render similar things pretty quickly in old mental ray years ago and now with everything went PBR I could only do it in GPU renders
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Re: how to render grass properly?

Unread postby sam » Mon Apr 15, 2019 7:04 pm

Which model of CPU do you have? Have you considered moving to a 32 core Threadripper?
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Re: how to render grass properly?

Unread postby kirkr22 » Mon Apr 15, 2019 9:47 pm

I have pretty old 3770k. Threadripper would be nice indeed. Together with taxes It's still too expensive although and worth of several super cool video cards.
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Re: how to render grass properly?

Unread postby sam » Mon Apr 15, 2019 9:57 pm

Ok I understand, well it's far to be the best CPU for Clarisse for sure.
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