physical sss buzzing

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hey

i'm busy setting up a leaf shader using the physical sss vop
now i'm getting the results i want using the Global Point Cloud, but when i render a sequence it's buzzing from frame to frame.

i've baked out one frame, set to PCmode to Read from file.
my auto size Factor is set to 16 and filter samples to 80
the best part of it is, i'm testing it on static geometry(460 leaves) and using a static camera. so essentially i'm rendering the same frame over and over, surly this should not buzz..?
it even buzzes when i set it to Cache positions/Normals/Areas, which i can't use since my camera is going to be moving, just did it as a test.

any tips on getting a solid render?

thx
jason
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Just checking, but you're not recreated the pc every frame are you? You can just write out one, and reference it…shouldn't buzz. The object can animate, just if you deform, it does indeed need a pc per frame.

tbh, I haven't gotten into deforming geo yet with SSS, it's on my list though. Supposedly with 11 it's much cleaner with potential buzzing issues with deforming data.

Cheers,

J.C.
John Coldrick
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nope just using the single *.pc file
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i am rendering it on the farm btw
all the OS's are the same(linux)

j
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I'd post a file. How thin a leaf are we talking here? Strikes me that a very thin one will take some special loving to get it to work nicely. Also, to ensure the farm isn't related, you can repro locally with a couple frames?

Cheers,

J.C.
John Coldrick
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yeah i'm rendering some frames locally now to test
the leaf is quite thin, so maybe this is the problem, i'll set-up a simpler scene to test and post

jason
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looks like it's most pretty much a point cloud auto size Factor issue, i bumped it up to 128 and it seems fine, but now it's just to slow for trees

at the moment i'm looking at changing my shader to use Physically based phase(henyeygreenstein), not as sexss as i want it, but the trees are more background in the shot

thx
jason
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Hi Jason,

Any chance you can post a simplified hip file? There should be absolutely no buzzing related to multi scattering for a static scene *if* each rendered frame is using the same point cloud. Likely I can help out with this if you can get me a hip. Thanks!
Ian Kerr
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hey Ian

sorry for the deeelay, busy busy busy

here's the simplified hip file
thx
jason

Attachments:
tree_dev_for_sesi_v01.hip (499.2 KB)

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Hi Jason,

The problem in this case is that the irradiance calculation (per-point) is not stable from frame to frame. The complex geometry of the leaves requires a higher number of samples to properly resolve shadows. You can fix this by increasing the sampling quality on your lights, but likely the best approach is to increase the “Global Light Quality” setting on the Physical SSS Vop. This may also mean that you can reduce your point cloud size. You might also try using the “Local and Global” multi-scattering model as this typically gives the best image vs. performance cost trade-off.

For future reference, I'll mention how I determined the cause of the problem, as it may help others debug similar problems. I first tried enabling caching of positions/normals/areas to test whether the point positions were changing from frame to frame (this would be bad). The problem did not go away, so I then tried enabling caching of irradiance, and this worked, since the irradiance was now constant from frame to frame. This led to the idea of increasing the global light quality to stabilize the irradiance calculation.

Hope this helps! Please let me know if you run into any other issues.

Cheers,
–Ian
Ian Kerr
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hey

thx for te debugging tips!!

i thought with PBR you shouldn't need to adjust your light's sampling quality?
i'll try bump up my SSS global light quality, i'm curious to see if i can actually use SSS for a couple thousand leaves!

thx again
jason
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Hi Jason,

Yes, light sampling quality is still very much important with PBR. Depending on your scene, some lights will require more samples than others to achieve low noise levels. Finding sampling qualities that are neither too high nor too low goes a long way towards establishing a good balance between image quality and render speed. Good luck!

–Ian
Ian Kerr
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this physical sss looks interesting. has anyone done a lesson or tutorial with it?

setup, differences, tips and tricks?

from the solution above, i gather the problem was not at all with the shaders, rather with the mantra settings?
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