Sub-Surface Scattering

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MetinSeven
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Re: Sub-Surface Scattering

Post by MetinSeven » Tue Nov 03, 2020 7:03 pm

Sharlybg wrote:
Tue Nov 03, 2020 6:13 pm
Can you just make a simpe scene of your Cycles SSS applied to luxball so that we can just convert it to Lux for you ? :idea:
There's no special node setup. Here's how the SSS material looks in Cycles:
Cycles SSS.png
Thanks in advance if you want to give it a try.
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B.Y.O.B.
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Re: Sub-Surface Scattering

Post by B.Y.O.B. » Tue Nov 03, 2020 7:11 pm

I have created a simple testscene.

In Cycles, the SSS is done with a principled (aka disney) shader.
  • Subsurface is set to 1.
  • Subsurface Radius is (1, 1, 1).
  • Subsurface color is red.
Note: In Cycles, the "Subsurface" slider is a multiplier for the subsurface radius. The radius itself is effectively RGB, meaning if you set it to (1, 0.5, 0.5) for example, red light will be scattered further than green and blue. See also https://docs.blender.org/manual/en/late ... ipled.html
Cycles
Cycles
I am not totally sure how to translate Cycles subsurface radius to LuxCore terms, so I just eyeballed the following and it seems to match:
  • Material null (so I can concentrate on the volume only first).
  • Homogeneous volume.
  • Multiscattering enabled (can't match Cycles otherwise, the volume stays too dark).
  • Absorption color set to the same red as Cycles.
  • Absorption depth is the max. thickness of the cube, 0.8 m.
  • Scattering color white.
  • Scattering scale 10.06.
Lux, only volume (null surface)
Lux, only volume (null surface)
Then, I tried to add the surface material. I think the most fitting thing would be glossytranslucent here, but even with diffuse color = black and transmission color = full white, it darkens the result considerably.
Lux with glossy translucent surface
Lux with glossy translucent surface
Note also that the path depth is important. Below a depth of 14, you can see visible darkening in the volume. I set the depth to 16 for all ray types in LuxCore.
Attachments
sss_test2.blend
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Re: Sub-Surface Scattering

Post by MetinSeven » Tue Nov 03, 2020 7:51 pm

Thanks a lot, B.Y.O.B., much appreciated!

I will have a thorough look at your scene tomorrow morning.

Yeah, the SSS Radius is an RGB value. I often plug the color in that slot as well, to emphasize the SSS color, as the RGB node has a three-fold vector output corresponding to the SSS Radius RGB channels.
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Re: Sub-Surface Scattering

Post by Sharlybg » Tue Nov 03, 2020 8:43 pm

Ok i've done my test here is what i have :

LUX
SSS_LUX.jpg
Cycles
SSS_CYCLES.jpg
Blend
SSS.blend
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Re: Sub-Surface Scattering

Post by Dez! » Wed Nov 04, 2020 2:35 am

Everything is correct. Great example, Sharlybg!
Luxcore has absolutely amazing volumes.
Need to retrain with Cycles.

Oh!
And how long ago was the ability to use volume nodes in material nodes added?
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Re: Sub-Surface Scattering

Post by MetinSeven » Wed Nov 04, 2020 10:34 am

Hi guys,

Tried the SSS example scenes, many thanks, BYOB and Sharlybg!

The scenes gave me some valuable new insights into Luxcore SSS.

Sharlybg, I'm curious why you used an Invert node in your SSS setup:
Image 1.png
Does the Scattering color absorb that color inside the volume, in stead of scattering it?

Also, a smart move to lighten the Material Output Shadow Color. That's one thing I forgot to try.

BYOB, would it maybe be a good idea to insert an energy multiplier in the volume node, to emphasize the SSS effect? And/or maybe implement an additive mode, so there will be brightening instead of darkening inside a volume?
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Re: Sub-Surface Scattering

Post by Dade » Wed Nov 04, 2020 2:34 pm

MetinSeven wrote:
Wed Nov 04, 2020 10:34 am
BYOB, would it maybe be a good idea to insert an energy multiplier in the volume node, to emphasize the SSS effect? And/or maybe implement an additive mode, so there will be brightening instead of darkening inside a volume?
If I'm understanding what you are asking, it would break the energy conservation principle (i.e. creating light from nowhere) so it is not possible.
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Re: Sub-Surface Scattering

Post by Dade » Wed Nov 04, 2020 2:37 pm

Dade wrote:
Wed Nov 04, 2020 2:34 pm
MetinSeven wrote:
Wed Nov 04, 2020 10:34 am
BYOB, would it maybe be a good idea to insert an energy multiplier in the volume node, to emphasize the SSS effect? And/or maybe implement an additive mode, so there will be brightening instead of darkening inside a volume?
If I'm understanding what you are asking, it would break the energy conservation principle (i.e. creating light from nowhere) so it is not possible.
Note: you may be able to obtain the same result by using volume emission.
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Re: Sub-Surface Scattering

Post by MetinSeven » Wed Nov 04, 2020 3:28 pm

Dade wrote:
Wed Nov 04, 2020 2:34 pm
MetinSeven wrote:
Wed Nov 04, 2020 10:34 am
BYOB, would it maybe be a good idea to insert an energy multiplier in the volume node, to emphasize the SSS effect? And/or maybe implement an additive mode, so there will be brightening instead of darkening inside a volume?
If I'm understanding what you are asking, it would break the energy conservation principle (i.e. creating light from nowhere) so it is not possible.
I was thinking: how come in real life certain materials have a certain light, almost glowing SSS, as if the light is perfectly scattered inside the volume without the surface or volume significantly decreasing the light strength. As BYOB mentioned, using a Glossy Translucent material darkens the SSS. Isn't there a scientifically viable way to retain light strength inside a volume in a more effective, but still physically correct way, like some real-life plastics seem to almost emit light due to SSS?

I'll try volume emission, thanks for the tip.
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Re: Sub-Surface Scattering

Post by MetinSeven » Wed Nov 04, 2020 3:51 pm

One more thing: I'd love to see a color input slot in the Material Shadow Color:
Image 1.png
Thanks.
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