SSS object intersection discoloration problem

[AWOLism]

I'm playing around rendering a character I'm sculpting, but I'm having an issue with the SSS skin material when intersected by other meshes.

To summarize:
- One head mesh with randomwalk SSS skin material with a Randomwalk Medium (diffuse transmission).
- Some hair meshes with just a regular material

Where the hair objects intersect the head mesh I get this greenish dark color, which probably makes some sense technically but looks quite bad.

My thought was that I might be able to get around this by using the Ray Switch node controlling the Opacity of the hair mesh material, but no combination of this gets me to the desired result.
The only way to get rid of the dark color tint around the hair mesh is by turning off both Diffuse and Shadow rays, but that of course removes all shadows from the hair meshes overall, and I don't want that.

While I could of course remove the intersecting parts of the hair meshes, ideally I would like to avoid that as it is extra work.

Is there any other way to solve this? Any help would be greatly appreciated.

Attaching screenshots and a simplified version of the scene if anyone wants to check it out.

Thanks in advance!
/ Andreas

[bepeg4d]

Hi,

If you multiply the Transmission color with a little bit of Dirt in the Skin material, the issue is solved:

SSS_intersection_problem_b.zip

(1.04 MiB) Downloaded 22 times

ciao,
Beppe

[AWOLism]

Thank you for the reply Bepe, I appreciate the effort, but I'm afraid I must not have been clear enough.

What I would like is basically the opposite of what you have achieved. I want no dark "outline" at all where the meshes intersect.
As stated, the "proper" way to do this would be to actually remove the parts of the hair meshes that intersect, but that can be a lot of work, so if I could manipulate it on a material level that would be optimal.
Kind of like how you can work with Nested Dialectrics on specular objects. I really thought the Ray Switch would be able to do this, but I can't figure it out.

Is there really no way to do this?

Cheers
Andreas

[AWOLism]

As I'm messing around with this further, I'm finding that Ray Epsilon has a massive effect on the overall look of the SSS, which is kind of messy.

Another discovery was that actually removing the intersections between the models did not help, I still get dark green halos around the hair objects, this is even weirder to me.

When changing the Transmission Type from Diffuse (which seemed like the reasonable choice for a waxy skin material) to Specular (with Fake Shadows on) this actually seems to remove most of the greenish halo, but it of course completely changes the look of the SSS, so that needs retuning, and I imagine this is even more time consuming to render?

[bepeg4d]

Ok, have a look at this different solution.
If you also add a Random Walk node to the Hair object with default settings and Transmission to 1, the dark greenish coloring goes away:

SSS_intersection_problem_b2.zip

(1.06 MiB) Downloaded 19 times

About Ray Epsilon issue with Random Walk, it is a known issue and should be fixed in one of the next core update.

ciao,
Beppe

[AWOLism]

Hi Beppe!

Oh, interesting, that actually seems like a good workaround. It does introduce a little translucency to the hair object, but if you bring up the density and turn down albedo and radius of the random walk medium that should make it effectively unnoticeable.

Can I ask, is the green halo considered a bug, and will the devs look at fixing it in the future?

Good to hear about the fix for the Ray Epsilon issue.

Thank you for the help!
Cheers
Andreas

[AWOLism]

Hey, I just wanted to bump my question regarding the green discoloration halo.
Is this something that can be fixed on a core level?
It is rather cumbersome and probably adds unnecessary render time to have to add SSS to the intersecting objects, so if it can be sorted out on a program level somehow that would be great.
Actually, I haven't had time to try out the 2026 version, is this something which would be solvable with the Trace Set functionality?

[bepeg4d]

Hi,

Sorry for the delay, and thanks for the reminder!

It turned out that the issue in this case is caused by Black Albedo color, if you change it to a mid dark red color, the issue is solved:

ciao,
Beppe

[AWOLism]

Hi Beppe!

Thanks for checking.
I have to test this more thoroughly further as soon as I have some time in the next couple of days, but my immediate thought is that altering the albedo of the skin will also change the overall look, which is not ideal.

Did a super fast test, and now it seems that controlling the opacity of the hair object with the Ray Switch node (diffuse and refraction set to 0) will actually remove the halo. Strange, as I feel that it didn't work the last time...

I do want to investigate the new options that Trace Sets provides also. Going to install 2026 soon and do some testing. I guess I will get back here if I have further questions.

Cheers
Andreas