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Inconsistent Subsurface Scattering Results

polycounter lvl 8
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Acute polycounter lvl 8

Been trying to pinpoint the reason different sections of geometry seem to be displaying subsurface scattering in different ways across a model I've been trying to render.

I want some intense SS to show up on these wing membranes, and I've had success getting it to work on other parts of the mesh. But on the wings the results just seem completely different despite using the same simple shader and settings.

As you can see here, the wing membrane the arrows are pointing at is barely showing any of the Scatter color, and is not being illuminated from behind as brightly as other parts of the models with similar thickness.

All the membranes here are 3-dimensional, rather than double sided planes, but deleting the other side of the mesh doesn't change the SS display results, so the actual thickness of the mesh doesn't seem to be playing a role.

Things tend to preview as expected with Draft Quality. It's in Full Quality (or with Ray Tracing enabled) that the SS disappears from this area of the mesh.

Anything seem weird with my Ray Tracing settings that could explain this? This is a new scene I've created to test the problem, so they should be pretty close to or at the default settings.

I've tried fiddling with scale settings but they don't seem to really resolve the issue. At smaller scales everything will get brighter overall, but the wing membrane still doesn't pick up color and illumination like the other meshes.

I'm wondering if there's something inherently messed up with my polygon faces in that area of the mesh? I've checked my vertex normals and they seem fine. Not sure what else could be the culprit. Nothing jumps out as me as looking weird in external 3D programs.

I appreciate any advice or guesses anyone has, thank you.


  • Acute
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    Acute polycounter lvl 8

    Definitely seems like it's something to do with the mesh. There's a distinct cutoff where the SS switches off on the wings when "use raytracing" is checked on. These two sections of the wing have different materials applied as they use different UV sets, but the material settings and texture types are identical.

    Draft Quality, no raytracing:

    Vs Full Quality w/raytracing:

  • iam717
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    iam717 greentooth

    have you checked all your lights (SHADOW option) or you just have one, i ran into issues and lights were usually the issue or even the HDRI at one point for me, also play with shadow offset settings, i heard fixes some oddities. trying to do the quick help attempt here for you, but hopefully they come around for you with a real solution.

    edit#1: https://us.v-cdn.net/5021068/uploads/MRZLTZCTZOXQ/image.png like in this image see the floor is replicating the issue you seem to be having which is why i mention lights or shadows. whats it called the shadow option like fog, turntable, that stuff, sry atm not on a pc where i can look.

  • Acute
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    Acute polycounter lvl 8

    I ended up contacting Marmoset support to figure this out.

    This ended up being an issue with Marmosets current inability (as of at least Nov 2022 when we had this discussion) to pass SSS through meshes whose sides consist of multiple materials.

    These dragons have multiple udims assigned to them. For example there's one material on one side of the wing for the topside UV's, and another material on the bottom side for the bottom UV's. Marmoset can't understand that these are the same enclosed volumetric mesh when different sides are composed of different materials, and so it's not able to properly pass SSS between the two sides. It can't simulate SSS without a volume, and for it to do that the mesh needs to be composed of the same material.

    There's no fix for this currently, I'd just have to combine the textures onto one big map so that they can use a single material. That way they could then share volumetric math caclulations for SSS. It only appears to work in Draft because Draft doesn't actually use Ray Tracing.

    SSS will only work when 2 criteria are met:

    all pieces must be 1 object
    2 all pieces that need to share SSS must share 1 material

    From Marmoset:
    "What you can see there is that SSS transmission will work for 1 pieces of meshes that share the material. This means that no calculations will occur between combined materials, as in the case of Material 1 and Material 2 of my picture > expressed by the X.

    This is a limitation we currently have. The only way around it is to reassign materials, so the wings have just 1 mat."

    Example of one mesh with 1 material applied:

    Figured I'd share this all in case it's useful for anyone else, as I don't think this info is on Marmoset's website.

  • EarthQuake

    @acute we released Toolbag 4.05 a couple of months ago. It's now possible for SSS to interact between multiple regions of a mesh with different material assignments.

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