Hi guys,
have an issue with Corona’s SSS behavior with multi/sub-object material and IDs (Corona 1.7 hotfix 2, 3ds max 2017)
Creating a sphere, dividing its polygons to ID 1 and ID 2.
Then applying multi/sub-object material to my sphere and assigning two similar materials, one for each ID.
When there are 2 CoronaMTLs with SSS off there is no problem, but when im enabling SSS option the seam appears
at the border of two different IDs. The same thing when i’m applying two similar CoronaSkinMtl to different ID slots.
But there is no seam when i put the only one material for both IDs. See the screnshots.
Also the thickness of the visible seam depends on the radius scale of SSS.
I’ve seen this problem before in Vray - many years ago.
In Arnold the problem is solved by “tagging multiple objects as belonging to the same SSS ‘set’ so that illumination will blur across object boundaries”
See here:
And see the last pict on the left “Without ‘shareSSS’”.
To solve this bug, we need this shareSSS feature in Corona!!!
I think once again this is the case of what should be considered “surface” and what should be considered “volume”, and to which of the objects each surface and volume should “belong”.
I imagine that having one object split into several material IDs is internally the same as having two separate objects (with the same or different mat IDs, this does not matter). In that case you get exactly the same result. (see screenshot)
So to me the way it works now is perfectly understandable, and I would say it is expected, however I can imagine some typical situations where this is unwanted - e.g. texturing a character.
That said, it is up to the dev team to decide whether this is something possible to improve, and if the required amount of work and time to improve this would be justified.
Maybe you could check with Vlado since you guys are on the same teams now. I know they had the same problem and found a solution for it several years back.
Respectfully, I would add that not solving this issue would be an absolute killer for character animation.
Just for information, here is a thread where Chaos is tracking a similar problem in Maya using alSurfaceShader.
Looks like they are solving it with surfaceID.
The solution that I’ve used for this problem is by masking with vertex colors. You can still have a polygonal selection, fill it with pure white, invert the selection and fill that with black. Then I used a vertex color map as a mask in the corona layered material. The result has no visible seams.
I see you are using standard Max bitmaps to try and use UDIM tiles with a Multimap. But can use CoronaBitmaps with a CoronaMultiMap if you set your face ID’s on your object the same integer as on the CoronaMultiMap (easy to do in an Unwrap modifier). That way you don’t have to use standard Max bitmaps loaders anymore.
