Wrong colors with trans

[lukas_fechtig]

Hi there,

we are experiencing significant issues with colors with Subsurface Scattering shaders. E.g. for a refractive apple or orange juice, the material turning blue or green, even when all input colors are only white or yellow. We could reproduce this error all in Vray SSS Shaders: VrayFastSSS, VrayFastSSS2 and ALVRaySurfaceMtl. Here an example with SSSFast2:





Any idea why this happening and how to avoid it? Is this a physical property that we do not understand or a bug? We are using an automated process for material creation, so "tweak until it is gone" is not an option - we need a way to ensure this does not happen.
If we understood correctly how to use the SSS inputs, it should be sufficient to set diffuse and specular to 0 and have the SSS and scatter color to the same color - right?

Thanks in advance!

[hermit.crab]

I presume your VRayColor's RGB Primitives are set to ACEScg since RGB seems to work fine. Not sure why this is happening, though. I will post again with my findings.

[^Lele^]

try leaving the scatter color to a mid gray, and see if that helps.

[lukas_fechtig]

Thanks for the help so far, but we still have the issue. I attached a minimal max file here (https://www.file-upload.net/download...rsSSS.max.html) as well as a screenshot below. Primaries are all sRGB, scatter color is a mid-gray, no over saturated colors are used. It seems like a purposeful calculation (if I change the color from yellow to blue, the effect is reversed) that leaves the side that is facing away from the light in the inverted color, but this is not how e.g. a green tinted glass or an orange juice would behave. Its harder to get the effect when using the "scatter coefficient + fog color" mode, but it seems so be impossible to create some materials that way.

[^Lele^]

It looks correct to me.
Lowering the scatter radius will make the tint inversion go away.

What material are you trying to achieve?
Have you tried the new SSS and volumetric modes?
They will work better for long SSS paths than the other three shaders.