KHR_materials_volume: missing extension support

[bhouston]

khronos-DragonAttenuation (KHR_materials_volume)

Reference image from Daussaul STELLAR:

Current V-Ray results:

[bhouston]

More examples of the differences when KHR_material_volume is not supported...

Stellar reference images of khronos-MosquitoInAmber (KHR_materials_ior, KHR_materials_volume):

Current V-Ray result - definitely lighter:

Stellar Reference image of khronos-TransmissionRoughnessTest (KHR_materials_ior, KHR_materials_volume):

Current V-Ray result - definitely lighter:

[bhouston]

This maps onto the V-Ray MTL volumetric properties: https://docs.chaos.com/display/VMAX/VRayMtl#heading-Volumetric

[jasondavies]

I'm going to work on this.

[jasondavies]

It would be helpful to know which properties these map to:

• thicknessFactor: The thickness of the volume beneath the surface. The value is given in the coordinate space of the mesh. If the value is 0 the material is thin-walled. Otherwise the material is a volume boundary. The doubleSided property has no effect on volume boundaries. Range is [0, +inf). Default=0.
• thicknessTexture: A texture that defines the thickness, stored in the G channel. This will be multiplied by thicknessFactor. Range is [0, 1]. Optional.
• attenuationDistance: Density of the medium given as the average distance that light travels in the medium before interacting with a particle. The value is given in world space. Range is (0, +inf). Default=+inf.
• attenuationColor: The color that white light turns into due to absorption when reaching the attenuation distance. Default=[1,1,1].

I'm looking at the various translucency properties on BRDFVrayMtl. I can set translucency=4 (Volumetric translucency). Then there's translucency_color and translucency_thickness, but not sure what to do with attenuation distance.

[bhouston]

thicknessFactor and thicknessTexture are not used in a path tracer because the path tracer can figure out the front and back surfaces of the material, thus you can ignore those unless they are 0, in which case we would switch to a thin walled material.

We could skip the thin walled material for now if you wanted and revisit it as a separate issue.

The attenuationDistance I suspect aligns with "depth" but we may have to convert units. The attention Color I believe aligns with "fog color."

I see this in section here: https://docs.chaos.com/display/VMAX/VRayMtl#heading-Volumetric

If the color is wrong, it may just need to be inverted, e.g. (1,1,1) - color. It should be pretty clear if this is the case because instead of orange (1, 0.5, 0 ), you'll get cyan (0, 0.5, 1 ). This is the difference between "absorption" versus "attenuation" colors I believe.

[bhouston]

I see this in the V-Ray docs for thin walled:

I think for volumetric fog (attentionDistance and attentionColor), these settings may work?

And I think you have to set the refraction ior based on the overall KHR_material_ior:

[bhouston]

And the ior for the material should default to 1.5 if KHR_materials_ior isn't specified according to the specification: https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_ior/README.md#extending-materials

[jasondavies]

[jasondavies]