Shaders: Improved Physical transmission model

[WestLangley]

We are modeling optical transparency, which only applies to dielectrics.

[WestLangley]

@donmccurdy Side note: We are currently modeling optical transparency. In that regard, I think the property would more-appropriately be named opticalTransparency.

Are there expected future enhancements for which the current nomenclature seems reasonable?

[donmccurdy]

Thanks for this change!

I'm hoping we can build toward the model defined by KHR_materials_transmission. Naming doesn't necessarily have to go along with that, but the term "transmission" seems to be gaining traction elsewhere:

• Blender Principled BSDF
• Autodesk Standard Surface

Aside from that, I'm hesitant to reuse the term "transparency" unless we want to rename .transparent and .opacity to .alphaMode and .alpha, or similar. Which we could do, but alone it doesn't feel worth the churn... maybe with NodeMaterial? I'm not excited about trying to communicate the non-relationship between .transparent and .opticalTransparency.

tl;dr "transmission" feels best to me, more for consistency with artist DCC tools than any other reason.

[donmccurdy]

In terms of future enhancements, also note the relationship between transmission and (still very much WIP) KHR_materials_volume.

[mrdoob]

Thanks!