I found out you can read MDL sources of the Iray materials supplied with DAZ Studio (there should be a huge cheer emanating from the audience now…). Now I finally understand what all that stuff in the “Iray Uber” base means.
“Translucency” dial does kick in diffuse transmission which is akin to glossy refraction (but only akin; it’s more like like single scattering). Then, supposing I get it right, the “SSS” dials will invoke actual multiple scattering, and “transmitted colour” is volume absorption colour.
The most complicated thing, to me, is finetuning those transmit/scatter distances to make sense because this is so different from the Jensen-based model and even from the albedo/DMFP one. Actually, Jensen’s (scatter/absorption distances for three major wavelengths; we’ve all seen it in, say, AoA’s SSS network) may be the least intuitive to use, but it makes the most sense from the physically based POV (as you would mostly only adjust multiplier colours, while leaving the scatter/absorption values more or less unchanged from the measurement-derived figures). In theory, the Iray volume parameters are similar, but not exactly. I know the scale conversion parameter from DS’s centimeter-based world to 3Delight’s millimeter-based one. But when it comes to Iray… I know nothing. Hence difficulties.
Now, I will also need to look closer at the scattering isotropy. DAZ skin presets go with negative values – which is kinda weird to me because I have long memorised that skin has strong _forward_ scattering. It’s referenced for example here:
But maybe these directions are reversed in Iray or this particular material at least? Maybe a negative number gives forward scattering?
…all this for a renderer I won’t be using much, but hey, I need to make sure I know what I won’t be using precisely. It’s a matter of damn honour.