good news (for me)

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:

http://docs.chaosgroup.com/display/VRAY3/VRayScatterVolume

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.

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2 thoughts on “good news (for me)

  1. That sounds really complicated. :/

    I still haven’t sat down to really play with this render engine. Heck, I’m still struggling with the whole SSS thing. Half the time, those skin texture materials result in some really awful renders. I suspect some of it is lighting related, but until I figure out how to correct it, I find myself falling back on simple surfaces and lights.

    I was looking at Baby Luna’s default materials yesterday, and trying to figure out why the diffuse texture map was plugged into the ambient channel.

    1. I have put all my knowledge of SSS into a lengthy ‘tutorial’ (more like treatise) which you can find in my Share CG gallery: http://www.sharecg.com/Mustakettu
      There are also light kits that I used for the images in the tutorial. So maybe you could take them apart and find the missing ingredient.

      I’m detailing SSS as found in omnifreaker’s UberSurface 1 and 2 shaders there. I’m not a big fan of AoA’s Subsurface shader mixer network that DAZ were pushing before Iray; I totally adore most other AoA’s products and freebies, but not that shader. Tofusan has one on ShareCG that is built around the same SSS shader mixer brick, but totally pwns AoA’s one re:usability and balance.
      http://www.sharecg.com/v/76970/gallery/21/DAZ-Studio/Subsurface-Skin-Shader

      As much as I respect the Hivewire (and DAZ) folks for their modeling/texturing/rigging skills, the way they set up 3Delight shaders is not optimal, to say the least.

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