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According to documentation Glossy Weight should be left alone, except for conversion of legacy materials.
Also, I'm not really sure about accuracy but using pure white as glossy color will increase the render time and iterations dramatically to achieve full convergence.
Uh, no. It means, that the Transmitted Color can't be determined by any kind of texture map, regardless of the format. The program code of the Iray renderer doesn't include methods to get color information out of a texture map. It needs plain values in linear space (the range from 0.0 to 1.0) for each of the three RGB color channels to know what it has to do.
Actually, the Roughness/Glossiness parameters would be the most appropriate thing to control the amount of glossiness. Regarding Iray Uber, one needs to know that the value for roughness/glossiness are internally squared (a method Disney implemented into their "principled" BRDF to get "a more perceptually linear change in the roughness"; like the Iray Material Plugin for 3ds Max, DAZ's Iray Uber has been build using some of the Disney methods), thus making a value of 0.5 actually only a 0.25.
So if you want to put in a known measured value for roughness into the Iray Uber, you need to calculate the square root to get the correct value. F.e., a NVIDIA example for skin rendering recommended a value of 0.3 for roughness. The square root of 0.3 is 0.547723, so that's the value you had to put into to receive an actual roughness of 0.3.
Using the "PBR Specular/Glossiness Mix", since DS 4.9.2.54 the Glossiness value will also get internally squared (wasn't like that in the earlier builds). So you'll also have to use the square root to get a correct known value. Glossiness is the inverse of roughness, so the glossiness value for the NVIDIA example would be 1.0 - roughness (1.0 - 0.547723 = 0.452277). That's because Glossiness will be, again internally, reverted to Roughness for the render process. MDL (NVIDIA Material Definition Language) does only know roughness, glossiness is a thing which isn't defined.
At this point two objects' reflections, one using the PBR Metallicity/Roughness Mix and the other the PBR Specular/Glossiness Mix, will look identical.
The Layered Weight parameters determine the strength (or level of influence) of a layer. You could use those to adjust your reflections, but a more accurate method would be to adjust your roughness/glosiness instead.
That sucks. I just tried it out and you're quite correct.
OK. Got the render time down to about 7 1/2 minutes. YAY! Still have this weird black bit behind his right trap and some weird triangle bit happening on top of his head.
So, for most things, glossy weight should be 1, glossy color should be pure white, and everything comes from roughness.
The triangle on top of the head looks something like an UV issue. Maybe the texture seams aren't aligned correct. I'd try and do some renders with Genesis 3 in that same scene, to make sure if that problem is only related to this figure.
The triangles (at least the one on the top of the head) are technically referred to as shadow terminator artifacts. They are endemic to Iray and similar renderers. The easiest recourse is to change the direction of the lighting; but if you'd like to avoid that, you can up the resolution of the base model.
Do a Google search or the term. There are some Iray-generic blog posts about it (which includes workarounds):
http://blog.irayrender.com/post/29042276644/shadow-acne-and-the-shadow-terminator
and you should find plenty of posts here about it. It comes up fairly regularly.
Not "most things", on "everything, that isn't an insulator" (metal). Only insulators are able to change the color of their reflections.
1. Yes. 2. Yes. 3. Almost...
Reflectivity (amount of light reflected) and Roughness (degree of diffusion of the reflected light due to irregularities on the reflecting surface).
There is a nice interactive example out there about the impact of Refractive Index and roughness on the reflections from a surface (scroll halfway down to the "WebGL Demo"). In our case, Glossy Reflectivity will play the part of the "Refractive Index, n".
I had been using various shades of gray to control how much glossy was showing.
I think the maps in the transmitted colors are supposed to be used to show subdermal veins, capillaries and fat clusters but it doesn't seem to work or the maps aren't quite right.
COOL. I have a Tumblr blog so I am now following this guys blog.
So seems the work around for getting rid of the Shadow Terminator issue is to drop using Bump maps and go with either Normals or Displacement maps (from what I'm reading)....
Thanks for this and related threads. While I'm often lost, it's helped get me on track to making pretty good skins.
On the subject of glossy colours, the bog standard iray water shader is mid-grey. Surely that's for a reason and might apply to skin as well?
On a different note, if you have the means to convert/reproject/bake UVs and are looking for high quality displacement and normal maps, I can't recommend these highly enough:
http://www.daz3d.com/macro-skin-for-genesis-and-genesis-2-female-s (8k normal maps - you can basically bin your glossy maps)
http://www.daz3d.com/displacement-for-victoria-6
The Glossy Color on the water and glass shaders are all plain white (like they should be, since they're dielectrics), what's mid-grey is the Base Color (aka diffuse color, or in PBR terms, the Albedo). Not completely sure, but I guess the mid-grey would be the result of a real-world measurement.
For skin we're using in most cases a diffuse texture map (which already includes the final diffuse color), so the Base Color in that case should be also white, except when you want that map to appear a bit darker.
My bad, I was thinking of the glossy reflectivity being at 50%.
I've found that using glossy weight 1, glossy color white, it often seems a good idea to adjust glossy reflectivity down. Is that appropriate?
This makes my head hurt. lol Why do they have to make it so complicated? They really need a more user friendly way of adjusting the settings.
That would be like changing a material's Refraction Index. Can you do that in the real world? (without applying a different material on top). Well, simple answer: you can do whatever you want or suits your needs, but the more you fiddle and change things "hardcoded" by laws of physics, the more you diverge from Physically Based Shading. If a material is too glossy for my taste, I usually add some more roughness. That's just the way real-world does.
That's why the gods gave us Vicodin.
Because they could? The real reasons know only the folks at Disney.
Once you get a grip on it, it's not really that complicated anymore, I found it harder to convert Poser materials to DAZ Studio.
A solution to avoid the value-conversion necessarity would be to switch to "Show Hidden Properties" on the Surfaces Tab's options menu, locate the "Settings" group there, and set the Roughness Squared and Glossiness Squared parameters from "On" to "Off". That way you can put your measured values into without the need of previously converting them. But you loose your "more perceptually linear change in the roughness" that way.
What I like the most about Iray (besides the nice renders it helps you create) is the ability to use real-world BRDF/BSSRDF measurements to make up your materials. The more reliable data you have, the less you'd have to "guess around".
Well, if you meant Glossy Reflectivity you were right to assume that it's at 0.5. But Glossy Reflectivity and Glossy Color are a different pair of shoes.
Glossy Reflectivity determines the amount of light which is reflected from a given material's surface due to its Refraction Index and the angle of incident.
Glossy Color is the color of the reflection, when light is reflected from a given material's surface. Dielectrics (anything not a metal) aren't able to change that color, so the color of the reflection will be of the same color the light has. Red for a red light, yellow for a yellow one, blue for a blue one, and so on.
So in our shader setup it has to be neutral (white) to let the reflection have the color the light has. If you change Glossy Color to mid-grey or something fancy, like a 222-243-255 light blue, 254-255-225 light yellow, or anything similar ridiculous, your white reflection color from a white light will be tinted grey, light blue, light yellow... or anything similar ridiculous.
And then you'll sit for hours on one and the same render, tweaking lighting and render settings, wondering why your figures' skin doesn't look right whatever you do... and begin to eat your keyboard.
I think one of things they could improve on the flow of the surface options is to move the Transmitted Color up under the area that makes more sense since the info that Arnold has imparted to us clearly shows that both are relevent to. Perhaps I'm wrong but don't understand why it's way way down on the list....
It would also be great if someone would make a shader preset that had all the correct values for skin, for nails, for the various eye components and inner mouth and teeth. I'd buy a set like that and not have it geared for any specific figure, it's off putting when it's for say Genesis 3. Not all of us use these figures.
Ok, back to nails for a moment. I still can't get finger and toe nails right. Unvarnished, mostly guys... pointers?
I really had to think about those surfuces you speak of, but the only surfaces that came to mind were... metals.
That could not only depend on your roughness settings alone. Translucency color and - strength as well as the SSS Reflectance Tint has an impact on the overall result. Not to forget the volume distribution function parameters. And/or the use of a not-so-optimal specular map...
For skin I always use Top Coat, for there are two different materials, with different optical properties, directly visible: the outermost stratum corneum tissue layer, and the sebum layer.
The Transmitted Color is a parameter belonging to the Volume Distribution Functions of MDL. So IMO it is located at the best place on the user interface as part of the "Volume" group along it "cousins". And you only need those when Thin Walled is turned to "Off".
Me personally, I really don't like much the way shaders are used in DS: having to select all the appropriate surfaces, apply the preset, select another set of surfaces, apply another preset... and so on.
But one could develop a set of shader presets and make a one-click material preset from those for each of the DAZ figures. Ideally, such a "General Skin Preset" should be added to the "Starter Essentials" package for each figure (in case of M4/V4 to the "Studio CF" package. One really don't needs it if he/she just only uses Poser). For G2F/G2M we already have one (even if it would need an intense overhaul).
But we would still have a bit of a problem with the specular maps from the older figure lines, which unfortunately come in many different dark and lighter greyscales. The ones for Victoria 6 start to look good at a Glossy Weight of 3.25, the ones for the Genesis 2 Base Female needs even 20.0 to show a convincing effect. Thanks to the bad habit of the "good ole" 3Delight times when everyone tended to cook his/her own soup instead of developing and agreeing to a general method of how things had to be done and stick to that.
(That are those who now put fancy, ridiculous colors into their Glossy Color parameter...
).
What?!? You don't use varnish on your nails?!? How outdated...
Jus' kidding! 
Let's see... human nails have an IOR between 1.54 and 1.56, so I'd try a Glossy Reflectivity value of 0.564976, 0.581507 or 0.598145. They also have a similar glossy appearance as skin (at least my have, unvarnished...
) so I'd try a Glossy Roughness value of 0.3 (with Roughness Squared and Glossiness Squared parameters "Off") or 0.547723. If they're still too glossy for your taste, increase roughness a few notches. Translucency Weight at around 0.4. No Top Coat, since an unvarnished nail wouldn't have one. If you use a conversion-to-Iray from an older line of figures, increase Base Bump to a value 4 to 5 times higher than that you get after converting (f.e.: if you have a 0.20 after converting, make it a 0.8 or 1.0). If you have a normal map, scratch the bump map, the normal is the better one.
Not skin but in some experiments I came across a great visual for the whole "don't use full saturation" thing. Super noisy, because its a basic test render, but It gets its point across
Its really evident when its refraction, but, yeah weird yellow bleeding is not pretty.
Don't use fully saturated colors folks!
Good tip! Thanks j cade.
That is a must for every render engine.
When you compare standard diffuse textures with proper albedo PBR textures, there's usually quite a big difference in saturation and contrast range. Using the vibrance adjustment in Photoshop by as much as -50% or even more can help a lot.
Even at the highest reflectivity option, I am still getting very flat, almost no gloss.
I think I need to look at something else, like the glossy weight or the maps.
What's a good estimation for glossy weight? I don't think the "specular maps" have improved all that much over the older ones, they may be a little lighter, but often they are not.
Hmmm, well OK about the Transmitted ....
Hmmm, I think you missed that little part I mentioned... I don't use the DAZ figures (rarely). I develop content for the Hivewire figures so would need to be a universal shader. Notice I didn't say Material preset. A shader COULD be universal. Just hold down Ctrl, tell it to ignore maps and you have a better (hopefully) look to your overall set up. For a content creator a few extra clicks to get things in place is better than hours and hours of pulling out my hair waiting on test renders.