Hi all,
We got bit by the migration’s bug, so I’ll recap the missing comments below.
Posted by: CarlG
On: 01-May-18 18:37
I almost never toy with hair, but I would prefer both; any new shader elements as standalone (such as diffuse and glossy) as well as a principled shader setup which combines everything (such as diffuse and glossy) in the correct fashion automatically (such as fresnel output influenced by roughness, which is not trivial for someone new to even consider). Some of the tricks we see would not be possible if all we had was the principled.
Posted by: FXR
On: 01-May-18 19:50
Aside from a good shader, I would love to see an option to “Extrude” hair segments, just like extruding bones to a rig. That would allow a user to "pull out the hair to follow an object surface more closely without intersecting the mesh.
Again, it would a option for regions like forehead bangs and “stray locks” Would be way more advantageous than initially adding extra segments to a hair particle setup. As even with extra segments, you still have to do most adjustments with the five basic segments. requires a lot of rendering and checking to determine look
Posted by: m9105826
On: 01-May-18 20:01
Have a look at the docs for Arnold’s newest hair shader:
https://support.solidangle.com/display/A5AFMUG/Standard+Hair?desktop=true¯oName=center
As far as I’m concerned, they’ve struck the perfect balance between artist friendly and art direction. Cycles’ existing solution had always been confusing because of its manual separation of the R and T terms, leaving artists without the technical know how to their own devices. As a result, I can’t count how many incorrect and non-energy-preserving setups I’ve seen shared throughout the community. Arnold’s devs saw this same issue in their community with their original shader, and I would love to see it addressed with a similar solution on our side.
Posted by: amyspark
On: 01-May-18 21:56
Hey all,
First and foremost: thank you for your comments! It seems that I have a lot to work ahead 
I’ll reply to everyone below.
—Quote (Originally by m9105826)—
Have a look at the docs for Arnold’s newest hair shader:
https://support.solidangle.com/display/A5AFMUG/Standard+Hair?desktop=true¯oName=center
As far as I’m concerned, they’ve struck the perfect balance between artist friendly and art direction. Cycles’ existing solution had always been confusing because of its manual separation of the R and T terms, leaving artists without the technical know how to their own devices. As a result, I can’t count how many incorrect and non-energy-preserving setups I’ve seen shared throughout the community. Arnold’s devs saw this same issue in their community with their original shader, and I would love to see it addressed with a similar solution on our side.
—End Quote—
I noticed that too when looking up how Cycles’s hair shading was done. I’ll make sure to build the shader along those lines.
—Quote (Origin> ally by rbx775)—
First off, congratulations for being accepted - and also thanks for taking part of course!
Your proposal surely is one of the more anticipated ones this year. Get ready for lots of love! 
For improvements: It would be cool to have the reflection and transmission control combined.
As it is right now you always have to mix reflection and transmission which seems a bit like unnecessary overhead.
Is that something that comes natural with Chiangs hair shading model ? Unified transmission+reflection ?
Regarding demo scenes there is fishy cat and the gooseberry project to get you set up.
https://www.blender.org/download/demo-files/
Im sure there will be alot of people providing test cases along the way!
All the best for your GSoC!
—End Quote—
AFAIK, Chiang’s model is based on d’Eon’s model (doi:10.1111/j.1467-8659.2011.01976.x (http://doi.org/10.1111/j.1467-8659.2011.01976.x)) which combines both (unlike Marschner, which has separate parameters for each mode). There are two key changes, though: the ability to adjust hair so as to simulate the animal hair’s undercoat, and the reparameterization of the underlying distribution’s roughness variables to make them perceptually manageable.
Posted by: zeauro
On: 02-May-18 06:29
- 1 for a principled hair shader node for easy set-up.
It should work with attributes of hair info node, UVmapped textures (bitmaps and procedural).
We may use textures as masks, values. It may be interesting to able to mix some properties of a node.
So if it makes sense to mix colors of some attributes of such principled hair shader node, it would make sense to have some elemental nodes to produce such blending.
We just need that to have an artistic control.
It is usual to duplicate same particle system on same area but with an other seed and other children settings to add variations.
So,multiple materials using this shader node in the scene should be supported.
It should work with weird geometry (spiral, braid). Models from Cosmos Laundromat movie should help.
We may sculpt objects and extruded curves to create hair. So, it should also work with models that are not using particles.
I don’t expect more from shading perspective.
The way hair particles geometry should be created has to evolve to become simpler. But it is out of scope and will not happen before end of this GSOC.
As long as, hair can be textured, current geometry is well handled and rendertime is not taking forever : mission would be accomplished.
Posted by: kynu
On: 02-May-18 08:08
—Quote (Originally by m9105826)—
Have a look at the docs for Arnold’s newest hair shader:
https://support.solidangle.com/display/A5AFMUG/Standard+Hair?desktop=true¯oName=center
—End Quote—
This is amazingly well done indeed!
Posted by: julperado
On: 02-May-18 10:13
Congratulations on getting accepted! This is great news
I’ve been working with a homemade node group trying to replicate the Arnold hair shader, so a big +1 to m9105826’s suggestion. An artist friendly hair ubershader that also gives enough control to the users would be awesome.
Here’s the file with my nodegroup (https://drive.google.com/file/d/1A795x0H4ytzzLAzYfhnUjD0-YXyMMh2n/view?usp=sharing) and some quick test renders, note that this is just a bunch of mix nodes so it’s probably incorrect and without proper energy conservation, plus it can get longer render times… 
but for most cases I’ve been getting good results.
Attachment 522931 (https://blenderartists.org/forum/attachment.php?attachmentid=522931)
I can’t wait to finally have a good hair shader for cycles, thank you in advance and I hope everything goes well with this project!
Posted by: rawalanche
On: 02-May-18 10:15
Yes, the Arnold one is a good reference of what a modern hair shader should look like. Corona and V-Ray have also implemented very similar hair materials recently.
Posted by: Geographic
On: 02-May-18 11:14
Well to be honestly, i’m ok width current shader options, what could use some coding love is how to make hair itself.
Some options never really made it, while in general its a bit hard to setup hair (pro’s create groups of hair comb each), while starters rarely do that and get poor results. currently its also a bit trouble some to setup animal fur, if you like to do it realistic. Same for physical hair dymanics… which perhaps touches flesh dynamics as well (muscles)…
For a another hair shader itself i could only say make it work nice width denoiser, and have it in cyclese and eevee.
Posted by: m9105826
On: 02-May-18 15:06
Did anyone asking for hair system features even read the thread? This is a GSoC project to very specifically develop an improved hair shader for Cycles. Everyone who has given it a few hours of their time is fully aware that the hair/particle system in Blender has been running on borrowed time for years now. That’s what the groom system improvements are for. That’s not what this project is about.
@S.Saprophyticus - thanks for the resources!
@sozap - largely along the same lines as the (missing) previous comments.
Everyone - the project is up at https://wiki.blender.org/index.php/User:LeonardoSegovia/GSoC_2018. I’ll post a summary there for reporting purposes later in the day.
. If you manage to find them (it should yield no hair at all or just a ball of black stuff), please send me a demo scene.
