[Soc-2018-dev] Weekly report #06 - Implementing a Hair Shader for Cycles

[Leonardo E. Segovia]

Week 6

Hi everyone! This is my report for the present week.
Objectives

We're still on Weeks 4-5:

   - Test shader functionality. If working, release test build.
   - Evaluate received feedback. Start bug fixes.

bzztploink and Lukas released the relevant builds on Monday.

I was stuck this weekend because of university constraints; however, today
was the end of the Digital Image Processing course, so I should have more
free time to devote to the project.
What's been done

This week, I implemented the agreed cleanup and organization changes. These
were breaking changes in some instances, outlined below.

   - Removed Lukas's debug "physical" parametrization and reordered the
   rest (rB20ed03722fad0954d961aeaa0a3b0b5df548afb7
   <http://developer.blender.org/rB20ed03722fad0954d961aeaa0a3b0b5df548afb7>,
   rB32ec7d77d7ebd236bd1f724ab735415f2d97389d
   <http://developer.blender.org/rB32ec7d77d7ebd236bd1f724ab735415f2d97389d>).

      - Parametrizations should now be in order of usability: Direct
      coloring (plug and play), Melanin concentration (for realistic results),
      and Absorption coefficient (if you have a paper, you can enter its values
      here).
      - This change means that existing .blends need to be adjusted.
   - Reordered and renamed sockets according to most useful to least (
   rB45abb2eee8030e1058182d30f1a6f232bbed8ab1
   <http://developer.blender.org/rB45abb2eee8030e1058182d30f1a6f232bbed8ab1>,
   rB06fb2a06738ccf0e9d90d41906ab18ea37bdb6f1
   <http://developer.blender.org/rB06fb2a06738ccf0e9d90d41906ab18ea37bdb6f1>
   ).
   - Refactors:
      - Randomization into factor +/- the given percentage (
      rBc36722f96287b226f02783925f52bb413c6b3c0c
      <http://developer.blender.org/rBc36722f96287b226f02783925f52bb413c6b3c0c>).

         - It is expected that some .blends need to be adjusted.
      - Melanin into Tungsten's quantity and redness ratio (
      rBf91635e64e50eaa50bd911af3f93da881eacf4d0
      <http://developer.blender.org/rBf91635e64e50eaa50bd911af3f93da881eacf4d0>,
      rBb005c15f6734ca20816d4820394f09667aaa853b
      <http://developer.blender.org/rBb005c15f6734ca20816d4820394f09667aaa853b>
      ).
   - Automagically get the RNG info even if the user hasn't socketed a Hair
   Info -> Random node (rB23d92659c7e9a603f40e121f671ca35b101a1831
   <http://developer.blender.org/rB23d92659c7e9a603f40e121f671ca35b101a1831>).

      - After this change, the only way to disable randomization is to *socket
      a Value node* and set it to 0.5 (just setting the value will be
      ignored), or set Randomization to 0.
   - Remove two possible sources of NaNs I and nirved experienced during
   debugging (rBc59ed14619e7b2e3bad075ec6e04edf0c7fc70ec
   <http://developer.blender.org/rBc59ed14619e7b2e3bad075ec6e04edf0c7fc70ec>
   ).

Next up

I don't expect to spend more time on coding (except for the reason below).
During the weekend and from now on, I'll begin porting the existing hair
tests to the new shader. I reiterate the previous request for .blend
hairstyles for this purpose -- it'll be great if you could post them to the
BA thread!
Questions

In the BA thread, jemain said:

I don’t know why but in real size (40 to 100 microns) the hair is melted
into a mass without details like molten resin. I have to put them at 300
micron to hope to have details.

I understand that this is a near-field shader, i.e. it should work fine at
that level of resolution. I'm not well versed in the innards of Cycles, so
I'd like to ask the community:

   - Firstly, how can one set the measurement units so as to achieve
   real-sized hair?
   - What could be possible sources of the lack of definition the poster
   mentioned? Could it mean a problem with the path tracer? Or is it a symptom
   of noise caused by our shader?


-- 
Lic. Leonardo E. Segovia
Departamento de Ciencias e Ingeniería de la Computación
Universidad Nacional del Sur
San Andrés 800 - Campus Palihue, B8000 Bahía Blanca, Argentina
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