[Soc-2018-dev] Weekly report #05 - Implementing a Hair Shader for Cycles

[Leonardo E. Segovia]

Week 5

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

This week I dealt with Weeks 4-5 objectives:

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

I released two builds this week:

   - one with the Week 4 features
   - the other almost mid-week after due to a bug (more on this later!).

Remember that I was partially out on Mon-Tue because of a public class I
had to prepare.
What's been done

   - In the BA thread, Zanzio found that renders were wildly inconsistent
   between CPU and GPU. This became noticeably if rendering with both at the
   same time.
      - Bug #1 was a precision issue when normalizing the coordinate frame
      in the presence of low roughness values (Git commit:
      rBe267191393fe5dbde91d5b428f4a6adaf6a3fa8f
      <https://developer.blender.org/rBe267191393fe5dbde91d5b428f4a6adaf6a3fa8f>
      ).
      - Bug #2, and the true reason for this, was that somehow the GPU
      kernel was reordering the second set of values. I don't know why it had
      such an effect, but it's been fixed. Thanks to nirved again for noticing!
      (Git commit: rB2d1eef87f6b61a4a2c8ff476d290c9d841661f6b
      <https://developer.blender.org/rB2d1eef87f6b61a4a2c8ff476d290c9d841661f6b>
      )
   - Did a bit of cleanup: store and retrieve consistently the values from
   the stack (Git commit: rBdc2827fd907d6c3a742ea2292e588521e2be55f9
   <https://developer.blender.org/rBdc2827fd907d6c3a742ea2292e588521e2be55f9>
   ).
   - Merged the latest master commits (Git commits:
   rB60a59fbae6b04f59e91f540eabc52102f354cd0a
   <https://developer.blender.org/rB60a59fbae6b04f59e91f540eabc52102f354cd0a>
   , rB535e2e4ade24801c123017ad5b1fe8e580c1d382
   <https://developer.blender.org/rB535e2e4ade24801c123017ad5b1fe8e580c1d382>
   , rB9b632addd34758035331399e69b3f619bc6d4b16
   <https://developer.blender.org/rB535e2e4ade24801c123017ad5b1fe8e580c1d382>
   .
      - Lukas changed the linear_rgb_to_gray function signature. (Git
      commit: rBbd37b18a0d6d33d2be3a2e96d1928356e85a0e2a
      <https://developer.blender.org/rB535e2e4ade24801c123017ad5b1fe8e580c1d382>
      ))

What I'll do later today, and during the weekend

Today, Brecht, Lukas, nirved and I discussed on IRC the current state of
the shader. This is a summary of the proposed changes:

   - Rename the Roughness parameters to (Longitudinal) "Roughness", "Radial
   Roughness" and "Undercoat Roughness"
   - Remove the "Physical" parameterization
   - Rename and reorder the rest to "Color" ("Direct coloring") "Melanin"
   ("Melanin Concentration"), and "Absorption coefficient"
      - The latter remains for debugging purposes e.g. if someone wants to
      use the coefficients straight from the papers
   - Remap Melanin concentration to represent:
      - Melanin -> quantity
      - Melanin Redness -> distribution
      - Brecht asked if Melanin could be done as a percentage of saturation
      (0 = white, 1 = black). I know it can be done that way (V-Ray), but there
      is no public, reliable mapping; for example, Tungsten's melanin
values have
      a similar representation, but Bitterli still uses d'Eon's mapping, so the
      Melanin value is still an absolute concentration quantity.
   - Redo the randomization to a relative tolerance
      - Formula: effective_roughness = clamp(roughness * (1.0f + (random -
      0.5f) * roughness_randomization/100.0f), 0.0f, 1.0f)
      - i.e. when randomized, the roughness value should be roughness ±
      roughness_randomization%-
   - Implement randomization by directly retrieving Hair Info -> Random
   unless the user sockets a different source.
      - This is easy with OSL (geom:curve_random), with SVM... not so much.
      I do have an idea, thanks to Principled Volume.
   - Reorder the sockets as follows:
      - Color
      - Roughness
      - Azimuthal Roughness
      - Undercoat Roughness
      - IOR
      - Offset
      - Randomize Color
      - Randomize Roughness
      - Random

Next up

If no bugs or feature requests arise in the next weeks, 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

None this week!

-- 
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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