[Soc-2018-dev] Weekly report #03 - Implementing a Hair Shader for Cycles

[Leonardo E. Segovia]

(As usual, the complete version is up at Blender-Wiki:
https://wiki.blender.org/index.php/User:LeonardoSegovia/GSoC_2018/Reports/Week_3
)
--

WEEK 3


Hi everyone! Yet another week's now gone. This is my report.


Objectives

We're still in Weeks 4-5:

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


What's been done

This week's been MUCH better!

-   It turns out that I had WITH_CYCLES_DEBUG on, that was the source of
    all the NaN noise in my side. I rebuilt Blender from the ground up;
    I still have RelWithDebInfo on.
-   Thanks to Lukas, bzztploink and artok, we could get test builds for
    all platforms! This enabled us to find some issues regarding:
    -   Linux compilation
    -   Missing initialization code
    -   Some stuff I didn't know about Cycles's innards

In terms of commits,

-   Me:
    -   Ronan Ducluzeau's noisy renders were caused by an mistake in
        Lukas's optimization of the trimmed logistic distribution. It's
        now fixed
        ({{GitCommit|rB2af62e27e2c894217b5d7daa880434447ea6ed35}}).
    -   Split off the azimuthal and longitudinal roughness factors to
        make debugging easier when needed
        ({{GitCommit|rB4a73f5ce584f49c68b2c64c74ef30188395088de}}).
    -   Added compatibility with OpenCL. Thanks to nirved for the
        patches and comments!
        ({{GitCommit|rB9d6333127623ebed7a275d382e63da1f87712aae}},
        {{GitCommit|rB00d0f3c23527b047f3211fd760080fdfcec4037b}})
    -   Added OSL compatibility. I know I don't allocate the memory for
        the closures correctly, I'll patch it in the weekend.
        ({{GitCommit|rB718d730bb409322dd9078273bfc0dc16b956a8d6}},
        {{GitCommit|rB718d730bb409322dd9078273bfc0dc16b956a8d6}},
        {{GitCommit|rB4774e906e19816b720c20271b9f9db28bb89debe}},
        {{GitCommit|rBb7a92fa75e56c2b76968e523892bfb7ee9a6f19e}},
        {{GitCommit|rBdf0ead4dabd0c0f07ff046b59c7d850005d59e60}},
        {{GitCommit|rBbf5a378c3c350384f18ce7daf6624ef0c56ab5ba}})
    -   Added the final missing feature from Chiang's formulation, the
        R-mode roughness modifier (formally called "Primary Reflection
        Roughness Modifier"). It enables the user to render a shiny coat
        on an otherwise rough patch of fur
        ({{GitCommit|rB25677d7738530d8f827e5ac960ccded6b128d897}},
        {{GitCommit|rBc3048b5f53aaeb5b860bf1760a7f1ef51f6c4701}},
        {{GitCommit|rBb3ebdf64459d7a99e9a848a49f7eaa250b8a07bd}}).
        -   I had to move the geometry data out of the main
            ShaderClosure to make space for this. I didn't know that I
            had to tell Blender how many closures I was using 😅, so all
            Final Renders returned black hair. This was found by YAFU on
            BlenderArtists and fixed by Brecht.
    -   Enabled Filter Glossy for the roughness coefficients. Brecht
        reports a ~75% improvement in render quality!
        ({{GitCommit|rB8087cefd03022759250f5d62eb4776fa6c55be4c}},
        {{GitCommit|rBdf1e50fa5d913d2a577a178d3f26f573795997ad}})
    -   Finally found out why Blender didn't listen to RNA's default for
        the hair parameterization. I added the correct initialization
        code ({{GitCommit|rB7780d306a416e0dcc37932698e70595bce58ea5e}}).
    -   Merged the latest master commits. Only a single conflict! (Week
        2: {{GitCommit|rB82e63de2842c2af8be5ec19a00752b110dbd0c57}},
        Week 3:
        {{GitCommit|rBfede34a58a8df630bb927ab9c017ab2cd87478b4}})
    -   Bonus:
        -   Turned off the Diffuse override. It doesn't seem to be
            needed after Brecht's curve_core_intersect patch
            ({{GitCommit|rB718d730bb409322dd9078273bfc0dc16b956a8d6}}).
        -   Added our shader to the Glossy closure checker as well
            ({{GitCommit|rB25677d7738530d8f827e5ac960ccded6b128d897}}).
-   Brecht:
    -   Fixed the hair offset calculation by avoiding
        curve_core_intersect & pals altogether
        ({{GitCommit|rBabfca9375c6ae75320b7538d547545350ad47c85}},
        {{GitCommit|rBff6883fbc330c5ac1333beaf38142d6b1d7728cf}},
        {{GitCommit|rBbaad410a5c5b15158b06d9b9a9d9c86522261cea}}).
    -   Added a few comments wrt. hair attenuation terms and offset
        angles
        ({{GitCommit|rBfc62d5a3b3622efa69a02377bc15ab6f6f1e389c}}).
    -   Fixed the range for the R-mode roughness modifier
        ({{GitCommit|rB3c56635da9579a297f3b32ce2cc0b9468d308199}}).
    -   Fixed the number-of-closures issue that came with the R-mode
        roughness modifier
        ({{GitCommit|rB1fac3b7da05ada27d8a41c114283394b5a505ebb}}).

Here are some images:

[[File:hair-ball-glossyfiltered.png|thumb|center|500px|A R-mode-modified
fur ball, also filtered with Filter Glossy. Rendered with Cuda on Win7
x64 in ~5min.]]

[[File:ZootopiaShaderNode-week-3.png|thumb|center|500px|The node's
current appearance and sockets.]]


Next up

Apart from finally settling the memory allocation flow for the OSL
shader, the original shader is mostly done. I could address some of the
proposed features in the BlenderArtists thread:

-   Define separately the eumelanin/pheomelanin coefficients, _à la_
    Arnold's Standard Hair
-   Perhaps add more features, such as tinting? Again, following
    Arnold's shader features.

If no more bugs arise, I may start with the rest of the objectives too.


Questions

None this time!


-- 
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