[Soc-2018-dev] Weekly report #10 - Implementing a Hair Shader for Cycles

[Leonardo E. Segovia]

Hey all, hey Brecht,

I spent the whole day yesterday dissecting the shader. I tried:
- selectively removing modes (azimuthal only, longitudinal only,  both)
- changing BSDF path labelling
- manual calculation of the alpha and attenuation angles
- whether the Bessel function / logarithm approximation was introducing
additional noise
- whether Lukas's changes in the logarithm approximation were the source of
the above

None of the above helped me figure out what causes the noise, only narrow
down where and when it happens:

- It only affects the TT mode of the shader
- It only affects the Glossy Indirect illumination pass
- It affects the whole hair length, not just those parts which are away
from the light source

I've uploaded a multi-layer EXR example at
https://framadrop.org/r/09_50tz4sM#1PkFSGOaENlBkpROKrLrjjy8HVi1pg4GqnfR1mqTkz8=
. You'll see that the Gloss Ind pass is absolutely noisy.

Best regards,
Leonardo

El mié., 25 de jul. de 2018 a la(s) 10:42, Leonardo E. Segovia (
leonardo.segovia at cs.uns.edu.ar) escribió:

> Hey all,
>
> Brecht: sadly, it is expected. I did more tests and it turns out that
> multiplying Radial Roughness by 3 gets a more similar look to the "before"
> render. Now, I think it'll be better to start separating the render into
> modes to see where the noise is actually coming from.
>
> Best regards,
> Leonardo
>
> El mié., 25 de jul. de 2018 a la(s) 09:45, Brecht Van Lommel (
> brechtvanlommel at gmail.com) escribió:
>
>> Thanks for the tests. To me the before renders still look better? While
>> 3x gives similar noise levels, the results are less accurate, the hair is
>> noticeably brighter.
>>
>> On Tue, Jul 24, 2018 at 10:40 PM Leonardo E. Segovia <
>> leonardo.segovia at cs.uns.edu.ar> wrote:
>>
>>> Hey all, hi Brecht,
>>>
>>> As regards noise and fireflies, they come from basically two places:
>>> - Low Roughness. This compress (longitudinally) the glints so much, they
>>> look like fireflies.
>>> - Low Radial Roughness. These make the hair look SO soft!
>>> - Very lightly colored hair (this one is non fixeable by us)
>>>
>>> Possible mitigations include:
>>> - Raising the clamp from 0.001 to 0.01 to at least have a reasonable
>>> glint when Roughness < 0.1
>>> - Filter Glossy
>>>
>>> I've already uploaded a fix for the former. I'd like to discuss the
>>> behavior of Filter Glossy.
>>>
>>> When doing the CPU profiling that you requested, I realised that Filter
>>> Glossy is executed after the closure setup (before closure setup =
>>> roughness, after = logistic parameters), which means it isn't affecting the
>>> roughness properly.
>>> rB52a0e67fe005
>>> <https://developer.blender.org/rB52a0e67fe0050683a888d6643ec2035bd2b3ea1c>
>>> is the commit that includes both mitigations, however, the new
>>> implementation is noticeably less effective. I've obtained decent results
>>> when applying fmaxf(3*roughness to both Roughness and Radial Roughness;
>>> you'll find a visual comparison of the fixes below. (The original picture
>>> is up at https://framapic.org/ksZ7ftOaGvqk/gS2Cv0cjrdUg.jpg.)
>>>
>>>
>>>
>>> I'm not sure if there is any need to strengthen Filter Glossy wrt.
>>> Radial Roughness in this way; please let me know what you think.
>>>
>>> Best regards,
>>> Leonardo
>>>
>>> El lun., 23 de jul. de 2018 a la(s) 08:16, Brecht Van Lommel (
>>> brechtvanlommel at gmail.com) escribió:
>>>
>>>> Hi,
>>>>
>>>>
>>>>>    - The regression test suite has not been uploaded yet to SVN.
>>>>>
>>>>
>>>> I've committed these as well now.
>>>>
>>>>
>>>>>    - It's been asked in the BA thread if I have extra objectives for
>>>>> the
>>>>>    shaders. Do you feel that more features/controls should be added?
>>>>>
>>>>
>>>> More features could be good, but also part of the GSoC plan was to
>>>> investigate optimizations for the shader, since it still is relatively slow
>>>> / noisy.
>>>>
>>>> So I think you should at least profile the shader, and look into
>>>> fireflies to see where they come from and if they are expected or not. I
>>>> don't know if there is anything major to find, but it's worth investigating.
>>>>
>>>>       - I came up with controls for each of the modes (R, TT, TRT,
>>>>> TRRT+).
>>>>>       - It's been also suggested to prune the UI into a "Basic" and
>>>>>       "Advanced" mode.
>>>>>
>>>>
>>>> I'd be hesitant to add features to break physical correctness and
>>>> energy conservation so early, before people have really tested the shader.
>>>> I rather users test it and then give us feedback saying they can't achieve
>>>> this or that look, and then we see what the best solution is.
>>>>
>>>> Maybe commit it to your branch and then we can always add it later if
>>>> there is a need for it.
>>>>
>>>>
>>>>> About the GSoC project deliverables:
>>>>>
>>>>>    - If I read the mail correctly, Google wants us students to
>>>>> summarize
>>>>>    and show off our work in a single place e.g. a blog post. How does
>>>>> Blender
>>>>>    handle this?
>>>>>       - My preference would be to add a page (similar to when one
>>>>> publishes
>>>>>       a paper) in amyspark.me explaining the work we did. We could
>>>>> embed
>>>>>       the resulting pictures, credits, etc. and the demonstration
>>>>> video from
>>>>>       Youtube.
>>>>>       - As for "Get the code", perhaps I could add links to the
>>>>> relevant
>>>>>       commits?
>>>>>
>>>>
>>>> A webpage is fine, for the code you can link to the git branch.
>>>>
>>>> Thanks,
>>>> Brecht.
>>>>
>>>>
>>>> --
>>>> Soc-2018-dev mailing list
>>>> Soc-2018-dev at blender.org
>>>> https://lists.blender.org/mailman/listinfo/soc-2018-dev
>>>>
>>>
>>>
>>> --
>>> 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
>>> --
>>> Soc-2018-dev mailing list
>>> Soc-2018-dev at blender.org
>>> https://lists.blender.org/mailman/listinfo/soc-2018-dev
>>>
>> --
>> Soc-2018-dev mailing list
>> Soc-2018-dev at blender.org
>> https://lists.blender.org/mailman/listinfo/soc-2018-dev
>>
>
>
> --
> 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
>


-- 
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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