[Soc-2018-dev] Weekly report #02 - Implementing a Hair Shader for Cycles

[Brecht Van Lommel]

I suggest to carefully check each part of the shader is working correctly.
For that you can create a simple scene with a single hair with a big
radius. Then you can light that with just a white background or a single
point light, without any indirect light bounces. If that's correct, add
more samples, more bounces, more hair, etc.

That way you can see if the results looks correct, compare with existing
BSDFs, and see where exactly it is more noisy or wrong. Getting some tests
builds for users is good, but I suggest to mainly focus on verifying the
correctness in an organised way at this point.

I'll have a look at the NaNs and assertions this weekend.


On Fri, May 25, 2018 at 9:46 PM, Leonardo E. Segovia <
leonardo.segovia at cs.uns.edu.ar> wrote:

> WEEK 2
>
>
> Hi everyone! The second week is gone, and this is my report.
>
>
> Objectives
>
> With the shader in place, I could move on to the following (Weeks 4-5):
>
> -   Test shader functionality. If working, release test build.
> -   Evaluate received feedback. Start bug fixes.
>
>
> What's been done
>
> Admittedly, not much 😢
>
> -   Lukas added a Diffuse override for using the shader with non-hair
>     geometry, in bsdf_principled_hair_setup. When using it with hair,
>     however, Blender didn't render anything _at all_. This was because
>     the ID of the shader was outside the range of CLOSURE_IS_BSDF. Our
>     shader is now treated as BSDF and transmissive
>     {{GitCommit|rBc4174a44dc311d2fd65ca74c4387bbc6f529df95}}
>
> -   With the BSDF in place, I tried to get reference renders. The main
>     problem is that Blender's hair primitive is returning NaNs in any or
>     all of the following:
>
>     -   the ray distance t
>     -   the intersection calculation, somewhere deep within
>         cardinal_curve_intersect or curve_intersect
>     -   the curve distance h (this one usually comes from the above)
>
>     Which results in one or more of the following:
>
>     -   crash when integrating indirect radiance
>         ("Non-finite sum in path_radiance_clamp_and_sum!")
>     -   crash when obtaining azimuthal_scattering (I have a scene to
>         obtain this exact crash)
>     -   assert(fabsf(dot(sd->dPdu, dPdCD)) < 1e-5f);. I do not know
>         (yet) what this one means.
>
>     I ring-fenced the shader from most of those
>     {{GitCommit|rB92030f83362fe567f78e519e0acca0700287a933}}}. Brecht
>     agreed to have a look at those weird NaNs, since they come from
>     outside our shader.
>
> -   In Phabricator's task for this project, Ronan Ducluzeau could get a
>     reference render without crashing. The shader returns a lot of noise
>     and fireflies.
>
>
> Next up
>
> Once we figure out why we receive bogus data from the ray intersection
> (and fix it), I'll be able to continue revising the shader.
>
> If possible, I'd like to get a dev build out in BlenderArtists for
> people to test. I don't expect to get great results yet, but it'd be
> great if we can ensure the hair primitive works properly. I don't have
> good enough bandwidth (3Mb) to upload a 200MB+ build unless I go to the
> uni, so I may ask either Lukas or bzztploink in IRC if they can upload
> them.
>
>
> Questions
>
> -   In re assertion above - what does it mean?
> -   What could be possible sources of noise in our shader? Compared with
>     a Diffuse shader (which doesn't use the hair primitive data), it's
>     obviously not working properly. I wonder if this and the other
>     issues are related.
>
> --
> 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
>
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