[Soc-2018-dev] Weekly Report #07 - Many Light Sampling

[Erik Englesson]

Thanks for the pointers to the functions!

Regarding GGX evaluation, yeah, it is unclear to me too. We'll figure it
out next week!

Thanks,
Erik

On 29 June 2018 at 21:07, Brecht Van Lommel <brechtvanlommel at gmail.com>
wrote:

> Some relevant Cycles functions are:
> * Pick a random BSDF with shader_bsdf_pick() (in case there are multiple)
> * Sample a random direction with bsdf_sample()
> * Estimate the BSDF roughness with bsdf_get_roughness_squared()
> * GGX evaluation is in bsdf_microfacet_ggx_eval_reflect(), can be copied
> and simplified to remove the pdf output, anisotropy, clearcoat, etc.
>
> It's not immediately clear to me how the GGX evaluation fits in. Normally
> BSDF evaluation would take a direction vector towards the light (and a view
> direction I and normal N which are constant for a shading point). But here
> we have this random BSDF direction, vector to the cluster center and
> maximum cosine...
>
>
> On Fri, Jun 29, 2018 at 6:04 PM Erik Englesson <erikenglesson at gmail.com>
> wrote:
>
>> Hi all,
>>
>> Here is my report for the week:
>>
>> *This week* I have been working on adding support for background lights
>> when using the light tree. Before this week, we had a certain fixed
>> probability of choosing either to sample the distant lights or the light
>> tree(background lights not supported).
>>
>> Now, a decision is made either to sample one of the three light groups:
>> the light tree, the distant lights or the background. After a group has
>> been chosen, the corresponding group sampling method is used. Deciding
>> which light group to sample is done by sampling a CDF that is based on the
>> energy of all the lights within each group. The tree is using the energy of
>> the root node, which is the aggregate energy of all the lights in the tree.
>> The distant and background lights uses an energy based on an approximation
>> of how much of their emitted energy hits the scene(as PBRT does).
>>
>> I also found and fixed some bugs. One bug was an indexing bug between
>> indices to the scene->lights array and the dscene->lights array. These
>> started to differ as soon as lights were disabled since only enabled lights
>> are sent to the device.
>>
>> The work done this week can be found in this commit
>> <https://developer.blender.org/rB2cdb08f4ffb3d7d24935d82ba32b1b1100ee2cd6>
>> .
>>
>> *Next week* I want to start on the first iteration of the traversal
>> splitting method. One of the things I find unclear right now is the
>> following in bold from the slide on page 20
>> <https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxja3VsbGF8Z3g6NWM0NmU2YWVlNjE3ODk1Yw>
>> :
>>
>> "Approximate solid angle of the cluster times BSDF peak
>> • BSDF peak is computed as follows:
>> 1. Sample a random direction from the BSDF
>> 2. Compute a conservative maximum cosine with the vector to the
>> cluster’s center
>> 3. *Evaluate a simple GGX model with the BSDF roughness*"
>>
>> I have a limited understanding of microfacet theory and I have not read
>> about the GGX model. I will have to look into this. Learning new things all
>> the time!
>>
>> If there is time, after having implemented a first pass of the splitting
>> method, I would like to see if I could do another iteration on the distant
>> light sampling. It would be nice for the choice between different distant
>> lights to depend on energy and maybe direction(the light_distant_sample()
>> function in kernel_light.h).
>>
>> Have a great weekend!
>>
>> Thanks,
>> Erik
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>>
>
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