The same random number was used when sampling from the volume segment
and from the direct scattering position, causing correlation issues with
light tree.
To solve this problem, we ensure the same light is picked for
volume segment/direct scattering, equiangular/distance sampling by
sampling the light tree only once in volume segment. From the direct
scattering position in volume, we sample a position on the picked light
as usual. If sampling from the light tree fails, we continue with
indirect scattering.
For unbiased MIS weight for forward sampling, we retrieve the `P`, `D`
and `t` used in volume segment for traversing the light tree.
The main changes are:
1. `light_tree_sample()` and `light_distribution_sample()` now only pick
lights. Sampling a position on light is done separately via
`light_sample()`.
2. `light_tree_sample()` is now only called only once from volume
segment. For direct lighting we call `light_sample()`.
3. `light_tree_pdf()` now has a template `<in_volume_segment>`.
4. A new field `emitter_id` is added to struct `LightSample`, which just
stores the picked emitter index.
5. Additional field `previous_dt = ray->tmax - ray->tmin` is added to
`state->ray`, because we need this quantity for computing the pdf.
6. Distant/Background lights are also picked by light tree in volume
segment now, because we have no way to pick them afterwards. The direct
sample event for these lights will be handled by
`VOLUME_SAMPLE_DISTANCE`.
7. Original paper suggests to use the maximal importance, this results
in very poor sampling probability for distant and point lights therefore
excessive noise. We have a minimal importance for surface to balance, we
could do the same for volume but I do not want to spend much time on
this now. Just doing `min_importance = 0.0f` seems to do the job
okayish. This way we still won't sample the light with zero
`max_importance`.
The current solution might perform worse with distance sampling, because
the light tree measure is biased towards equiangular sampling. However,
it is difficult to perform MIS between equiangular and distance sampling
if different lights are picked for each method. This is something we can
look into in the future if proved to be a serious regression.
Pull Request: https://projects.blender.org/blender/blender/pulls/119389
fdc2962beb