The refactor in 97d9bbbc97 changed the way q is computed in the spherical triangle sampling code. While the new approach is more efficient and saves a few operations, it introduces numerical precision issues for skinny/small (spherical) triangles.
Therefore, this change moves the computation of q back to the method from the paper, while keeping the more efficient solid angle computation.
Pull Request: https://projects.blender.org/blender/blender/pulls/119224
it is not clear from which point the `cos_theta_u` should be computed in
volume segment, so the original implementation was mixing the closest point
and the point where the minimal angle is formed.
Use the closest point on segment as a conservative measure.
Pull Request: https://projects.blender.org/blender/blender/pulls/119965
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
By restricting the sample range along the ray to the valid segment.
Supports
**Mesh Light**
- [x] restrict the ray segment to the side with MIS
**Area Light**
- [x] when the spread is zero, find the intersection of the ray and the bounding box/cylinder of the rectangle/ellipse area light beam
- [x] when the spread is non-zero, find the intersection of the ray and the minimal enclosing cone of the area light beam
*note the result is also unbiased when we just consider the cone from the sampled point in volume segment. Far away from the light source it's less noisy than the current solution, but near the light source it's much noisier. We have to restrict the sample region on the area light to the part that lits the ray then, I haven't tried yet to see if it would be less noisy.*
**Point Light**
- [x] the complete ray segment should be valid.
**Spot Light**
- [x] intersect the ray with the spot light cone
- [x] support non-zero radius
Pull Request: https://projects.blender.org/blender/blender/pulls/119438
Ref: #118534
turns out `in_volume_segment` does need to be checked. If the ray origin
lies on the wrong side of the mesh light, part of the ray could still be
lit by the other side, so the sample should not be considered invalid.
Pull Request: https://projects.blender.org/blender/blender/pulls/119529
if emission sampling is not set, we do not use MIS weight when sampling
from the BSDF, but we were still drawing samples from the light,
resulting in double contribution.
Pull Request: https://projects.blender.org/blender/blender/pulls/118534
wi is the viewing direction, and wo is the illumination direction. Under this notation, BSDF sampling always samples from wi and outputs wo, which is consistent with most of the papers and mitsuba. This order is reversed compared with PBRT, although PBRT also traces from the camera.
When rendering in the viewport (or probably on instanced objects, but I didn't
test that), emissive objects whose scale is negative give the wrong value on the
"backfacing" input when multiple sampling is enabled.
The underlying problem was a corner case in how normal transformation is handled,
which is generally a bit messy.
From what I can tell, the pattern appears to be:
- If you first transform vertices to world space and then compute the normal from
them (as triangle light samping, MNEE and light tree do), you need to flip
whenever the transform has negative scale regardless of whether the transform
has been applied
- If you compute the normal in object space and then transform it to world space
(as the regular shader_setup_from_ray path does), you only need to flip if the
transform was already applied and was negative
- If you get the normal from a local intersection result (as bevel and SSS do),
you only need to flip if the transform was already applied and was negative
- If you get the normal from vertex normals, you don't need to do anything since
the host-side code does the flip for you (arguably it'd be more consistent to
do this in the kernel as well, but meh, not worth the potential slowdown)
So, this patch fixes the logic in the triangle emission code.
Also, turns out that the MNEE code had the same problem and was also having
problems in the viewport on negative-scale objects, this is also fixed now.
Differential Revision: https://developer.blender.org/D16952
There has been an attempt to reorganize this part, however, it seems that didn't compile on HIP, and is reverted in
rBc2dc65dfa4ae60fa5d2c3b0cfe86f99dcb5bf16f. This is another attempt of refactoring. as I have no idea why some things don't work on HIP, it's
best to check whether this compiles on other platforms.
The main changes are creating a new struct named `MeshLight` that is shared between `KernelLightDistribution` and `KernelLightTreeEmitter`,
and a bit of renaming, so that light sampling with or without light tree could call the same function.
Also, I noticed a patch D16714 referring to HIP compilation error. Not sure if it's related, but browsing
https://builder.blender.org/admin/#/builders/30/builds/7826/steps/7/logs/stdio, it didn't work on gfx1102, not gfx9*.
Differential Revision: https://developer.blender.org/D16722
Uses a light tree to more effectively sample scenes with many lights. This can
significantly reduce noise, at the cost of a somewhat longer render time per
sample.
Light tree sampling is enabled by default. It can be disabled in the Sampling >
Lights panel. Scenes using light clamping or ray visibility tricks may render
different as these are biased techniques that depend on the sampling strategy.
The implementation is currently disabled on AMD HIP. This is planned to be fixed
before the release.
Implementation by Jeffrey Liu, Weizhen Huang, Alaska and Brecht Van Lommel.
Ref T77889
* Split light types into own files, move light type specific code from
light tree and MNEE.
* Move flat light distribution code into own kernel file and host side
building function, in preparation of light tree addition. Add light/sample.h
as main entry point to kernel light sampling.
* Better separate calculation of pdf for selecting a light, and pdf for
sampling a point on the light. The selection pdf is now also stored in
LightSampling for MNEE to correctly recalculate the full pdf when the
shading position changes but the point on the light remains fixed.
* Improvement to kernel light storage, using packed_float3, better variable
names, etc.
Includes contributions by Brecht Van Lommel and Weizhen Huang.
Ref T77889
