**Problem**:
Area lights in Cycles have spread angle, in which case some part of the area light might be invisible to a shading point. The current implementation samples the whole area light, resulting some samples invisible and thus simply discarded. A technique is applied on rectangular light to sample a subset of the area light that is potentially visible (rB3f24cfb9582e1c826406301d37808df7ca6aa64c), however, ellipse (including disk) area lights remained untreated. The purpose of this patch is to apply a techniques to ellipse area light.
**Related Task**:
T87053
**Results**:
These are renderings before and after the patch:
|16spp|Disk light|Ellipse light|Square light (for reference, no changes)
|Before|{F13996789}|{F13996788}|{F13996822}
|After|{F13996759}|{F13996787}|{F13996852}
**Explanation**:
The visible region on an area light is found by drawing a cone from the shading point to the plane where the area light lies, with the aperture of the cone being the light spread.
{F13990078,height=200}
Ideally, we would like to draw samples only from the intersection of the area light and the projection of the cone onto the plane (forming a circle). However, the shape of the intersection is often irregular and thus hard to sample from directly.
{F13990104,height=200}
Instead, the current implementation draws samples from the bounding rectangle of the intersection. In this case, we still end up with some invalid samples outside of the circle, but already much less than sampling the original area light, and the bounding rectangle is easy to sample from.
{F13990125}
The above technique is only applied to rectangle area lights, ellipse area light still suffers from poor sampling. We could apply a similar technique to ellipse area lights, that is, find the
smallest regular shape (rectangle, circle, or ellipse) that covers the intersection (or maybe not the smallest but easy to compute).
For disk area light, we consider the relative position of both circles. Denoting `dist` as the distance between the centre of two circles, and `r1`, `r2` their radii. If `dist > r1 + r2`, the area light is completely invisible, we directly return `false`. If `dist < abs(r1 - r2)`, the smaller circle lies inside the larger one, and we sample whichever circle is smaller. Otherwise, the two circles intersect, we compute the bounding rectangle of the intersection, in which case `axis_u`, `len_u`, `axis_v`, `len_v` needs to be computed anew. Depending on the distance between the two circles, `len_v` is either the diameter of the smaller circle or the length of the common chord.
|{F13990211,height=195}|{F13990225,height=195}|{F13990274,height=195}|{F13990210,height=195}
|`dist > r1 + r2`|`dist < abs(r1 - r2)`|`dist^2 < abs(r1^2 - r2^2)`|`dist^2 > abs(r1^2 - r2^2)`
For ellipse area light, it's hard to find the smallest bounding shape of the intersection, therefore, we compute the bounding rectangle of the ellipse itself, then treat it as a rectangle light.
|{F13990386,height=195}|{F13990385,height=195}|{F13990387,height=195}
We also check the areas of the bounding rectangle of the intersection, the ellipse (disk) light, and the spread circle, then draw samples from the smallest shape of the three. For ellipse light, this also detects where one shape lies inside the other. I am not sure if we should add this measure to rectangle area light and sample from the spread circle when it has smaller area, as we seem to have a better sampling technique for rectangular (uniformly sample the solid angle). Maybe we could add [area-preserving parameterization for spherical
ellipse](https://arxiv.org/pdf/1805.09048.pdf) in the future.
**Limitation**:
At some point we switch from sampling the ellipse to sampling the rectangle, depending on the area of the both, and there seems to be a visible line (with |slope| =1) on the final rendering
which demonstrate at which point we switch between the two methods. We could see that the new sampling method clearly has lower variance near the boundaries, but close to that visible line,
the rectangle sampling method seems to have larger variance. I could not spot any bug in the implementation, and I am not sure if this happens because different sampling patterns for ellipse and rectangle are used.
|Before (256spp)|After (256spp)
|{F13996995}|{F13996998}
Differential Revision: https://developer.blender.org/D16694
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
