Light tree more noisy with saturated light and surface colors #112217
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Reference: blender/blender#112217
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System Information
Operating system: Linux-6.4.15-x64v4-xanmod1-x86_64-with-glibc2.35 64 Bits, X11 UI
Graphics card: NVIDIA GeForce RTX 2070 Super/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 535.104.05
Blender Version
Broken: version: 4.0.0 Alpha, branch: main, commit date: 2023-09-10 22:17, hash:
fca8df9415b13.6.2 release shows same behaviour
Worked: (newest version of Blender that worked as expected)
Short description of error
I built a simple scene for testing Agx vs Filmic vs ACES:
A simple grid and three point lights above it. The grid is saturated blue (RGB 0,0,1) and the lights are saturated red, green and blue.
Not really a real world situation but it unveiled a strange performance regression with the Light Tree.
If you only have a single light visible during preview rendering all is fine and snappy. As soon as you have e.g. the blue and the red or all three of them visible, sampling around the non-blue light sources becomes extremely grainy and takes quite a while to resolve.
Switching the Light Tree off gives instant and clean results.
This also happens in 3.6.2 and with CPU rendering.
Exact steps for others to reproduce the error
I don't know if this is really a bug or just a corner case of how the Light Tree works.
This is a known limitation. When the light tree is sampled, different lights are assigned different importances based on their "perceived intensity" (based on the energy the light is emitting, and how far away the light is). At the moment, this calculation does not take into consideration the colours of the lights, or the shader of the object being sampled (with the exception of whether or not the shader on the object has a transmissive component or is a volumetric shader).
As a result of this, the ray hits the blue ground, and the the light tree calculates the importance of each light (ignoring the colours of the lights and the colour of the material being sampled), then samples the light. Due to it ignoring the colour factor, the red and green lights have high importance on the ground near them, meaning they get sampled significantly more than the blue light, which results in noise in this specific scene (Because the green and red lights contribute very little to the shading of the surface, while the blue light does. But the green and red lights still get assigned high importance).
Some work could be done to improve this (E.G. In the light importance calculator there is a "shader function approximation scaler" which scales the importance of lights based on the relationship between the shader and the light. At the moment it's always set to 1.0. But a function could be added to adjust the importance of different lights based on material and light colours.)
Note: The shader function approximation scaler is the variable
f_afound inlight_tree_importance()here: https://projects.blender.org/blender/blender/src/branch/main/intern/cycles/kernel/light/tree.h#L219I'll leave the final call on this report to @weizhen or @brecht (Keep it open as a known limitation, close the report, or implement a proper shader approximation to handle cases like this better.)
Thanks for the clarification. I never had any problems with this in real production scenes so far.
Performance regression with Light Tree onto Light tree more noisy with saturated light and surface colors