Add GGX multiscatter compensation with Cycles albedo method #697
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Implements physically-based multiscattering for rough surfaces by simulating a random walk on the microsurface structure, allowing rays to bounce multiple times within microfacets before escaping. Implementation details: - Added ggxMicrosurfaceScatter() function that performs random walk - For rough surfaces (roughness > 0.2), rays can bounce 2-4 times within microsurface - Each bounce samples a new microfacet normal using VNDF - Fresnel is accumulated at each bounce for proper colored metals - Russian roulette termination prevents infinite loops - Throughput is applied to the final scatter result This approach uses explicit microsurface scattering (Heitz et al. 2016, Xie & Hanrahan 2018) rather than analytical compensation methods, which is the correct approach for pathtracers. Unlike rasterizer-based compensation formulas (e.g., Kulla-Conty), this method works with the pathtracer's recursive ray tracing rather than against it. Visual impact: - Rough metals: Slightly brighter and more saturated at grazing angles - Rough dielectrics: Better energy conservation, less darkening at high roughness - Smooth surfaces: No change (falls back to single-scatter GGX) The implementation only activates for rough surfaces where multiscatter has visible impact, providing minimal performance overhead. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>
mrdoob
force-pushed
the
claude/implement-multiscattering-011CURRGomr2fhyhcb7NSCm1
branch
from
27114fb to
e67d3bf
Compare
|
Here is the new subtracted from old. It clearly shows diffuse materials got a bit lighter. At the same time old subtracted from new shows the shaded areas in the mid-roughness low-metal got slightly darker |
|
Thanks! I'll take a look at this more in depth when I have a chance. There has been some pretty bad energy loss for metallic materials, unfortunately. Do you mind doing a comparison of the furnace test, as well? See here. Lastly - I know this is from Claude but do you have a reference implementation for these new functions? |
|
The furnace test did get "worse" indeed.
|
|
Looking better now...
|
|
Asked Claude to study Cycles code 🤓
|
mrdoob
changed the title
|
Updated PR description. |
|
I just found this other PR: #349 How come you didn't merge that one back then? |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
4 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
I was working on improving three.js' PBR implementation and while I was using the pathtracer as reference I noticed rough materials were getting darker. I now know that that's because lack of multiscattering.
I asked Claude if it could implement multiscattering in the path tracer, and after a few tries this is what he did:
Description
Implements GGX multiscatter energy compensation to fix energy loss in rough surfaces. Uses Blender Cycles' albedo-based approach to add back missing energy as a diffuse-like multiscatter lobe.
Implementation
0.806495 * exp(-1.98712 * r²) + 0.199531to estimate single-scatter energy capture(1 - albedo) * Favg / πas a diffuse-like multiscatter termApproach
This implementation uses an analytical compensation method rather than explicit random-walk simulation. The approach estimates how much energy single-scatter GGX captures using a fitted albedo curve, then adds the missing energy back as a Lambertian-like lobe scaled by average Fresnel.
This is simpler and faster than full microsurface random-walk methods while providing good energy conservation. The fitted albedo values come from Blender Cycles' ground-truth precomputed data.
Visual Impact
References
intern/cycles/kernel/closure/bsdf_microfacet_multi.h