Cycles: Ray Visibility > Diffuse is different than using Is Diffuse Ray in shader #118484
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Reference: blender/blender#118484
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System Information
Operating system: Linux-6.6.3-arch1-1-x86_64-with-glibc2.38 64 Bits, X11 UI
Graphics card: Mesa Intel(R) HD Graphics 520 (SKL GT2) Intel 4.6 (Core Profile) Mesa 23.2.1-arch1.2
Blender Version
Broken: version: 4.1.0 Alpha, branch: main, commit date: 2024-01-25 14:57, hash:
ca767644ebe3Short description of error
In Cycles, using the Ray Visibility > Diffuse object property to make an object invisible to diffuse rays produces different results than using a Light Path node to do the same thing.
I expected these to be the same, but this could be a problem with my expectation too. It does work for Camera rays and the manual suggests the same effect can be achieved with nodes ("In terms of performance, using these options is more efficient than using a shader node setup that achieves the same effect.").
Exact steps for others to reproduce the error
Open this .blend and go into Rendered View: DiffuseRayTest.blend
The lower left plane is with Ray Visibility > Diffuse off. The lower right plane has a shader that uses Is Diffuse Ray to simulate it. The two upper planes show the same thing works for Camera rays (at least in this test).
Apparently emission rays are considered Diffuse by the Ray Visibility filter of an Object in Cycles, but is not considered diffuse by the Light Path Node.
Maybe it should be Diffuse in both cases?
Needs information from developers... Cc. @brecht.
For reference, this issue only occurs in the Next Event Estimation (NEE) part of the light sampling.
Looking at the code, this appears to be a known limitation, presumably from performance enhancing code.
Source:
af4b09a61c/intern/cycles/kernel/light/sample.h (L67)So what's happening in Cycles is:
integrate_surface_direct_light()).integrate_surface_direct_light()with the functionlight_sample_shader_eval())light_sample_shader_eval()we encounter the code from above, that passes NULL rather than path_flags tosurface_shader_eval().surface_shader_eval()being the function that figures out how the node setup of the light influences it's output.I'll leave the final decision on how to handle this report up to other people.
In the meantime I have a recommendation for @scurest. You can remove mesh lights from NEE to get the desired results. This does come at the cost of increased noise in various situations, but this might be a trade off you're willing to take. To do this, follow these steps:
Materials Propertiestab in the Properties editor.SettingsSurface, setEmission SamplingtoNone. The light will no longer be sampled by NEE, but can still be sampled by forward path tracing (which does work with your node setup)