Cycles: small light sources with multiple importance sampling enabled breaks the Huang BSDF #113728

New Issue

Fernando Alcala · 2023-10-14T20:31:08+02:00

Fernando Alcala commented

2023-10-14 20:31:08 +02:00

System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 536.23

Blender Version
Broken: version: 4.0.0 Beta, branch: blender-v4.0-release, commit date: 2023-10-06 00:22, hash: 887570065fa4

Short description of error
Small area lights, small non-zero radius spot and point lights, small mesh lights, or a sun light with multiple importance sampling enabled causes spontaneous fireflies and unresolvable noise with the Huang BSDF.

Exact steps for others to reproduce the error
Use the animal fur example blend.
Turn off denoising.
Change to object mode, select the hair, open a shader editor, and change Chiang to Huang.
Render and notice the fireflies.
Turn multiple importance sampling off for the "Key" light.
Render and notice no fireflies.

Chiang MIS	Huang MIS	Haung MIS off

It is most apparent with light hair colors.

Why do I say it is unresolvable? Here is a 40,000sample render with "Key" multiple importance sampling on, it is still noisy. Interestingly, the denoised image still has some noise.

noisy	denoised	noisy high contrast

I know Weizhen isn't here.

**System Information** Operating system: Windows-10-10.0.19045-SP0 64 Bits Graphics card: NVIDIA GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 536.23 **Blender Version** Broken: version: 4.0.0 Beta, branch: blender-v4.0-release, commit date: 2023-10-06 00:22, hash: `887570065fa4` **Short description of error** Small area lights, small non-zero radius spot and point lights, small mesh lights, or a sun light with multiple importance sampling enabled causes spontaneous fireflies and unresolvable noise with the Huang BSDF. **Exact steps for others to reproduce the error** Use the animal fur example blend. Turn off denoising. Change to object mode, select the hair, open a shader editor, and change Chiang to Huang. Render and notice the fireflies. Turn multiple importance sampling off for the "Key" light. Render and notice no fireflies. | Chiang MIS| Huang MIS | Haung MIS off| |--|--|--| |![chiang MIS on.png](/attachments/9aba07d4-ba5e-4cd7-871e-d76c40814e25) |![huang MIS on.png](/attachments/4226c47d-a9d9-4101-85ca-e809bf6fe5e0) |![huang MIS off.png](/attachments/2ff569df-4e44-4a5d-bca4-893ea44a2a24) | It is most apparent with light hair colors. Why do I say it is unresolvable? Here is a 40,000sample render with "Key" multiple importance sampling on, it is still noisy. Interestingly, the denoised image still has some noise. | noisy | denoised | noisy high contrast | |--|--|--| |![melanin0.25 light path defaults MIS on 40000samples.png](/attachments/bc8f76f5-670e-43fe-bbd0-a2850630c9ce) |![melanin0.25 light path defaults MIS on 40000samples denoised.png](/attachments/7ec773d0-1936-4de4-8262-cd9a92a018ba) |![melanin0.25 light path defaults MIS on 40000samples vhc1.7.png](/attachments/c2e725a6-8a60-4dd4-a372-6fa4ccfa254b) | I know Weizhen isn't here.

huang MIS off.png

558 KiB

huang MIS on.png

560 KiB

melanin0.25 light path defaults MIS on 40000samples.png

1.1 MiB

melanin0.25 light path defaults MIS on 40000samples vhc1.7.png

1.3 MiB

chiang MIS on.png

539 KiB

melanin0.25 light path defaults MIS on 40000samples denoised.png

996 KiB