Cycles: Certain Emission Sampling settings will render differently between Light Tree On vs Off #115997

New Issue

Alaska · 2023-12-10T07:59:43+01:00

Alaska commented

2023-12-10 07:59:43 +01:00

System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 4090/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 537.42

Blender Version
Broken versions:
3.6.5
4.0.1, branch: blender-v4.0-release, commit date: 2023-11-16 16:40, hash: d0dd92834a08
Worked: Not Sure

Short description of error
Cycles has a feature for selecting the emissive mesh sampling method. The option is for picking whether the front, back, both, or neither, of the triangle should be included in direct light sampling.

When this setting is configured improperly (E.G. Selecting "Back" when only the Front of the triangle is facing the object you're lighting up), then the Light Tree and non-Light Tree sampling method will render differently. Specifically the non-Light Tree will render the light brighter than expected. Here are some images demonstrating that:

Light Tree On	Light Tree Off

This is unexpected behaviour. And quick testing points to the front of the mesh being sampled by both direct light sampling and random sampling and being added together rather than "blended" and this is the reason for why the "Back + Light Tree Off" option renders brighter. But I could be wrong.

Exact steps for others to reproduce the error
Mesh Sampling Light Tree Test File.blend

Download the .blend file above.
Switch between different configurations of Mesh Emission Sampling and Light Tree On vs Off and compare them, taking note of their brightness.

In the .blend file attached, "Back" emission sampling and Light Tree Off renders brighter than expected. All other combinations are fine.

Note: The Light Tree option can be found at Render Proerties -> Sampling -> Lights -> Light Tree
Note: The mesh emission sampling option can be found at Material Properties -> The emissive material -> Settings -> Surface -> Emission Sampling

**System Information** Operating system: Windows-10-10.0.22621-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 4090/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 537.42 **Blender Version** Broken versions: 3.6.5 4.0.1, branch: blender-v4.0-release, commit date: 2023-11-16 16:40, hash: `d0dd92834a08` Worked: Not Sure **Short description of error** Cycles has a feature for selecting the emissive mesh sampling method. The option is for picking whether the front, back, both, or neither, of the triangle should be included in direct light sampling. When this setting is configured improperly (E.G. Selecting "Back" when only the Front of the triangle is facing the object you're lighting up), then the Light Tree and non-Light Tree sampling method will render differently. Specifically the non-Light Tree will render the light brighter than expected. Here are some images demonstrating that: |Light Tree On|Light Tree Off| |-|-| |![Light Tree On.jpg](/attachments/3e27aa99-de57-44ff-a204-7aa566b5dabd)|![Light Tree Off.jpg](/attachments/e61d8e0f-acf7-4c51-b5bd-ba053b0aa200)| This is unexpected behaviour. And quick testing points to the front of the mesh being sampled by both direct light sampling and random sampling and being added together rather than "blended" and this is the reason for why the "Back + Light Tree Off" option renders brighter. But I could be wrong. **Exact steps for others to reproduce the error** [Mesh Sampling Light Tree Test File.blend](/attachments/e2b85838-122f-45fd-a35f-4ffd931c8a0d) 1. Download the .blend file above. 2. Switch between different configurations of Mesh Emission Sampling and Light Tree On vs Off and compare them, taking note of their brightness. In the .blend file attached, "Back" emission sampling and Light Tree Off renders brighter than expected. All other combinations are fine. Note: The Light Tree option can be found at `Render Proerties -> Sampling -> Lights -> Light Tree` Note: The mesh emission sampling option can be found at `Material Properties -> The emissive material -> Settings -> Surface -> Emission Sampling`

Mesh Sampling Light Tree Test File.blend

959 KiB

Light Tree Off.jpg

120 KiB

Light Tree On.jpg

154 KiB

Alaska added the

labels 2023-12-10 07:59:44 +01:00

Alaska changed title from ~~Cycles: Incorrect Emission Sampling settings will render differently between Light Tree On vs Off~~ to Cycles: Certain Emission Sampling settings will render differently between Light Tree On vs Off

2023-12-10 13:20:28 +01:00

Pratik Borhade commented

2023-12-12 12:01:39 +01:00

@Alaska hi, can confirm when light tree is disabled.

And quick testing points to the front of the mesh being sampled by both direct light sampling and random sampling as the reason for why the "Back + Light Tree Off" option renders brighter

Not sure. Because same "bright render" I observed when backface of the emissive surface was facing the plane (and emissive sampling was set to "Front").

@Alaska hi, can confirm when light tree is disabled. > And quick testing points to the front of the mesh being sampled by both direct light sampling and random sampling as the reason for why the "Back + Light Tree Off" option renders brighter Not sure. Because same "bright render" I observed when backface of the emissive surface was facing the plane (and emissive sampling was set to "Front").

Pratik Borhade added

and removed

labels 2023-12-12 12:01:52 +01:00

Alaska commented

2023-12-12 12:09:30 +01:00

And quick testing points to the front of the mesh being sampled by both direct light sampling and random sampling as the reason for why the "Back + Light Tree Off" option renders brighter

Not sure. Because same "bright render" I observed when backface of the emissive surface was facing the plane (and emissive sampling was set to "Front").

For the scene I was testing, if the front of a triangle was facing a diffuse surface, and the emission sampling setting was set to back, then the issue would occur.

But if you then rotate the mesh such that the back of the triangle was facing the diffuse surface, and you change the emission sampling option to Front, the same issue will occur.

So basically, the issue only occurs if the Emission Sampling setting is set to the opposite of the direction the mesh light is facing relative to the object being lit up.

> > And quick testing points to the front of the mesh being sampled by both direct light sampling and random sampling as the reason for why the "Back + Light Tree Off" option renders brighter > > Not sure. Because same "bright render" I observed when backface of the emissive surface was facing the plane (and emissive sampling was set to "Front"). For the scene I was testing, if the front of a triangle was facing a diffuse surface, and the emission sampling setting was set to back, then the issue would occur. But if you then rotate the mesh such that the back of the triangle was facing the diffuse surface, and you change the emission sampling option to Front, the same issue will occur. So basically, the issue only occurs if the `Emission Sampling` setting is set to the opposite of the direction the mesh light is facing relative to the object being lit up.

Pratik Borhade commented

2023-12-12 12:12:58 +01:00