reducing principled IOR which should only affect transmission, is also incorrectly modifying the reflection IOR and reducing the affect of reflections. #110476

New Issue

michael campbell · 2023-07-25T20:47:43+02:00

michael campbell commented

2023-07-25 20:47:43 +02:00

System Information
Operating system: Windows-10-10.0.19044-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 535.98

Blender Version
Broken: version: 4.0.0 Alpha, branch: main, commit date: 2023-06-30 00:04, hash: e73a9a6a7d7b
Worked: (unknown)

Short description of error
setting ior low reduces reflections.

Exact steps for others to reproduce the error

Open attached blend file, turn on cycles viewport. The reflection is missing because transmissive IOR is set low. I understand the glossy IOR (which is automatically calculated in this pbr workflow, rather than being exposed) cant be less than the transmissive IOR to avoid parts of the material rendering black, but it should be allowed to be higher so it can retain reflections regardless of the transmission IOR being low.

Double checked the manual and it does confirm it should only affect transmission. When building shaders manually, I decouple the two IOR's and then limit the glossy IOR to never fall below the transmission IOR but allow it to be turned up higher.

In the attached file, increasing the ior or turning off transmission will bring the reflections back.

Cheers 👍

**System Information** Operating system: Windows-10-10.0.19044-SP0 64 Bits Graphics card: NVIDIA GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 535.98 **Blender Version** Broken: version: 4.0.0 Alpha, branch: main, commit date: 2023-06-30 00:04, hash: `e73a9a6a7d7b` Worked: (unknown) **Short description of error** setting ior low reduces reflections. **Exact steps for others to reproduce the error** Open attached blend file, turn on cycles viewport. The reflection is missing because transmissive IOR is set low. I understand the glossy IOR (which is automatically calculated in this pbr workflow, rather than being exposed) cant be less than the transmissive IOR to avoid parts of the material rendering black, but it should be allowed to be higher so it can retain reflections regardless of the transmission IOR being low. Double checked the manual and it does confirm it should only affect transmission. When building shaders manually, I decouple the two IOR's and then limit the glossy IOR to never fall below the transmission IOR but allow it to be turned up higher. In the attached file, increasing the ior or turning off transmission will bring the reflections back. Cheers 👍

IOR breaks reflection roughness.blend

6.8 MiB

michael campbell added the

labels 2023-07-25 20:47:44 +02:00

michael campbell changed title from ~~reducing IOR which should only affect transmission, is also incorrectly modifying the reflection IOR and reducing the affect of reflections.~~ to reducing principled IOR which should only affect transmission, is also incorrectly modifying the reflection IOR and reducing the affect of reflections.

2023-07-25 20:48:09 +02:00

YimingWu commented

2023-07-26 02:32:12 +02:00

Interesting find... I'm not sure if this effect is related to how refraction could turn gradually into full-reflection physically, so adjusting refraction IOR should indeed affect reflection?

Interesting find... I'm not sure if this effect is related to how refraction could turn gradually into full-reflection physically, so adjusting refraction IOR _should_ indeed affect reflection?

michael campbell commented

2023-07-26 02:51:43 +02:00

I believe it's correct that the reflections should become less visible when decreasing the IOR , but in 3d, providing the glossy IOR doesn't drop below the refractive IOR, then you're golden (avoids black areas with neither glossy or transmission). Handy for architectural glass and plastic bottles etc if you don't want to build from individual nodes with layer weight + transparent/glossy bsdf's.

The only reason I bring it up is because the decoupling has been super useful in other engines, and the Blender user manual suggested it should also only affect refraction rather than reflection.

I believe it's correct that the reflections should become less visible when decreasing the IOR , but in 3d, providing the glossy IOR doesn't drop below the refractive IOR, then you're golden (avoids black areas with neither glossy or transmission). Handy for architectural glass and plastic bottles etc if you don't want to build from individual nodes with layer weight + transparent/glossy bsdf's. The only reason I bring it up is because the decoupling has been super useful in other engines, and the Blender user manual suggested it should also only affect refraction rather than reflection.

Pratik Borhade commented

2023-07-26 07:55:50 +02:00

The reflection is missing because transmissive IOR is set low

Hi, thanks for the report. I can confirm. Looks like the material is dark when IOR is close to 1.
AFAIK from physics, IOR of air is 1 and for air I don't expect any reflection to happen.
I maybe wrong here, checking further.

> The reflection is missing because transmissive IOR is set low Hi, thanks for the report. I can confirm. Looks like the material is dark when IOR is close to 1. AFAIK from physics, IOR of air is 1 and for air I don't expect any reflection to happen. I maybe wrong here, checking further.

Pratik Borhade commented

2023-07-26 08:23:29 +02:00

reducing principled IOR... reduces the affect of reflections

This is likely expected as said above (but not sure about complete dark result). Also, reflection/refraction is also controlled by ior value, see:
https://projects.blender.org/blender/blender/src/branch/main/source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl#L61

  float fresnel = (do_multiscatter != 0.0) ? btdf_lut(NV, roughness, ior).y : F_eta(ior, NV);
  float glass_reflection_weight = fresnel * transmission;
  float glass_transmission_weight = (1.0 - fresnel) * transmission;

@brecht hi, can you check whether this is a bug?

> reducing principled IOR... reduces the affect of reflections This is likely expected as said above (but not sure about complete dark result). Also, reflection/refraction is also controlled by ior value, see: https://projects.blender.org/blender/blender/src/branch/main/source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl#L61 ``` float fresnel = (do_multiscatter != 0.0) ? btdf_lut(NV, roughness, ior).y : F_eta(ior, NV); float glass_reflection_weight = fresnel * transmission; float glass_transmission_weight = (1.0 - fresnel) * transmission; ``` @brecht hi, can you check whether this is a bug?

Pratik Borhade added

Module

Render & Cycles

Status

Needs Info from Developers

and removed

Status

Needs Triage

labels 2023-07-26 08:23:42 +02:00

Brecht Van Lommel commented

2023-07-26 12:36:45 +02:00

This is by design. I don't think the manual contradicts it either, it says the IOR is for transmission and enabling transmission should affect multiple components for energy conservation. Beyond that this is a feature request outside the scope of the bug tracker.

Brecht Van Lommel closed this issue

2023-07-26 12:36:47 +02:00

Blender Bot added

Status

Archived

and removed

Status

Needs Info from Developers

labels 2023-07-26 12:36:49 +02:00

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