Cycles Blackbody does not correctly pick up the scene_linear color space #106201

New Issue

Jason Clarke · 2023-03-28T05:51:51+02:00

Jason Clarke commented

2023-03-28 05:51:51 +02:00

System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 531.18

Blender Version
Broken: version: 3.5.0 Beta, branch: blender-v3.5-release, commit date: 2023-03-17 23:05, hash: 73509d1bc1cb

Short description of error
Cycles' blackbody shader does not produce correct/expected results when used with a wide-gamut render space such as ACEScg. It appears to be outputting linear-sRGB values even when the rendering space is some other color space.

Here's a sample file using the default color config: (output using ACES "Utility - Linear - sRGB" > "Output - sRGB"

Here's a sample file using the ACES 1.3 config: (output using ACES "ACES - ACEScg" > "Output - sRGB"

The saturation difference between these two renders is not expected. (and indeed, it does not occur with the sky texture! The sky texture produces very similar results with both configs)

Treating the ACEScg render as linear sRGB produces a much more correct result, suggesting the blackbody shader is outputting values in linear sRGB rather than the space defined by the OCIO scene_linear role:

(output using ACES "Utility - Linear - sRGB" > "Output - sRGB", even though the source image is ACEScg)

One would expect that treating an ACEScg render as linear sRGB would produce a very desaturated result compared to an actual sRGB render, not a more similar result! (indeed, with the sky texture you get a more desaturated result, exactly as expected)

Exact steps for others to reproduce the error
Here's the blend file these were generated from: blackbody_bar.blend

This should produce fairly similar results between the stock config and ACES 1.3 configs here: https://github.com/AcademySoftwareFoundation/OpenColorIO-Config-ACES/releases/tag/v1.0.0

Instead, it produces much more saturated results with ACES

Compare to this file with the sky texture, which DOES produce similar results with both configs: sky_test.blend

According to #68926 both the sky texture and the blackbody texture should be picking up the scene_linear space. However, it seems that only the sky texture is actually doing so.

**System Information** Operating system: Windows-10-10.0.22621-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 3080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 531.18 **Blender Version** Broken: version: 3.5.0 Beta, branch: blender-v3.5-release, commit date: 2023-03-17 23:05, hash: `73509d1bc1cb` **Short description of error** Cycles' blackbody shader does not produce correct/expected results when used with a wide-gamut render space such as ACEScg. It appears to be outputting linear-sRGB values even when the rendering space is some other color space. Here's a sample file using the default color config: ![blackbody_bar_sRGB0000.jpg](/attachments/dd951ae6-c9e2-4cd0-b601-ce16429e9e46) (output using ACES "Utility - Linear - sRGB" > "Output - sRGB" Here's a sample file using the ACES 1.3 config: ![blackbody_bug_ACEScg0000.jpg](/attachments/87ca6de9-ab10-47fd-ab8b-e4f5d2fea212) (output using ACES "ACES - ACEScg" > "Output - sRGB" The saturation difference between these two renders is not expected. (and indeed, it does not occur with the sky texture! The sky texture produces very similar results with both configs) Treating the ACEScg render as linear sRGB produces a much more correct result, suggesting the blackbody shader is outputting values in linear sRGB rather than the space defined by the OCIO scene_linear role: ![blackbody_bug_aces_treated_as_sRGB0000.jpg](/attachments/5bd47a50-3737-44a2-b343-487fa49eb75a) (output using ACES "Utility - Linear - sRGB" > "Output - sRGB", even though the source image is ACEScg) One would expect that treating an ACEScg render as linear sRGB would produce a very desaturated result compared to an actual sRGB render, not a more similar result! (indeed, with the sky texture you get a more desaturated result, exactly as expected) **Exact steps for others to reproduce the error** Here's the blend file these were generated from: [blackbody_bar.blend](/attachments/2430fd33-e439-4dac-8905-85cde3e784f1) This should produce fairly similar results between the stock config and ACES 1.3 configs here: https://github.com/AcademySoftwareFoundation/OpenColorIO-Config-ACES/releases/tag/v1.0.0 Instead, it produces much more saturated results with ACES Compare to this file with the sky texture, which DOES produce similar results with both configs: [sky_test.blend](/attachments/83b790e5-30cb-4475-ae79-1492a92d7b3a) According to https://projects.blender.org/blender/blender/issues/68926 both the sky texture and the blackbody texture should be picking up the scene_linear space. However, it seems that only the sky texture is actually doing so.

sky_test.blend

788 KiB

blackbody_bar.blend

836 KiB

blackbody_bar_sRGB0000.jpg

95 KiB

blackbody_bug_aces_treated_as_sRGB0000.jpg

87 KiB

blackbody_bug_ACEScg0000.jpg

98 KiB

Jason Clarke added the

labels 2023-03-28 05:51:51 +02:00

Zijun Zhou commented

2023-03-29 19:45:05 +02:00

Comparing formed images by different view transforms doesn't actually show whether it works corectly or not. You need to export EXR and plot the chromaticity to see it.

As I tested, the Blackbody in latest 3.6 is working properly with a BT.2020 working space:

It has been working since this bug was taken care of: #98036

Comparing formed images by different view transforms doesn't actually show whether it works corectly or not. You need to export EXR and plot the chromaticity to see it. As I tested, the Blackbody in latest 3.6 is working properly with a BT.2020 working space: ![image](/attachments/67e03aee-3c02-4f34-ae0d-777b726b12b9) It has been working since this bug was taken care of: https://projects.blender.org/blender/blender/issues/98036

image.png

56 KiB

Jason Clarke closed this issue

2023-03-29 20:08:13 +02:00

Blender Bot added

and removed

labels 2023-03-29 20:08:15 +02:00

Sign in to join this conversation.