Cycles AOVs: Wrong, unclamped RGB values when Alpha/Transparency map used in Shader #75123
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Reference: blender/blender#75123
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System Information
Operating system: Linux-5.3.0-42-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GTX 780 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 435.21
Blender Version
Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-03-12 05:06, hash:
375c7dc4ca
Worked: (optional)
Short description of error
Custom AOVs write un-clamped, wrongly multiplied RGB values when a texture is used for alpha (Principled BSDF) or to mix with Transparent BSDF.
Exact steps for others to reproduce the error
Download and extract the attached zip file.
aov_alpha.zip
Open aov_alpha.blend and Render a still image.
In the image editor, use the "Sample" tool on the Render Result.
The G channel value is 2.0, exceeding the RGB range.
(See also "aov_alpha_screenshot.jpg")
For comparison:
Activate the alternative Material Output node (in the "Without Alpha" frame).
Hit render.
The AOV output is correct, not exceeding the clamp range.
For more complex scenes/AOVs:
The values will be wrongly added based on the erroneous alpha multiplier, e.g. for this Geometry Position based AOV:
Added subscriber: @Plyro
Added subscriber: @JohannesLubke
Added subscriber: @troy_s
There is no such thing as “exceeding RGB range” in various situations.
This isn’t the clearest bug report. What exactly is expected and what is causing the problem?
Added subscriber: @BartekMoniewski
I can confirm this.
Simplified file here:
aov_alpha_edit.blend
AOV alpha is 0.0 or 1.0 (+ AA) depending on mesh occlusion. Color values are added (binary, not multiplied by alpha) if main shader have alpha above 'Alpha Threshold' from Pass settings (docs.blender.org/manual/en/latest/render/layers/passes.html).
Alpha Threshold for AOV should be either hardcoded to 0.0 or samples should be blended, not added.
There is no way to render propper albedo pass from transparent shader without changing all other passes Alpha Threshold.
Changed status from 'Needs Triage' to: 'Confirmed'
Added subscriber: @lichtwerk
Can confirm, looks like a bug to me.
Added subscriber: @ThomasDinges
Still an issue with Blender 3.0. (Checking on older Cycles reports atm, that may be fixed with Cycles X).
Added subscriber: @ManuelGrad
Issue still present in Blender 3.4.