Setting alpha channel in compositor affects RGB channels (white artifacts) #72740

New Issue

Daniel · 2019-12-28T00:27:12+01:00

Daniel commented

2019-12-28 00:27:12 +01:00

System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 432.00

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-12-04 11:32, hash: f1aa4d18d4
Worked: probably never

Short description of error
When combining RGB channels with alpha channels in the compositor, this generates white artifacts on the output RGB channels.

Exact steps for others to reproduce the error
Use some RGB image and either use the set alpha, separate rgba and combine rgba setup (as shown in the image) and connect this to the composite, viewer or some file output node for the viewer or compositor output one can also use the dedicated alpha output, all methods result in white artifacts on the RGB channels, this causes problems when one wants to use the compositor for channel packing involving the alpha channel (combining four greyscale maps into a single RGBA image).

alpha_fail_on_rgb.blend
(toggle the backdrop option in the compositor to trigger the processing)
Another example that uses actual render layers:
rendered_alpha_artifacts.blend

**System Information** Operating system: Windows-10-10.0.18362 64 Bits Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 432.00 **Blender Version** Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-12-04 11:32, hash: `f1aa4d18d4` Worked: probably never **Short description of error** When combining RGB channels with alpha channels in the compositor, this generates white artifacts on the output RGB channels. **Exact steps for others to reproduce the error** Use some RGB image and either use the set alpha, separate rgba and combine rgba setup (as shown in the image) and connect this to the composite, viewer or some file output node for the viewer or compositor output one can also use the dedicated alpha output, all methods result in white artifacts on the RGB channels, this causes problems when one wants to use the compositor for channel packing involving the alpha channel (combining four greyscale maps into a single RGBA image). ![image.png](https://archive.blender.org/developer/F8247003/image.png) [alpha_fail_on_rgb.blend](https://archive.blender.org/developer/F8248223/alpha_fail_on_rgb.blend) (toggle the backdrop option in the compositor to trigger the processing) Another example that uses actual render layers: [rendered_alpha_artifacts.blend](https://archive.blender.org/developer/F8248239/rendered_alpha_artifacts.blend)

Daniel commented

2019-12-28 00:27:12 +01:00

Added subscriber: @bestdani

Jacques Lucke commented

2019-12-28 13:52:51 +01:00

Added subscriber: @JacquesLucke

Jacques Lucke commented

2019-12-28 13:52:51 +01:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Jacques Lucke commented

2019-12-28 13:52:51 +01:00

I assume the issue here is that you use an alpha mode for the image that is not correct for your use case. I cannot check that because you did not upload the .blend file. Please always provide a simple test file with the images packed into it for reports like this.

The white border are usually related to premultiplied alpha afaik.

I assume the issue here is that you use an alpha mode for the image that is not correct for your use case. I cannot check that because you did not upload the .blend file. Please always provide a simple test file with the images packed into it for reports like this. The white border are usually related to premultiplied alpha afaik.

Daniel commented

2019-12-28 16:08:52 +01:00

I have just added a blend file with even some little bit simpler (and cc0) images.

It also reminds me of premultiplied alpha, but in this case it happens on the output images and there's no option to change the alpha behavior at all for this afaik.

I'm also not sure if this is intended or not, so if this is an actual bug or not. At least I was not expecting that setting an alpha channel like that would affect the RGB channels of an image and I guess in many use cases where one wants to pack for example roughness together with a rendered / baked albedo or normal into a single RGBA image this is probably not what is wanted and expected to happen.

I have just added a blend file with even some little bit simpler (and cc0) images. It also reminds me of premultiplied alpha, but in this case it happens on the output images and there's no option to change the alpha behavior at all for this afaik. I'm also not sure if this is intended or not, so if this is an actual bug or not. At least I was not expecting that setting an alpha channel like that would affect the RGB channels of an image and I guess in many use cases where one wants to pack for example roughness together with a rendered / baked albedo or normal into a single RGBA image this is probably not what is wanted and expected to happen.

Jacques Lucke commented

2019-12-29 12:09:10 +01:00

Changed status from 'Needs User Info' to: 'Needs Triage'

Jacques Lucke commented

2019-12-29 12:09:10 +01:00

Added subscriber: @Jeroen-Bakker

Jacques Lucke commented

2019-12-29 12:09:10 +01:00

Unfortunately, I also don't know that. @Jeroen-Bakker might be able to answer that color management question for the compositor.

Richard Antalik commented

2020-01-17 16:17:30 +01:00

Added subscriber: @iss

Richard Antalik commented

2020-01-17 16:17:30 +01:00

looks like quite different issue, but similar in result #69106 (White outline around alpha in texture paint (when using multiple layers))

Brecht Van Lommel commented

2020-01-18 18:30:23 +01:00

Added subscriber: @brecht

Brecht Van Lommel commented

2020-01-18 18:30:23 +01:00

Changed status from 'Needs Triage' to: 'Archived'

Brecht Van Lommel closed this issue

2020-01-18 18:30:23 +01:00

Brecht Van Lommel self-assigned this 2020-01-18 18:30:23 +01:00

Brecht Van Lommel commented

2020-01-18 18:30:23 +01:00

The Blender compositor uses premultiplied / associated alpha. That means if you want to apply an alpha mask to an image, that must affect not only A, but also be multiplied into the R, G, B channels.

Rather than manually manipulating the channels like this, it's easier to use for example an Alpha Over node to to mix between a given image and a fully transparent RGBA color.

The Blender compositor uses premultiplied / associated alpha. That means if you want to apply an alpha mask to an image, that must affect not only A, but also be multiplied into the R, G, B channels. Rather than manually manipulating the channels like this, it's easier to use for example an Alpha Over node to to mix between a given image and a fully transparent RGBA color.

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Setting alpha channel in compositor affects RGB channels (white artifacts) #72740