compositor rgb curve node is modifying the alpha channel. #110384

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michael campbell · 2023-07-22T18:54:23+02:00

michael campbell commented

2023-07-22 18:54:23 +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: 3.6.0, branch: blender-v3.6-release, commit date: 2023-06-27 08:08, hash: c7fc78b81ecb
Worked: (unknown)

Short description of error
compositor rgb curve node is modifying the alpha channel.

Exact steps for others to reproduce the error

open attached file
go to the compositor
render a still frame
mute and unmute the rgb curve node

You'll notice the white background which is added by the alpha over node, becomes grey when the rgb curve node is un-muted, indicating the rgb curve node is actually modifying the alpha channel rather than just the RGB channels as expected. It can be worked around using the alpha into the fac of the alpha over node, but that defeats the purpose of an alpha over node.

**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: 3.6.0, branch: blender-v3.6-release, commit date: 2023-06-27 08:08, hash: `c7fc78b81ecb` Worked: (unknown) **Short description of error** compositor rgb curve node is modifying the alpha channel. **Exact steps for others to reproduce the error** 1. open attached file 2. go to the compositor 3. render a still frame 4. mute and unmute the rgb curve node You'll notice the white background which is added by the alpha over node, becomes grey when the rgb curve node is un-muted, indicating the rgb curve node is actually modifying the alpha channel rather than just the RGB channels as expected. It can be worked around using the alpha into the fac of the alpha over node, but that defeats the purpose of an alpha over node.

rgb curve node breaking alpha.blend

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michael campbell added the

labels 2023-07-22 18:54:24 +02:00

Evan Wilson commented

2023-07-22 21:29:51 +02:00

Thank you for the report @3di.

The curve node is set to Extend Extrapolated. The line continues straight, making the color at X=0 negative, instead of 0 as expected. Background of 1.0 white plus a negative foreground color ends up gray.

To fix this, either set the Extend mode to Extend Horizontal, or add another point to the curve at (0, 0).

Does that make sense?

Thank you for the report @3di. The curve node is set to `Extend Extrapolated`. The line continues straight, making the color at X=0 negative, instead of 0 as expected. Background of 1.0 white plus a negative foreground color ends up gray. To fix this, either set the Extend mode to `Extend Horizontal`, or add another point to the curve at (0, 0). Does that make sense?

third_point.png

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Extend Extrapolated.png

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Extend Horizontal.png

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Evan Wilson added

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labels 2023-07-22 21:30:04 +02:00

michael campbell commented

2023-07-22 22:45:00 +02:00

Hi, thanks, but the issue is the RGB curve node shouldn't be affecting the Alpha channel at all, it's meant specifically for RGB channels only as the name suggests. You would never want the alpha to change when you're modifying the colour channels. When you want to modify the Alpha then you would connect the alpha pass to an RGB curve node or similar, and modify prior to a set alpha node to replace the alpha that's burned into the combined pass.

michael campbell commented

2023-07-22 22:53:07 +02:00

Additionally, you generally need the curve to be extend extrapolated for correct results on the RGB channels. So setting it to Extend horizontal isn't possible.

Extend horizontal = incorrect results (see parade scope on the right)

Extend extrapolated = correct results (except the alpha is now broken):

Additionally, you generally need the curve to be extend extrapolated for correct results on the RGB channels. So setting it to Extend horizontal isn't possible. Extend horizontal = incorrect results (see parade scope on the right) ![image](/attachments/7b91d26b-4dfe-4035-8aa1-155222ee3a56) Extend extrapolated = correct results (except the alpha is now broken): ![image](/attachments/f2f8401e-a6ac-46be-badb-26cff00a064b)

image.png

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image.png

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Evan Wilson commented

2023-07-22 22:58:38 +02:00

The curve node isn't modifying the alpha channel. Looking at the output of the curve node alone (by right-clicking the alpha section in the Image Viewer window), you can see that Alpha is still 0.0, while the RGB values are -0.56849. It is the adding of this negative color value that turns the output gray, not a change in the alpha value.

Negative_color.png

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Gray_result.png

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michael campbell commented

2023-07-22 23:09:30 +02:00

hmm, I've just checked, and the alpha can display as a negative. So hmm, something's not right. Why would the alpha over show gray if the alpha is set to 0? It should be letting pure white though where the alpha is 0.

michael campbell commented

2023-07-22 23:13:56 +02:00

Perhaps the bug is that if the colour channels are negative, then the alpha over node doesn't respect the alpha's value? Alpha 0 should mean the alpha over uses the background completely, regardless of the rgb values of the foreground image right?

Evan Wilson commented

2023-07-23 01:17:29 +02:00

Additionally, you generally need the curve to be extend extrapolated for correct results on the RGB channels. So setting it to Extend horizontal isn't possible.

Ah yes, I was only just referring to a fix for your example case, and not the general case. Adding an addition point at (0, 0) on the curve widget would be the more general fix.

> Additionally, you generally need the curve to be extend extrapolated for correct results on the RGB channels. So setting it to Extend horizontal isn't possible. Ah yes, I was only just referring to a fix for your example case, and not the general case. Adding an addition point at (0, 0) on the curve widget would be the more general fix.

Evan Wilson commented

2023-07-23 01:21:30 +02:00

Perhaps the bug is that if the colour channels are negative, then the alpha over node doesn't respect the alpha's value? Alpha 0 should mean the alpha over uses the background completely, regardless of the rgb values of the foreground image right?

Ah, this not the case,. Alpha 0 simply means that it doesn't block the background color from coming though, and says nothing about the foreground colors (unless the box is checked, or the premultiply sliders are used, both of which are legacy options.)

> Perhaps the bug is that if the colour channels are negative, then the alpha over node doesn't respect the alpha's value? Alpha 0 should mean the alpha over uses the background completely, regardless of the rgb values of the foreground image right? Ah, this not the case,. Alpha 0 simply means that it doesn't block the background color from coming though, and says nothing about the foreground colors (unless the box is checked, or the `premultiply` sliders are used, both of which are legacy options.)

michael campbell commented

2023-07-23 01:43:49 +02:00

Oh I see. I misunderstood how the alpha over node actually worked then. Sorry about that.

michael campbell closed this issue

2023-07-23 01:44:28 +02:00

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labels 2023-07-23 01:44:29 +02:00