Compositing: viewer node doesn't correctly preview 'set alpha' output #83353
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Reference: blender/blender#83353
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System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce RTX 2060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.23
Graphics card: Radeon RX550/550 Series ATI Technologies Inc. 4.5.13596 Core Profile Context 20.11.3 27.20.11027.5002
Blender Version
Broken: version: 2.92.0 Alpha, branch: master, commit date: 2020-12-09 08:14, hash:
3a23f18e02Short description of error
See screenshot below. Expected result: the circled viewer shows the cube silhouette in red. Instead we see solid red output
When the output of the set alpha node is connected to the composite node the viewer does display the expected result.
set_alpha_bug.blend
Added subscriber: @info-27
Compositing: viewer node doesn't correctly preview imageto Compositing: viewer node doesn't correctly preview 'set alpha' outputAdded subscriber: @rjg
Changed status from 'Needs Triage' to: 'Needs User Info'
I can't reproduce this is the release version of 2.91.0 and you seem to be using a pre-release from 2020-10-15. Please download the release version and check if you can still reproduce the problem.
I opened the test file in version: 2.91.0, branch: master, commit date: 2020-11-25 08:34, hash:
0f45cab862. I get the same result as in the opening post.Changed status from 'Needs User Info' to: 'Needs Developer To Reproduce'
Changed status from 'Needs Developer To Reproduce' to: 'Confirmed'
Added subscriber: @iss
I can reproduce getting solid red image in both cases. I would suspect though that this is because I am using slightly buggy GPU driver. So I am not sure if this is bug in Blender.
This issue was referenced by
9dbea1db66Added subscriber: @temeddix
I have the same problem. The problem lies in not only the viewer node but also image editor. It doesn't show the alpha correctly.
I'll attach the file that describes the problem. Hit render and you'll see what I mean.
icon.blend
To Summarize (2.91):
Screenshot:
Saved Image:
The way Set Alpha works should be addressed by
9dbea1db66. Together with D10110 applied or manually replacing the old Set Alpha with the new one gives correct results for both the project from the original report and @temeddix.I believe this can be closed as resolved, but I would suggest @info-27 and @temeddix to test Blender 2.93/2.92 once the patch D10110 has landed and see if this fixes the issue for you as well.
I've done some tests and it seems the problem still remains in 2.92.
Using 'Replace Alpha' option in the new 'Set Alpha' node still makes the image editor show the wrong alpha.
Backdrop alpha was fine in 2.91, but now it's not in 2.92. It became broken.
Summarize(2.92):
Screenshot:
Saved Image File:
Based on your description and the tests on my own system the node behaves correct. The output from the RGB Curve node's color socket has RGB values greater zero in the areas where the result is supposed to be transparent. When the Set Alpha node is used with mode Replace, this only modifies the values in the alpha channel. Keep in mind the compositor is using premultiplied alpha, meaning that the output of Set Alpha in the outer area now have fully transparent but emissive pixels similar to a candle flame. That is the case because the pixels have alpha set to zero, but RGB value that aren't zero. Consequently you still see color displayed in the output despite the alpha being zero as this is not a screen-referred / straight alpha workflow in Blender. You will still see the "expected" result when exporting to a format like PNG though, which uses doesn't support premultiplied alpha and uses straight alpha instead.
What you actually want is to mask the outer pixels, so the RGB values need to be set to zero as well. That is why you need to use the Apply Mask mode for Set Alpha. This also sets the RGB values to zero, masking them out.
@info-27 @iss This is also the case for your original post. When I said that I couldn't reproduce, I should've specified that I always get a completely red square. As explained above this is the expected result when the alpha is just replaced (Replace was the mode that the old node always used).
Changed status from 'Confirmed' to: 'Resolved'
@info-27 Should the viewer node incorrectly display the background as transparent when it actually isn't please let us know and I can reopen the ticket.