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Image Editor: Alpha (like Bloom) not showing properly again
Closed, ResolvedPublicBUG


System Information
Operating system: Linux-5.4.0-7642-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: Quadro RTX 4000 with Max-Q Design/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 440.100

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-21 08:49, hash: rBd72b5785eb02
Worked: 2.91.0 Alpha around September 15th

Short description of error
The improvement done recently as part of the Image Editor refactor, that shows correct alpha values is no longer working.

I made a video about it on September 15th, compiled that same day and it worked fine. But as of recently it no longer works.

Exact steps for others to reproduce the error
The easiest way to test is with EEVEE bloom, attached this simple file I used in the demo video:

Event Timeline

Issue introduced by rB78ea6302f1c4: Fix T80746: Image blur in compositor creates halo from alpha

The render result is tagged in the shader as being not premultiplied. That seems to be incorrect. As the data is premultiplied. Need to check the actual state in on the render result, and how this is transferred to the shader.

Philipp Oeser (lichtwerk) changed the task status from Needs Triage to Confirmed.Sep 21 2020, 2:58 PM
Philipp Oeser (lichtwerk) changed the subtype of this task from "Report" to "Bug".

Ok what I have found out so far.

The float buffer of this file when rendering is straight alpha. For the bloom effect to be visible it should be rendered to premultiplied. Changing the alpha association will render the alpha_bug.blend incorrectly. This points to a more indepth issue that we need to tackle.

The reason why this wasn't visible before was that the pure emissive colors were also rendered for straight alphas what isn't correct. I need to get the whole picture clear to find out which transformation is done when.

  • If you render, save to openEXR and load the openEXR back, the bloom is drawn correctly.