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Alpha display problem in the Image Editor
Confirmed, NormalPublicBUG


System Information
Operating system: Linux-5.4.0-45-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.51.06

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-01 12:39, hash: rB526fbdec3b6a
Also broken in the 2.90 and 2.83.5 LTS!

Short description of error
The Alpha channel display in the Image Editor gets completely garbled if you switch it to show only the Alpha channel, change the current frame by pressing cursor left or right and the pressing F12 again. This is what the Alpha should look like:

And here's how it looks after changing the frame:

Exact steps for others to reproduce the error

  • Open the attached blend file (basically an empty scene with the default cube):

  • Press F12 to render and to open the Image Editor. Everything looks fine.
  • Switch the Image Editor to show the Alpha channel:

  • Press cursor right or left to change the current frame
  • Press F12 again
  • The Alpha is a total mess

This happens in 2.83.x, 2.90 and the latest master. Both in CPU and GPU mode.
When saving the garbled image from the Image Editor (to a PNG e.g.) the Alpha is fine. So it looks to me like it's only a display problem?

Event Timeline

Here with 2.91, I can reproduce without even changing frame. Just open Blender, enable Transparency and render. Then switch render viewport to Alpha and render again.
Note that it only happens with Cycle and the alpha channel actually seems fine when switching back to Colour and Alpha mode.

Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Confirmed.EditedThu, Sep 3, 3:08 PM
Germano Cavalcante (mano-wii) changed the subtype of this task from "Report" to "Bug".

I can confirm in Cycles, because of the tile rendering.
The problem happens when you view the Alpha channel before rendering is complete.
So it must be a problem to update the image.