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Image Editor "Erase Alpha" brush erases all channels
Confirmed, HighPublicBUG


System Information
Linux 64-bit

Blender Version
Broken: 2.92
Note: In versions 2.83 and earlier, Alpha was not premultiplied.

Short description of error
Erasing the alpha on float images erase the RGB channels as well.

Exact steps for others to reproduce the error

  1. Open the attached file. (Left image is a Non-float while right image is Float, both have Alpha channel. We are seeing only the RGB channels).
  2. Use the brush "Erase Alpha" to erase part of the non-floating-point image (on the left).
  3. Use shift-click to sample the pixel-values of this image. The RGB channels have NOT actually been affected and the image only appears to have been premultiplied.
  4. Use the brush "Erase Alpha" on the floating-point image (on the right).
  5. Use shift-click to sample the pixel-values of this image. ALL channels were erased!

It may look like the same bug as T86866 but it isn't! The brush works as expected; it's the RGB display that's wrong.

Event Timeline

Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Confirmed.Fri, Mar 26, 4:58 PM
Germano Cavalcante (mano-wii) triaged this task as High priority.
Germano Cavalcante (mano-wii) changed the subtype of this task from "Report" to "Bug".

Thanks for the report, I can confirm the problem.

I don't know what the industry standard is but I think the other channels shouldn't be erased when you use an "Erase Alpha" brush.
In any case, this should be consistent across different image formats.

For the number of related reports I dare to set this bug as a high priority (for now).

(Personally, I think the pre-multiplication should be optional).
(I also mentioned about this on T86764).

@Sam Brubaker (rocketman), with the edits I made, I would say that this report was even more similar to the T86866 (sorry about that).
But the idea of the pre-multiplied alpha has already been described in other reports.
So I'll merge then.

Germano Cavalcante (mano-wii) renamed this task from Image Editor appears to multiply the RGB channels by the Alpha channels when non-float images are displayed in RGB to Image Editor "Erase Alpha" brush erases all channels.Fri, Mar 26, 5:22 PM

It's a funny issue because the behavior of the Erase Alpha brush appears to be ONE bug when in fact there are two different bugs. And now I don't know which bug report is which! This is exactly what I was afraid would happen, lol