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Workbench Transparent Film Alpha Bug
Closed, ResolvedPublic

Description

System Information
Operating system: Window 7 SP1
Graphics card: nVidia 660 GTX

Blender Version
version: 2.80 (sub 74), branch: blender2.7, commit date: 2019-06-09 21:43, hash: 030c7df19da9, type: Release
build date: 09/06/2019, 17:18

Short description of error
Alpha of rendered transparent workbench image is wrong.

Exact steps for others to reproduce the error
Open the default scene and look through the camera. Change the render engine to "Workbench" and activate the Transparent flag under the Film rollout.
Render the scene, inspect the pixels and you will discover that the alpha of the pixels of the cube are not entirely solid, as they should be.

Event Timeline

Jeroen Bakker (jbakker) triaged this task as Confirmed, Medium priority.

Ok, during rendering the AA-buffers are incorrect. Changing AA quality will change the Alpha value to use. Note that Single pass AA and No AA will make the Alpha completely 0.0

@Jeroen Bakker (jbakker) Thanks for looking into this (I was the person that posted about it on Blender Artists).

Unfortunately, with your suggested workaround (and with a more complex scene) I get either a completely transparent scene, or only the outlines if I enable them. (Using Blender 2.8 from yesterday on MacOS).

After some testing, the only "workaround" I can find is bumping AA samples to 32, which makes the Alpha 96.5% instead of 87.5% when using 8 samples. Still not really usable, but I'll work like this for now.

The work around is to use viewport rendering. This does not have these artifacts. Only final rendering is not working.

I bisecting the issue are found that rBede4f3196985: DrawManager: OpenGL State Image Rendering initiated the error. Will investigate to find out what works for both cases (hair rendering + transparency)