HDRP Alpha output problem

monty_python · August 12, 2020, 1:32pm

Hi,
I am having problems with alpha when using Unity 2020.1.1f1 + HDRP. I have configured both my Color Buffer Format and Post-processing Buffer Format to use R16G16B16A16 based on this page:
https://docs.unity3d.com/Packages/com.unity.render-pipelines.high-definition@7.2/manual/Alpha-Output.html

However it doesn’t seem to produce correct result. Setting output to Target texture produces following alpha channel (HDRP 8.2.0):

It seems like alpha it is not being post-processed (judging based on flickering edges produced most probably by lack of TAA post-processing) and also it contains some incorrect color information rather than actual alpha.

I have tested also HDRP 9.0.0 preview 33 and it similarly outputs non post-processed alpha (even though this time it is at least proper alpha information).

Please does anyone have a clue what am I doing wrong? I think this might be some kind of bug with alpha channel output, but I am not sure. I am using just a default scene that opens with new HDRP project. In order to output alpha I have set Camera Background Type to Color (0,0,0,0). Thanks.

unity_Ki_viE1vxNTlA · September 25, 2020, 6:02pm

Turnoff the Stop Nans in camera below temporal Anti-aliasing settings.

SebLagarde · October 1, 2020, 10:44am

Hi, thanks to have report this issue, should be fixed by this PR: Fix alpha not having TAA applied to it. by FrancescoC-unity · Pull Request #2069 · Unity-Technologies/Graphics · GitHub

eliashodel · October 1, 2020, 2:32pm

There seems to be a similar issue with the motion blur post processing effect. when ever i turn on motion blur the alpha channel becomes a greyscale version of the image.

no motion blur:

with motion blur:

monty_python · January 2, 2022, 5:15pm

Hi,
I am experiencing some alpha related issues when using Temporal anti-aliasing. All issues mentioned above seem to be solved, but I can’t get rid of new “ghosting” effect caused by TAA. Problem could be easily reproduced with moving camera and couple of objects in the scene (behaves the same in both 2021.2.7f1 and 2022.1.0b2).
Please is there a way to use TAA without such ghosting? Thanks.