Forward+: TAA or FSR2 on a transparent viewport causes the viewport to turn opaque

[Calinou]

Godot version

4.0.beta7

System information

Fedora 36, Vulkan Forward Plus, AMD Radeon RX 6900 XT

Issue description

When enabling temporal antialiasing, the viewport becomes opaque if it's currently transparent.

This does not occur with FXAA and MSAA, both of which support transparent viewports (with some visible artifacts though).

TAA disabled	TAA enabled

If it's not possible to support TAA on transparent viewports, I suggest making the transparent viewport property take priority (so it disables TAA), and print a warning if both TAA and transparent viewport are enabled.

Steps to reproduce

• Enable Display > Window > Size > Transparent, Display > Window > Per Pixel Transparency > Allowed and Rendering > Transparent Background in the advanced Project Settings.
• Notice how the editor 3D viewport (and the running project) features transparency.
• Enable Use TAA in the Project Settings.
• Notice how the editor 3D viewport (and the running project) loses transparency.

Minimal reproduction project

test_taa_transparent_viewport.zip

[viksl]

I can confirm this in case it's needed. Happened to me to just yesterday in beta17 win 10 with a subviewport.

[DarioSamo]

From a brief look into it it seems like the TAA resolve step intentionally ignores the alpha component as it resolves the color, tonemapping, history, etc. It seems taking alpha into account would also expand the memory used during execution.

There is no obvious solution as to how to extend this in the shader as it'd need to use a separate path for mixing the alpha (to avoid tonemapping) and would probably need another variant of the shader to not cause a performance impact when transparent output isn't necessary (which is the more common use case).

Adding a warning and disabling TAA for this use case might be the more reasonable approach here.

[Ryhon0]

Also seems to happen with FSR 2 enabled. FSR 1 works properly

[hippogyz]

FSR 1.0 with scale less than 1 also turns background to opaque.

This is my system info:
Godot v4.3.stable.mono - Windows 10.0.22631 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 4070 (NVIDIA; 31.0.15.3161) - AMD Ryzen 9 7900X 12-Core Processor (24 Threads)