FXAA doesn't work properly on the Mobile Renderer

[mrjustaguy]

Tested versions

Reproduced in v4.5.dev.gh-102330 [edc231eaf] and 4.5 dev 4 and 4.4 Stable

System information

Godot v4.5.dev (edc231eaf) - Windows 11 (build 26100) - Multi-window, 1 monitor - Vulkan (Mobile) - dedicated AMD Radeon RX 6600 (Advanced Micro Devices, Inc.; 32.0.21001.9024) - Intel(R) Core(TM) i3-10105F CPU @ 3.70GHz (8 threads)

Issue description

FXAA doesn't function properly when Using the Mobile Renderer.
It seems like it's having a Much harder time detecting edges using the Mobile Renderer.

Steps to reproduce

Enable FXAA in a project that uses the Mobile Renderer, and disable the Background in a default empty scene, observe the Grid in Forward+ as a comparison.

Minimal reproduction project (MRP)

N/A

[Calinou]

I can't reproduce this on a MRP I made for testing this. Can you provide comparison images that are easily reproducible locally?

Testing project: test_antialiasing_thin_3d_meshes.zip

Forward+

No AA	FXAA

Mobile

No AA	FXAA

"Ground truth" (8× MSAA + 2.0 render scale + TAA + no FXAA):

[mrjustaguy]

I just created a new project, one with forward+ and one with mobile, and turned fxaa on/off after disabling the background between the two projects. aside from that, nothing was changed, no camera moves and stuff.

FXAA does work on the mobile renderer, but it lets through WAY more aliasing, as can be compared when you don't disable the backgrounds and look at the grid.

I don't have time right now, I'll post images later as an edit to this post

[BlueCube3310]

FXAA is often not applied on mobile when the post process step happens in a subpass.