1836726 - Subpixel AA cannot be force-disabled to work around performance issues

Status: RESOLVED FIXED

(Core :: Graphics: WebRender, enhancement, P2)

Description

[Kestrel]

Subpixel AA text rendering significantly reduces scrolling performance on some sites like reddit. This was especially noticeable for me on Ubuntu 23.04 with AMD RS880 (R600) when testing builds after Bug 1600472 (Disable allowing sacrificing subpixel anti-aliasing for small screens) which killed scrolling smoothness, later partly addressed by Bug 1635610 (Enable per scroll root picture cache slices by default) but it is still not as smooth as before.

It's not possible to do a comparison on the latest Nightly version since there is no way to turn subpixel AA off in WR, only force-enable with gfx.webrender.quality.force-subpixel-aa-where-possible = true. gfx.text.disable-aa = true does not use grayscale and does not seem to affect WR, giving no performance benefit. Changing the system setting /org/gnome/desktop/interface/font-antialiasing to grayscale or none has no effect and fontconfig settings also don't work (Bug 1726312).

Subpixel AA is expensive and unnecessary in many cases, it is not needed for high resolution screens (Bug 1698701), it is disabled on Android (Bug 1527807) and WR often sacrifices it for performance anyway. Exposing an about:config setting to let users disable it for themselves would help identify and work around associated performance issues.

Comment 1

[Bob Hood [:bhood]]

Glenn, could this be a quick-fix for Nightly, or are there deeper concerns with providing this?

Comment 2

[Glenn Watson [:gw]]

It's likely to be something fairly simple, I'll take a look this week.

Comment 3

[Glenn Watson [:gw]]

Attachment: Bug 1836726 - Allow disabling subpixel AA in WR via preference

Previously, this was hard coded to off for android, on for everything
else. With this change, it's controlled by a preference which
defaults to off for android, and on for everything else. This allows
users to disable subpixel AA on desktop for performance reasons.

Comment 4

[Glenn Watson [:gw]]

This is a straightforward change, so I've put a patch up to add this functionality. Thanks for the detailed report.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/a46936121982
Allow disabling subpixel AA in WR via preference r=gfx-reviewers,lsalzman

Comment 6

[Iulian Moraru]

https://hg.mozilla.org/mozilla-central/rev/a46936121982