[MrJul]

What does the pull request do?

This PR disables CompositionOptions.UseRegionDirtyRectClipping by default.

It's been found to cause slowdowns in various scenarios, read #21538 (comment) for details.

Users on embedded systems might still want to turn it on, but users with actual GPUs should benefit from the setting being off by default.

This will be the new default in Avalonia 12.1.

Fixed issues

• Fixes I found that the rendering frame rate is very low in versions above 12. #21538

[kekekeks]

We should probably keep it on for SW rendering targets. (need benchmarks though).

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0066268-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

We should probably keep it on for SW rendering targets. (need benchmarks though).

Win32, software mode, average CPU frame render time:

Test	UseRegionDirtyRectClipping = false	UseRegionDirtyRectClipping = true
> 1000 lines (#21538)	10.0 ms	9.7 ms
2 borders (#21538 (comment))	7.1 ms	6.7 ms
RenderDemo Hit Testing	15.8 ms	18.5 ms

So it seems that keeping it enabled in software mode is a bit better in simple cases but worse in complex ones.
I'm sure examples can be found where it's tremendously better, but this is where it should probably be manually enabled.

[maxkatz6]

Ideally, we might need to document rendering/composition options here too AvaloniaUI/avalonia-docs#998
cc @luke-whos-here

[qwertychouskie]

Does this also apply to Linux Embedded when using direct DRM rendering? Unfortunately it seems that documentation is very lacking for what options/tunables apply, which makes diagnosing performance issues (#21580) difficult.

Also, is there a way to enable/disable this option (and other related ones) using environment variables for easy testing, or is it required to change the source code and re-compile?

[MrJul]

Does this also apply to Linux Embedded when using direct DRM rendering?

DRM uses the GPU, so you'll get the slow software mask fallback by default when regions are enabled. So yes, setting the flag explicitly to false using .With(new CompositionOptions() { UseRegionDirtyRectClipping = false }) or waiting for this PR should improve things there. You don't need to recompile Avalonia.