Could bgfx draw on two monitors with proper vsync on each?

[piermorel]

I need to display different things on different monitors for a scientific project (currently two monitors, but potentially three). My current trials with bgfx are not satisfactory (missed frames everywhere despite rendering only a single triangle), but I'm wondering if it's due to bgfx's whole design philosophy or just due to an implementation bug on macOS (see #1867 ).

It seems clear that drawing on two monitors with vsync on each is impossible with a single threaded solution, as the two monitors are never in perfect sync even if they are running at the same frequency (and it gets much worse if they refresh at different frequencies, say one 144Hz monitor and on 60Hz one). But I'm not clear at all on how to use one thread per monitor in bgfx or if it's possible at all, since bgfx::frame() doesn't allow to specify what we want to render and wait on.

I do know it's possible to have a dual monitors with different frequencies and proper vsync working with macOS and Metal (the trick seems to have one commandQueue per monitor and running each in a separate thread), but could this be achieved with bgfx, whether on macOS or on any other platform/API (I can change) ?

[bkaradzic]

Currently internal rendering submission thread is single-threaded. It would be possible to split it into multiple threads, but this is very specific use case.

[piermorel]

Yes that's what I was afraid of, thanks for the confirmation. Would I be able to do this by calling "internal" bgfx functions (I'm seeing some kick() and flip() in the source) or would it be way more involved than that ?

As for the use case, mine is definitely specific, but multi-monitor/window apps are not that uncommon (sim games, 3D editors).

[bkaradzic]

Yeah, you could hack something in to work for you. Write code separate from bgfx that just does that, flips swap chain on different threads with vsync on. Then replace bgfx swap chain present with your implementation.

[piermorel]

Thanks, I was just thinking of something that could be even simpler (but certainly not cleaner) given my poor skills : could I use bgfx fully offscreen to render textures corresponding to my monitors as fast as possible (without vsync) in a single thread, and just do a bit of Metal code to display the latest rendered textures on the corresponding monitors with vsync ? I display only very simple stuff so rendering is fast.

I guess the question is then how can bgfx be used for fully offscreen rendering, and how to retrieve the Metal texture ?