Question regarding bgfx's present (or SetFBO)

[preetishkakkar]

Hello,

I am investigating an issue where a compute pass (which has its own view id) seems to cause DirectX 11 renderer to present, this is due to the fact that bgfx seems to call setFrameBuffer irrespective of whether a pass is a draw or compute pass, causing it to present even in cases when it can be avoided.

Please see attached bgfx's code snippet image for reference

I think we should be calling these only in the case where it's not a compute pass, i.e. we should add a check at line 5657

Please let me know if it's expected behavior or if it's a bug in bgfx?

[bkaradzic]

This is correct behavior. Compute is executed first during view pass. View pass might contain both.

In this particular case if you just have compute, you could create 1x1 frame buffer and assign it to view to prevent present.

[preetishkakkar]

Thanks for the clarification, appreciated. Are you suggesting that compute & draw pass can share same view id? If yes, would the compute pass use the fbo that is set by draw pass? Also since you mentioned compute is run before draw, can I assume appropriated sync is handled by bgfx? Can compute & draw overlap on gpu?

[pezcode]

would the compute pass use the fbo that is set by draw pass

A framebuffer is set per view. It doesn't matter if the view uses compute or draws, as long as there is at least one compute/draw/blit, the framebuffer is bound (and presented if it's the backbuffer).

Also since you mentioned compute is run before draw, can I assume appropriated sync is handled by bgfx? Can compute & draw overlap on gpu?

All synchronization is handled by bgfx. The order is blit -> compute -> draw with no overlap between stages. Blits and compute dispatches are executed serially in submission order, draws are possibly sorted.