[Interest] Qt3D FPS computations aspects

Oleg Evseev ev.mipt at gmail.com
Thu Oct 8 15:58:36 CEST 2020

Hi all,

In our 3d application we used to check the performance using FPS display
based on qt3d-planets example: frames are accumulated in onTriggered action
of FrameAction component of root 3d entity and displayed every second:

We look at fps on several devices: windows, android and embedded linux
(using eglfs). And it seemed to be ok.

Now on the new x86 yocto linux device with weston 2.0.0 wayland we see
unusual fps behaviour.

Static Scene3D with several quick control buttons on top of the scene.
1) FPS is stable and around 60 fps if the application is drawn as weston
window with screen size regardless of the Qt3DRender policy
2) if the application is drawn fullscreen ("visibility: Window.FullScreen"
or "flags: Qt.FramelessWindowHint" is set), fps is 60 on start but then
drop down and stays around 30 (and sometimes jumps for a while to around
40, 50 or 60) if Qt3DRender policy OnDemand.
I don't see any lags. If 3d scene is changing (camera rotation around 3d
model) fps increases to 50-60.
3) If Qt3DRender policy is Always then stable 60 fps is shown.
4) Also 60 fps when a quick item is opened above  the OnDemand 3d scene.

So for now it looks like FrameAction is triggered rarely while OnDemand
policy is on. So lower fps in fact shows better performance?
Can this logic take place only for some device? Or this is something else?
We've never seen such behaviour on other devices while OnDemand policy was
usually used.

Thanks in advance for any thoughts.

With regards, Oleg.
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