[Interest] QOpenGLWidget's performance drop

[sivan nanthiran]

Hi Tor,

I have created a bug report (https://bugreports.qt.io/browse/QTBUG-100943)
as you requested. Please note that my Qt patch only forces the
QOffscreenSurface to be window-backed.

Regards,
Sivan

On Wed, Feb 16, 2022 at 6:58 PM Tor Arne Vestbø <Tor.arne.Vestbo at qt.io>
wrote:

> Thanks for the detailed report Sivan. Can you file an issue about in
> https://bugreports.qt.io/ with the sample code, and a diff of the changes
> you made to improve performance? Thanks!
>
> On 16 Feb 2022, at 11:53, sivan nanthiran <nanthiran2005 at gmail.com> wrote:
>
> Hi,
>
> Recently I have upgraded from Qt-5.4.1 to Qt-5.15.2. The following is my
> system configuration:
> - MacOS
> - External display connected to External GPU, and made as the primary
> screen, connected to a macbook
>
> In my simple testApp, I have 2 QOpenGLWidget and 1 QWidget. I just simply
> render a triangle in those QOpenGLWidgets which changes color every 16ms.
>
> The rendering performance in my app used to be 60fps as it was rendering
> on the external GPU. However, after Qt-5.15 upgrade, I noticed the
> performance is about 45fps and there are load in my macbook's Intel GPU.
>
> Upon investigating, I found out that its due to QOffscreenSurface's change
> to be not window-backed anymore. Since QOpenGLWidget internally uses
> QOffscreenSurface, I believe this is causing the rendering to not happen on
> the screen's GPU. When I hack the code to make the QOffscreenSurface to
> window-backed, I'm able to get the same performance as before.
>
> So is there any clean way to achieve the same performance I used to
> achieve? If not, are there any effects if I force the QOffscreenSurface to
> be window backed for MacOS?
>
> Thanks in advance.
>
> Regards,
> Sivan
>
>
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