[Interest] QQuickWindow::update to render custom OpenGL in QtQuick2

[Preet]

Hi

I want to render things with OpenGL directly in a QQuickItem (like
http://doc-snapshot.qt-project.org/qt5-dev/qtquick/scenegraph-openglunderqml.htmlshows)
but I get the impression that doing so means every other item in the
scene is updated at the rate that I choose to call QQuickWindow::update()
from my custom QQuickItem.

I want to avoid the overhead of painting everything again and again if the
items in the scene haven't changed, especially if I'm updating my object at
30 or even 60fps.

With a QQuickItem that uses a QSGNode I can set DirtyStateBits and then
call QQuickItem::update() to force an update to *just that* item. I can't
do this with the tutorial I linked above though, because it's not using a
node.

* Does QQuickWindow::update() truly force a repaint to everything in the
window even if it hasn't changed? If so...

* Is there a way to use partial updates instead?

I'm aware I can render to an FBO, but that takes away the 'easy' full
screen MSAA I get with the direct method.


Preet
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