[Interest] Mouse/Pen input latency in Qt Quick and render loop

[Daljit Singh]

To follow up on this, I was looking at the source code for the renderloop in the Qt repositories and I found this comment here https://github.com/qt/qtdeclarative/blob/dev/src/quick/scenegraph/qsgrenderloop.cpp#L79:
[https://avatars2.githubusercontent.com/u/159455?s=400&v=4]<https://github.com/qt/qtdeclarative/blob/dev/src/quick/scenegraph/qsgrenderloop.cpp#L79>
qt/qtdeclarative<https://github.com/qt/qtdeclarative/blob/dev/src/quick/scenegraph/qsgrenderloop.cpp#L79>
Qt Declarative (Quick 2). Contribute to qt/qtdeclarative development by creating an account on GitHub.
github.com


- Uses one QRhi (and so OpenGL Context, Vulkan device, etc.) to render multiple windows.
- This assumes multiple screens can use the OpenGL context.
- Animations are advanced using the standard timer, so QML animations run as expected even when
vsync throttling is broken.

Does that mean that with Qt6 the "threaded" render loop used by the QML SceneGraph has animations correctly even when v-sync is disabled?
________________________________
From: Daljit Singh <daljit97 at live.com>
Sent: 15 August 2020 01:08
To: interest at qt-project.org <interest at qt-project.org>
Subject: Mouse/Pen input latency in Qt Quick and render loop

Hi, I am developing a cross platform application for displaying pen input. The project requires real-time rendering of digital ink (stylus input) and hence the latency of the rendered strokes is very important. To render the strokes I am using a custom QQuickItem together with QSGRenderNode. Unfortunately, drawing the ink results in significant latency as the input is rendered with some delay (e.g. comparing it to something like OneNote I would say there seems to be around 10-15ms more delay). I came across a previous mail thread here https://lists.qt-project.org/pipermail/interest/2014-March/011692.html where it is mentioned that this is largely due to VSync and it's possible to disable this by calling QSurfaceFormat::setSwapInterval(0). This works well and reduces the latency significantly, unfortunately it also introduces another problem: the render loop for the Qt Quick scenegraph must be set to "basic" otherwise QML animations go a bit crazy.

My application requires that some of the ink to be rendered when the user is scrolling a large canvas (for which I am currently using Flickable), but using the basic render loop results in the animation being quite jerky (so scrolling is not smooth at all) as while the strokes are rendered the animation system is stopped (at least this is my understanding). Is there a way to overcome what I described? I only require the app to be in "low-latency" mode while the user is drawing on the screen and for all other scenario I would like to stick with the threaded render loop.


Best,

Daljit
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