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MEASURING THE FRAME RATE IN QT/QML

September 19, 2021September 19, 2021 lucaMeasuring the Frame Rate in Qt/QML 0
Comment


--------------------------------------------------------------------------------

Tags: QML, Qt

I frequently need to measure the frame rate in a Qt/QML application to have an
idea of how heavy the changes I’m making are. In the past, I found two
approaches that seemed to answer the question: one is using a simple QML item,
measuring the frequency of changes of a property. The other is using a custom
QQuickItem, measuring the frequency of the calls received to refresh it. I never
used the first approach because it did not feel safe, while I typically used the
second.

Instead of copy/pasting code into my projects repeatedly, I thought I could add
it to my collection of simple procedures in
https://github.com/carlonluca/lqtutils, making it simple to integrate when
needed. I therefore asked myself which one to include.

By thinking a bit about it, I thought of a different simpler approach using the
signal QQuickWindow::frameSwapped(). This is what Qt doc says about it:

> This signal is emitted when a frame has been queued for presenting. With
> vertical synchronization enabled the signal is emitted at most once per vsync
> interval in a continuously animating scene. This signal will be emitted from
> the scene graph rendering thread.

This seemed a reasonable approach, simple to implement, very concise and
lightweight. The resulting code is in the repo. Doc is in:
https://github.com/carlonluca/lqtutils#lqtutils_uih.


IS THIS CORRECT?

I was wondering if my approach was correct, so I thought I could compare with
the other approaches. I took the snippets from this SO question. I tried the
three techniques separately and also tried to add them together in the same code
for a comparison of the values. For the test I used my QML app here:
https://github.com/carlonluca/Fall. In the branch
https://github.com/carlonluca/Fall/tree/fps_comparison I added all the
components to compare. This is the result:



As it can be seen, the method based on the QQuickPaintedItem gives the same
result of the one based on the frameSwapped() signal. The other one is very
different. The explanation for this is probably based on the fact that the Qt
scene graph on Linux renders through the threaded render mode. In fact, forcing
a single-threaded render mode makes the three techniques agree on the
measurement (second run in the video).

Compared to a custom QQuickPaintedItem, the frameSwapped() technique seems to be
more lightweight, so it is the one I implemented in my repo.

Bye!


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