QScreen::devicePixelRatio returns incorrect values(integer value) on Wayland with fractional scaling in both Qt5 and Qt6 (Manjaro).

What did you do?

I want to get devicePixelRatio with QScreen in Wayland of Manjaro. However, it always returns an integer value, even though I am running using a fractional scaling ratio.

This is my display setup

This is my Qt code

I use code below to query the devicePiexlRatio using Qt5.

#include <QApplication>

#include <QScreen>

#include <QDebug>



int main(int argc, char *argv[]) {

    QApplication app(argc, argv);



    // Get the primary screen

    QScreen *screen = QApplication::primaryScreen();

    if (!screen) {

        qDebug() << "Error: No primary screen detected.";

        return 1;

    }



    // Query and print devicePixelRatio

    qreal pixelRatio = screen->devicePixelRatio();

    qDebug() << "Device Pixel Ratio:" << pixelRatio;

    qDebug() << "Screen Name:" << screen->name();

    qDebug() << "Physical DPI:" << screen->physicalDotsPerInch();

    qDebug() << "Logical DPI:" << screen->logicalDotsPerInch();



    return app.exec();

} 

I tested it with high dpi env var enabled or disabled, neither worked.

The result is

$./test_pixelratio

Device Pixel Ratio: 2

Screen Name: "eDP-1"

Physical DPI: 126.192

Logical DPI: 96


 

What did you expect to happen?

An accurate devicePixelRatio, in my case, should be 1.25.

What happened instead?

An integer value. It returns 2.

What I am suspecting

You didn't support fractional scaling ratio in QtWaylandScreen. I read the code, though it is qt6.

It had implemented fractional_scale_v1, the wayland protocol that supports the fractional scaling ratio. And it had a fractional scaling manager. But it is only integrated into the qt wayland related window class, not QtWaylandScreen.

I might also miss some other widgets or components that still fully supports fractional scaling ratio.