How to check screen dpi before a QApplication exists?

Vadi2

That won't work, you have to enable or disable it before the application is created.

mrjj

Hi
I have no hires screen to test on so just asking.
You are saying that

#include "mainwindow.h"
#include <QApplication>
#include <QScreen>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    if ( a.screens().at(0)->geometry().width() > 2000) // 2000 is just random value. 
        QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
    else {
        QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling, false );
    }
    MainWindow w;
    w.show();

    return a.exec();
}

Does not fix the issue , turn on/off the scaling ?

Vadi2

Don't need a high-res display: the bug is on normal, non-high res displays. Using Qt 5.6 - 5.11, enable AA_EnableHighDpiScaling and watch everything blow up in size :(

and yes, setting the property after the initialisation doesn't seem to affect anything.

mrjj

@Vadi2
hi
and i assume that Qt::AA_DisableHighDpiScaling suffer the same thing as not change anything
if done in main ?

Vadi2

I'm not sure what do you mean - but I've already tried setting Qt::AA_DisableHighDpiScaling after my application was created and it was too late.

mrjj

@Vadi2
Yes that was what i meant.
It seems to be changed in Qt5.12 as i cant get it to show as (too) big.

When using Qt5.6, how do you enable AA_EnableHighDpiScaling ?
Just in case i want to test on that version.

Vadi2

Just do it before the QApplication

int main(int argc, char *argv[])
{
    QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

With it on (bad):

With it off (good):

mrjj

Hi
Oddly enough i installed Qt5.6 and tried on windows 10 but
didnt change the Widgets even with (Qt::AA_EnableHighDpiScaling, true);

Anyway, i was wondering if a workaround would be

int main(int argc, char *argv[])
{
    bool EnableHighDpiScaling = false;

    {
        QApplication a(argc, argv);
        if ( a.screens().at(0)->geometry().width() > 1090) // check somehow. DPI i assume
            EnableHighDpiScaling = true;
    }

    QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling, EnableHighDpiScaling);

    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

It cost an extra construction of a QApplication but the added startup time should be negligible ?

Vadi2

This might be a Linux-only bug, as Windows does HiDPI differently (as does macOS).

I'll try your suggestion!

Vadi2

Works, thanks a lot!