Changing DPI scaling size of display make Qt application's font size get rendered bigger

Question

I have created some GUI application using Qt. My GUI application contains controls like push button and radio button. When I run the application, buttons and fonts inside button looks normal. When I change the DPI scaling size of display from 100% to 150% or 200%, font size of controls rendered bigger but not control size (pushbutton, radio button) irrespective of resolution. Due to this the text inside controls were cut off. please see the attached image.

Qt application look when DPI scaling size set to 100%

Qt application look when DPI scaling size set to 200%

I am running my application in some tablets also. In tablets, DPI scale value should be more than 150% else everything will be shown very small.

I searched in the web for creating UI application in Qt irrespective of resolution and DPI scale value but no luck. So I am posting my questing here. Please let me know if there is some way to get rid of this.

Answer 1

High DPI support is enabled from Qt 5.6 onward.

Setting QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling) in your application source code allows automatic high-DPI scaling.

NOTICE: To use the attribute method, you must set the attribute before you create your QApplication object:

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QApplication app(argc, argv);   
    return app.exec();
}

Answer 2

Using layouts correctly can help.

http://qt-project.org/doc/qt-4.8/layout.html

Telling the OS that you handle DPI changes, will prevent weird font changes that you weren't expecting.

http://msdn.microsoft.com/en-us/library/ms701681(v=vs.85).aspx

For spacing critical places, you can check the size of your rendered font, and then set the minimum size of your object based on the resulting size of your text.

http://qt-project.org/doc/qt-4.8/qfontmetrics.html#details

https://blog.qt.digia.com/blog/2009/06/26/improving-support-for-higher-dpi-on-vista/

You could try checking with other built in measurements from Qt:

http://qt-project.org/doc/qt-4.8/qpaintdevice.html#widthMM

http://qt-project.org/doc/qt-4.8/qpaintdevice.html#logicalDpiX

If you are using QML, try for pristine layouts of only anchor based placement.

http://qt-project.org/doc/qt-4.8/qml-anchor-layout.html

QApplication has some settings that are somewhat related.

http://qt-project.org/doc/qt-4.8/qapplication.html#setDesktopSettingsAware

You could manually specify the font, too.

http://qt-project.org/doc/qt-4.8/qapplication.html#setFont

Hope that helps.

Answer 3

I had a fixed size window which was not large enough to fit all the text it contained when Windows accessibility settings where applied to scale up all text sizes. Windows does this via dpi increases. I fixed this by retreiving the os scaling factor and then adjusted the size of the my window and some of it's layouts (which I couldn't get to scale automatically for some reason).

Here's how I got the dpi scale (in a file called "WindowsDpiScale.h"):

#ifndef WINDOWSDPISCALE_H
#define WINDOWSDPISCALE_H

#include <QtGlobal>

#ifdef Q_OS_WIN

#include <windows.h>

const float DEFAULT_DPI = 96.0;

float windowsDpiScale()
{
    HDC screen = GetDC( 0 );
    FLOAT dpiX = static_cast<FLOAT>( GetDeviceCaps( screen, LOGPIXELSX ) );
    ReleaseDC( 0, screen );
    return dpiX / DEFAULT_DPI;
}

#endif //Q_OS_WIN

#endif // WINDOWSDPISCALE_H

And then, how I applied it in my case:

...
#include "WindowsDpiScale.h"

MainWindow::MainWindow( QWidget *parent )
    : QMainWindow( parent )
{
...
    // Enlarge the window and various child widgets to accomendate
    // OS display scaling (i.e. accessibily options)
    setScaleToOsSettings();
...
}

void MainWindow::setScaleToOsSettings()
{
#ifdef Q_OS_WIN
    setScale( windowsDpiScale() );
#endif
}

void MainWindow::setScale( float scale )
{
    // Resize the window
    this->setFixedSize( (int)(scale * this->maximumWidth()),
                        (int)(scale * this->maximumHeight()) );

    // Resize the layouts within the stacked widget
    foreach( QVBoxLayout * layout,
             windowUi_->pagerStackedWidget->findChildren<QVBoxLayout *>() )
        layout->parentWidget()->setFixedSize(
            (int)(scale * layout->parentWidget()->contentsRect().width()),
            (int)(scale * layout->parentWidget()->contentsRect().height()) );
}