CODEWITHSUNDEEP
c++qtqml
peibye
peibye

Reputation: 71

Changing Qt Scale Factor

I'd like to change my Qt app's scale based on the resolution of my devices screen. I'm trying to determine the resolution of the device and set 

QApplication::setAttribute(Qt::AA_EnableHighDpiScaling); // DPI support
QCoreApplication::setAttribute(Qt::AA_UseHighDpiPixmaps); // HiDPI pixmaps
qputenv("QT_SCALE_FACTOR", scaleAsQByteArray);

before my QApplication is initialized, scaleAsQByteArray to 1 if it's around the size where my app looks decent, and > 1 depending on the difference between my intended resolution and the large display resolution of the current device.

This doesn't seem to be possible, at least from what I understand, as you need an initialized QApplication to get screen information, but I also can't use qputenv after that happens.

Does anyone have a solution for how I could go about setting the scale factor of a QApplication based on screen resolution? Or to simplify things, how to get the height and width of the device monitor before I initialize the QApplication window.

I was thinking about using a Scale {} in qml: hiding the main Window and showing the scaled version instead, though this might have some performance drawbacks, but that's basically what I'm trying to do.

Upvotes: 1

Views: 10108

Answers (1)

peibye
peibye

Reputation: 71

I think I found a cheeky solution here. I ended up creating a throwaway QApplication just to get the width of the display, then set the scale factor and create the actual QApplication.

QApplication* temp = new QApplication(argc, argv);
QRect screenGeometry = QApplication::desktop()->screenGeometry();
double width = screenGeometry.width();
// assumes that the default desktop resolution is 720p (scale of 1)
int minWidth = 1280;
delete temp;

double scale = width / minWidth;
std::string scaleAsString = std::to_string(scale);
QByteArray scaleAsQByteArray(scaleAsString.c_str(), scaleAsString.length());
qputenv("QT_SCALE_FACTOR", scaleAsQByteArray);

QApplication app(argc, argv);

The app I'm work on has a ton of hard-sized objects, and I don't have the time to change everything to use layouts, so this is a great solution so far. Hope this helps someone else!

Upvotes: 4

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