Raspberry Pi Qt5 Set physical screen size

Question

I am developping a qt5 application on my raspberry pi on raspbian using cross compilation.

When I run it, I get a black screen trying display a Pop-up which is a QFrame.

I suppose that it cannot be positioned because I have error messages arriving at the beginning :

EGLFS: Unable to query physical screen size, defaulting to 100 dpi.

EGLFS: To override, set QT_QPA_EGLFS_PHYSICAL_WIDTH and QT_QPA_EGLFS_PHYSICAL_HEIGHT (in millimeters).

EGLFS: Unable to query screen depth, defaulting to 32.

EGLFS: To override, set QT_QPA_EGLFS_DEPTH.

When I disable the QFrame, the application is launched in full-screen by default, whereas I didn't set it.

I tried to set these environment variables with :

int main(int argc, char *argv[])
{
    QByteArray width = qgetenv("QT_QPA_EGLFS_PHYSICAL_WIDTH");
    width.setNum(1920);

    QByteArray height = qgetenv("QT_QPA_EGLFS_PHYSICAL_HEIGHT");
    height.setNum(1080);

    QApplication a(argc, argv);

    MainWindow w;
    w.show();
    w.launchGame();

    return a.exec();
}

But the error messages are still displayed.

The main window is a QWidget and I defined his size using :

setFixedSize(1280, 720);

My question is, how to set these variables correctly or do you have another solution ?

Answer 1

Raspberry Pi 3 Kernel 4.14.27-v7+

Qt 5.10.1 cross-compiled from Ubuntu 16.04 host

I had to add the following to my .profile on my Rpi

Open .profile

sudo nano ~/.profile

I added these lines at end

# physical display properties
export QT_QPA_EGLFS_PHYSICAL_WIDTH=520
export QT_QPA_EGLFS_PHYSICAL_HEIGHT=326

Then reload bash profile with (or reboot)

source .profile

The following link has descriptions for all the EGLFS environment variables:

http://doc.qt.io/qt-5/embedded-linux.html