Raspberry Pi Qt5 Set physical screen size - environment-variables

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

Answer 2

I just put something like
export QT_QPA_EGLFS_PHYSICAL_WIDTH=myWidth
QT_QPA_EGLFS_PHYSICAL_HEIGHT=myHeight
QT_QPA_EGLFS_DEPTH=myDepth
in my .bashrc to get rid of those errors. Also the physical width and height are in millimeters not pixels, and screen depth is color depth of your display (18 in my case).