Get physical screen size in Qt

Question

I'km working in Qt, i need help to get the physical size of the screen (monitor),

In Qt one can get a QDesktopWidget from QApplication, I mean:

QDesktopWidget *mydesk = QApplication::desktop();

The QDesktopwidget has some methods to get the resolution in pixels and some to get the the size in milimethers:

mydesk-> widthMM(); mydesk->heightMM();

However, this does not correspond to the physical size, when I measure my screen with a ruler, there is a considerable difference.

Also one can get the DPI measurement and calculate the size of the screen:

mydesk->physicalDpiX(); mydesk->physicalDpiY();
double Winches = (double)mydesk.width() / (double)mydesk.physicalDpiX();
double Hinches = (double)mydesk.Height() / (double)mydesk.physicalDpiY();

where mydesk.width() and mydesk.height() give the size in pixels(resolution)

However the measurement is also wrong and very close to mydesk.widthMM() and mydesk.heightMM()

Also I have triyed mydesk.logicalDpiX() and it has similar results.

Solution

The following method worked for me to get accurate physical size of my screen in mm.

QSizeF screenSize = QGuiApplication::primaryScreen()->physicalSize();

However, in Qt documentation I found the following sentence:

Depending on what information the underlying system provides the value might not be entirely accurate.

So it might not work for your system.