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.

Answer 1

Here is my (quick and dirty) example. It seems to work for me and I hope it works for you. I'm assuming you can take care of main.cpp on your own. I did this on a MacBook Air 11.6" and substituted a picture of a dime for the USA icon included with OS X:

#ifndef WINDOW_H
#define WINDOW_H

#include <QtGui>

class Window : public QWidget
{
    Q_OBJECT

public:
    QWidget *canvas;
    QSlider *slider;
    QPixmap pixmap;

private:
    qreal zoom;
    qreal pixels;
    qreal px_width;
    qreal px_height;
    qreal mm_width;
    qreal mm_height;

public:
    Window();
    void paintEvent(QPaintEvent *);

public slots:
    void setZoom(int);
};

Window::Window()
{
    QHBoxLayout *layout = new QHBoxLayout;

    canvas = new QWidget;
    slider = new QSlider;
    slider->setMinimum(0);
    slider->setMaximum(100);
    slider->setValue(50);

    layout->addWidget(canvas);
    layout->addWidget(slider);

    this->setLayout(layout);

    if(!pixmap.load(":/resources/USA.gif"))
    {
        qDebug() << "Fatal error: Unable to load image";
        exit(1);
    }

    QObject::connect(slider, SIGNAL(valueChanged(int)), SLOT(setZoom(int)));
}

void Window::paintEvent(QPaintEvent *event)
{
    QPainter paint;
    paint.begin(this);
    paint.scale(zoom, zoom);
    paint.drawPixmap(0,0, pixmap);
    paint.end();
}

void Window::setZoom(int new_zoom)
{
    zoom = (qreal)(50+new_zoom) /50;
    pixels = pixmap.width() * zoom;

    QDesktopWidget desk;

    px_width = desk.width() / pixels;
    px_height = desk.height() / pixels;
    mm_width = px_width * 17.9;
    mm_height = px_height * 17.9;

    qDebug() << "Zoom: " << zoom;
    qDebug() << "desk.widthMM:" << desk.widthMM();
    qDebug() << "px_width: " << px_width;
    qDebug() << "px_height: " << px_height;
    qDebug() << "mm_width: " << mm_width;
    qDebug() << "mm_height: " << mm_height;

    this->repaint();
}

#include "moc_window.cpp"

#endif // WINDOW_H