你如何将鼠标坐标转换为“mm”

Question

I am trying to convert the distance from a mouse press to mouse release into mm. I already calculated the distance but am not sure how to convert to the correct units.

void Widget::mousePressEvent(QMouseEvent* event)
{
    if (event->buttons() & Qt::LeftButton)
    {
        xAtPress = event->x();
        yAtPress = event->y();
        qDebug() << "Mouse X press: " << xAtPress;
        qDebug() << "Mouse Y press: " << yAtPress;
    }
}
void Widget::mouseReleaseEvent(QMouseEvent* event)
{
    if (event->button() == Qt::LeftButton) 
    {
        int xAtRelease = event->x();
        int yAtRelease = event->y();
        qDebug() << "Mouse X release: " << xAtRelease;
        qDebug() << "Mouse Y release: " << yAtRelease;
        int dx = xAtRelease - xAtPress;
        int dy = yAtRelease - yAtPress;
        qDebug() << "Dx: " << dx;
        qDebug() << "Dy: " << dy;


        //Create some kind of conversion function here.


    }
}

Answer 1

In short:
You can't (at least not accurately and reliably).

In long:
In order to perform even the most basic conversion you need to know the conversion factor between pixels and mm. And in order to calculate this factor you need to know the resolution of the screen (eg via QScreen ) and the real size of the screen - which is is the problematic part. QScreen even provides a physicalSize() function which returns the size of the screen (in mm) - but then warns

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

which essentially means: In the best case your monitor correctly provides that information and your OS correctly handles that information so you will get sensible data; more likely you will get utter nonsense; worst case you get values that look plausible - but actually are subtly incorrect.

In essence there is no reliable way to query that information from the OS, so you can't do it.