Tracking mouse coordinates in Qt

Question

Let's say I have a widget in main window, and want to track mouse position ONLY on the widget: it means that left-low corner of widget must be local (0, 0).

Q: How can I do this?

p.s. NON of functions below do that.

widget->mapFromGlobal(QCursor::pos()).x();
QCursor::pos()).x();
event->x();

Lamont

Answer 1

I am afraid, you won't be happy with your requirement 'lower left must be (0,0). In Qt coordinate systems (0,0) is upper left. If you can accept that. The following code...

setMouseTracking(true); // E.g. set in your constructor of your widget.

// Implement in your widget
void MainWindow::mouseMoveEvent(QMouseEvent *event){
    qDebug() << event->pos();
}

...will give you the coordinates of your mouse pointer in your widget.

Flying

I know that, and the exact thing I want to do is to transform it to lower left corner. – Lamont

You asked this here: #18551662. This is the 'do it yourself' solution. I assumed, with this question here, you asked for an easier Qt way. I must disappoint you, I am not aware of a way to make Qt switch its coordinate system easily. You will have to do the coordinate transformations yourself. If I were you, I'd try to live with the Qt system. Just to move (0,0) from upper to lower corner will hurt only your program's performance unnecessarily. – Flying

OK, let's consider that upper-left corner of WIDGET is (0,0). It anyway give wrong output. – Lamont

How far wrong? Works perfectly for me. Upper left corner is (0,0), upper right corner is (width,0), lower left corner is (0, height), lower right corner is (width,height). Tracking only within the widget. – Flying

Answer 2

If all you want to do is to report position of the mouse in coordinates as if the widget's lower-left corner was (0,0) and Y was ascending when going up, then the code below does it. I think the reason for wanting such code is misguided, though, since coordinates of everything else within said widget don't work this way. So why would you want it, I can't fathom, but here you go.

#include <QtWidgets>

class Window : public QLabel {
public:
    Window() {
        setMouseTracking(true);
        setMinimumSize(100, 100);
    }
    void mouseMoveEvent(QMouseEvent *ev) override {
        // vvv That's where the magic happens
        QTransform t;
        t.scale(1, -1);
        t.translate(0, -height()+1);
        QPoint pos = ev->pos() * t;
        // ^^^
        setText(QStringLiteral("%1, %2").arg(pos.x()).arg(pos.y()));
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Window w;
    w.show();
    return a.exec();
}

Mycorrhiza