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.