How to use mouseMoveEvent on paintEvent on Qt 5?

Question

I'm new on Qt and c++, so I'm having some difficulties. I'm trying to create a widget that can get the mouseMoveEvent position and draw an ellipse on my pixmap on mouse position. Below you can see the code:

#include "myimage.h"
#include <QPainter>
#include <QPen>
#include <QColor>
#include <QMouseEvent>
#include <QDebug>

Myimage::Myimage(QWidget *parent) : QWidget(parent)
{
    setMouseTracking(true); // E.g. set in your constructor of your widget.
}



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

}

void Myimage::paintEvent(QPaintEvent * event)
{
    event->accept();
    QPixmap pixmap2("/home/gabriel/Qt_interfaces/OpenCVTests/Webcam_PyQt5/Images/Court_top_View.jpg");

    QRect rectangle(0, 0, width()-1, height()-1);

    QPainter painter(this);
    painter.drawRect(rectangle);
    painter.drawPixmap(5, 5, width()-10, height()-10, pixmap2);


    painter.drawEllipse(pos(), 10 ,10 );
}

The mouse position is being printed on console, but no ellipse on image.

Could you help me?

Regards,

Gabriel.

Answer 1

According to the doc:

pos : QPoint

This property holds the position of the widget within its parent widget.

If the widget is a window, the position is that of the widget on the desktop, including its frame.

...

Access functions:

QPoint pos() const void

move(int x, int y)

void move(const QPoint &)

As we see this data we do not want it, a possible solution is to create a variable that stores the value of the position obtaining through QMouseEvent and update the painting through the function update(), in addition the first time the Widget there should be no ellipse so we check that the position has been assigned through the function isNull() of QPoint, as I show below:

*.h

private:
    QPoint mPoint;

*.cpp

Myimage::Myimage(QWidget *parent)
    : QWidget(parent)
{
    setMouseTracking(true);
}

void Myimage::mouseMoveEvent(QMouseEvent *event)
{
    mPoint = event->pos();
    update();
}

void Myimage::paintEvent(QPaintEvent *)
{
    QPixmap pixmap2("/home/gabriel/Qt_interfaces/OpenCVTests/Webcam_PyQt5/Images/Court_top_View.jpg");

    QRect rectangle(0, 0, width()-1, height()-1);

    QPainter painter(this);
    painter.drawRect(rectangle);
    painter.drawPixmap(5, 5, width()-10, height()-10, pixmap2);

    if(!mPoint.isNull()){

        painter.drawEllipse(mPoint, 10 ,10 );
    }
}