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Drawing polygon using event filter in Qt

  • I am working on a feature in my GUI that allow user to draw polygon by clicking the points.
    First the user needs to enable this function by clicking a button. After that the user clicks any four points on the screen. When the fourth point is clicked, all the 4 points will be connected to form a polygon.

    However the polygon is not appearing after the fourth is clicked. Instead there is a message of "QPainter::begin: Paint device returned engine == 0, type: 1". What is the problem in my code?

    @bool QTGraphicsShape::eventFilter(QObject *obj, QEvent *event)
    double static x[4],y[4];
    int static i;
    if ((event->type() == QEvent::GraphicsSceneMouseRelease) && (Draw3Points == true)) {
    QGraphicsSceneMouseEvent mouseEvent = static_cast< QGraphicsSceneMouseEvent >( event );
    QPointF img_coord_pt = mouseEvent->scenePos();
    x[i] = img_coord_pt.x();
    y[i] = img_coord_pt.y();
    if (i >= 4)
    Draw3Points = false;
    static const QPointF points[4] = {
    QPointF(x[0], y[0]),
    QPointF(x[1], y[1]),
    QPointF(x[2], y[2]),
    QPointF(x[3], y[3])
    QPainter painter(this);
    painter.drawPolygon(points, 4);
    return true;
    } else {
    return QObject::eventFilter(obj, event);

    void QTGraphicsShape::on_pushButton_clicked()
    Draw3Points = true;

  • Qt Champions 2017

    Any reason you are using eventFilter for this. Event filter is not the right place for you to draw. Either you use paint or paintEvent(..) method make this happen.

  • Hi,
    I have reimplemented paint method to draw a polygon like this.


    void OverlayPolygon::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
    qDebug() << "OverlayPolygon->paint";
    QPen p(colorBG);

    // polyPoints is a QList<QPointF*>
    QPointF pointArr[polyPoints.size()];
    for(int i = 0; i < polyPoints.size(); i++){
    painter->drawConvexPolygon(pointArr, polyPoints.size());



    First collect all points and then pass them to painter in reimplemented paint method.
    Hope it helps...

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