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;

  • 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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