Problem with QGraphicsPolygonItem and drag their corners with mouse after move

Question

i have been working on a QT application using the QGraphicsPolygonItem. First i created a class called DiagramScene who inherites the QGraphicsScene. First i create the item and put red corners in every point of the Polygon:

        item = new DiagramItem(myItemType, myItemMenu);
        item->setBrush(myItemColor);
        addItem(item);
        item->setPen(QPen(myLineColor, 2));
        item->setPolygon(mouseEvent->scenePos());
        item->setSelected(true);
        emit itemInserted(item);
        this->putPointsOnItem(item);

The the putPointsOnItem function:

void DiagramScene::putPointsOnItem(DiagramItem *item) {
    qDebug() << "putPointsOnItem";
    removeControlPoints();
    QPolygonF p = item->polygon();
    poly_points.clear();
    poly_points = p;
    qDebug() << p;
    for (int i = 0; i < p.size(); i++) {
        QPointF point = item->mapToScene(p[i]);
        QGraphicsEllipseItem *ellipse = addEllipse(QRectF(point.x() -5, point.y() -5, 8, 8), QPen(Qt::red, 3), QBrush(QColor("red")));
        ellipse->setFlag(QGraphicsItem::ItemIsMovable);
        ellipse->setZValue(100000000);
        corner_points.append(ellipse);
        real_corners.append(item);
    }
}

Then i check if the corner is selected:

void DiagramScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent) {

    if (mouseEvent->button() != Qt::LeftButton) {
        return;
    }

    this->finded = false;
    for (int i = 0; i < this->corner_points.length(); i++) {
        QGraphicsEllipseItem *item = this->corner_points[i];
        qDebug() << item;
        if (item->contains(mouseEvent->scenePos())) {
            item->setSelected(true);
            const QList<QGraphicsItem *> items = selectedItems();
            if (items.length()>0) {
                DiagramItem *startItem = qgraphicsitem_cast<DiagramItem *>(items.first());
                startItem->setFlag(QGraphicsItem::ItemIsMovable, false);
                selectedItem = startItem;
            }
            this->selected_corner = item;
            this->selected_corner_point = this->poly_points[i];
            selected_corner_number = i;
            finded = true;
            break;
        }
    }
    if (this->finded == true) {
        QGraphicsScene::mousePressEvent(mouseEvent);
        qDebug() << "POLY POINTS FINDED:";
        qDebug() << poly_points;
        return;
    }

And when the user move the mouse i check the selected corner and put the mouse position into one of the points:

void DiagramScene::mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent) {

    
    QGraphicsScene::mouseMoveEvent(mouseEvent);

    QString coord = QString("%1, %2").arg(mouseEvent->scenePos().x()).arg(mouseEvent->scenePos().y());
    parentMain->setCoordinates(coord);

    if ((mouseEvent->buttons() & Qt::LeftButton)) {

        const QList<QGraphicsItem *> items = selectedItems();
        
        if (finded == true) {
        
            finded = false;

            for (int i = 0; i < corner_points.size(); ++i) {
                if (corner_points.at(i) == selected_corner) {
                    poly_points[selected_corner_number] = mouseEvent->scenePos();
                    finded = true;
                    break;
                }
            }
            if (finded) {
                qDebug() << poly_points;
                selectedItem->setPolygonItem(poly_points);
                return;
            }

Until that everithing works fine, but when the user move the item, the position dont work as i expected. For now i'm going crazy for many days and need help with this. Thanks a lot!

I think the problem is here:

poly_points[selected_corner_number] = mouseEvent->scenePos();

problem image described

Answer 1

Your comments pointed me in the right direction and I found the solution:

poly_points[selected_corner_number] = selectedItem->mapFromScene(mouseEvent->scenePos());

Thanks a lot!