Painting problem for QGraphicsItem

I have to draw polygons on a scene. The polygon coordinates (in meters) are defined in a file like this:

POLYGON LAYER 3
21510.021 -77875.788 21767.976 -78175.582 22040.392 -78262.410
22762.365 -78488.670 23023.361 -78840.834 23401.474 -78810.424
23574.386 -78526.898 23190.596 -78121.994 22981.393 -77899.388
22236.109 -77426.189 21510.021 -77875.788

A Polygon resides in layers which can be any from 1 to 10.

I create Layers which are QGraphicsItem and add them to scene using scene.additem().
Then i create Polygons which are also QGraphicsItem and set the corresponding layer as its parent item.

Code Polygon.cpp:

Code:

---

Polygon::Polygon(QList<QPointF> *polygonPointList, qreal minX, qreal maxX, qreal minY, qreal maxY, QColor color)
{
    _pen = new QPen();
    _pen->setColor(color);
    _rectWidth = maxX-minX;
    _rectHeight = maxY-minY;
    _rectX = -(_rectWidth)/2.0;
    _rectY = -(_rectHeight)/2.0; 
 
    _polygonPath = new QPainterPath();
    _polygonPath->moveTo(polygonPointList->takeAt(0));
 
    while(!polygonPointList->isEmpty()){
        _polygonPath->lineTo(polygonPointList->takeAt(0));
    }
}
 
QRectF Polygon::boundingRect() const
{
    qreal adjustment = 2.000;
    return QRectF(_rectX-adjustment,_rectY-adjustment,_rectWidth+2*adjustment,_rectHeight+2*adjustment);
}
 
void Polygon::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    painter->setPen(*_pen);
    painter->drawPath(*_polygonPath);
    return;
}

---

Problem:
The polygons get painted on the scene but when I scroll the view they don't get repainted correctly.I think
this may be due to incorrect boundingRects. What is the problem with the code above??

Why reinvent the weel? Just use QGraphicsPolygonItem.