How do I remove previous paint and repaint in QGraphicsItem?

lansing

I have things drawn on the my QGraphicsItem object, and I have a wheelEvent that's going to draw new stuff on the item. When the wheelEvent happened, I wanted the old drawing to be removed and painted with the new stuff. I tried using update(), but it only adds the new drawing on top of the old one, it doesn't remove the old one.

void MyGraphicsItem::wheelEvent(QGraphicsSceneWheelEvent *event)
{
        if (event->delta() > 0) {
            setZoomFactor(zoomFactor + 1);            
        }
        else {
            setZoomFactor(zoomFactor - 1);            
        }

        setNewStuff();
        update();
}

Christian Ehrlicher

You could to clear your old background by your own -> e.g. with QPainter::fillRect()

lansing

@Christian-Ehrlicher

Can you elaborate more? I tried creating a clearPaint() inside the QGrpahicsItem class and called it before the update() in the wheelEvent but nothing changed.

void MyGraphicsItem::clearPaint()
{    
    QPainter *painter = new QPainter;
    QBrush blueBrush(Qt::blue);
    painter->fillRect(boundingRect(), blueBrush);
}

void MyGraphicsItem::wheelEvent(QGraphicsSceneWheelEvent *event)
{
    # ...
    setNewStuff();
    clearPaint();
    update();
}

jsulm

@lansing said in How do I remove previous paint and repaint in QGraphicsItem?:

QPainter

QPainter should be used in paintEvent

Christian Ehrlicher

You can only paint inside QGrpahicsItem::paint()

lansing

Okay I got it. Using fillRect() in the paint() worked.

void MyGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{    
    QBrush darkYellowBrush(Qt::darkYellow);
    QBrush redBrush(Qt::red);

    painter->fillRect(boundingRect(), darkYellowBrush);
    painter->drawline(...)
}

Every time an is update() called from other functions, fillRect() will paint the empty rectangle first and then do all the drawing after.

Thanks for the help.

Christian Ehrlicher

Then please mark the issue as solved, thx.

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