painting on QGraphicsView

Hi,

I have to design a canvas where users can drag & drop images as background and paint by tablet.

I use view/scene to drag & drop the background images. After switching mode, user sketch can be captured in a pixmap after tablet events.

However, how can I blend the pixmap (user sketch) into the background images ?

Code:

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class TabletCanvas : public QGraphicsView

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Code:

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void TabletCanvas::drawForeground(QPainter *painter, const QRectF& rect)
{
    QPixmap _m(pixmap); // the pixmap keeps user sketch
    //_m.setMask(pixmap.createMaskFromColor(Qt::white, Qt::MaskInColor)); 
    _m.setAlphaChannel(pixmap.createMaskFromColor(Qt::white, Qt::MaskOutColor)); 
 
    painter->drawPixmap(0, 0, _m);
}

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The above code works like to "paste" sketch on the images.
Attachment 8992

I want to blend the sketch and 'background' image pieces.

How to do it ?
1. Can I get the scene-coordinate pixamap in QGraphicsView::paintEvnet() or drawForeground() ?
2. Or the user sketch pixmap should be another item in scene ?

Please give me some suggestions, thank you