declarative alpha changer - c++

Question

I'd like to write a QDeclarativeItem FadeEdges that I instantiate from QML, such as in this example:
FadeEdges {
Text {
id: sometext
text: "some text"
}
}
After sometext repaints, I'd like to reduce the alphas (from fully opaque to transparent) of the pixels it painted at its edges. So its rendered edges will appear faded. My question is what mechanisms, if any, are available to make this change in child alpha values.
I've tried to install event filters and setting setFiltersChildEvents(). No PaintEvents seem to be sent.

Answer 1

It is not an exact answer to my own question but I suppose it will do for starters. Only the QGraphicsEffect::draw member function needs to be overridden (margins_ is a QMargins member variable):
void FadeEdgesEffect::draw(QPainter* painter)
{
QLinearGradient lg;
lg.setColorAt(qreal(0), QColor(0, 0, 0, 0));
lg.setColorAt(qreal(1), QColor(0, 0, 0, 255));
qreal const width(boundingRect().width());
qreal const height(boundingRect().height());
QPoint offset;
QPixmap pixmap(sourcePixmap(Qt::LogicalCoordinates, &offset));
{
QPainter p(&pixmap);
p.setCompositionMode(QPainter::CompositionMode_DestinationIn);
if (margins_.left())
{
lg.setStart(qreal(0), qreal(0));
lg.setFinalStop(margins_.left(), qreal(0));
p.fillRect(QRectF(0, 0, margins_.left(), height), lg);
}
// else do nothing
if (margins_.right())
{
lg.setStart(qreal(width), qreal(0));
lg.setFinalStop(width - margins_.right(), qreal(0));
p.fillRect(QRectF(width - margins_.right(),
0, margins_.right(), height), lg);
}
// else do nothing
if (margins_.bottom())
{
lg.setStart(qreal(0), height);
lg.setFinalStop(qreal(0), height - margins_.top());
p.fillRect(QRectF(0, height - margins_.bottom(),
width, margins_.bottom()), lg);
}
// else do nothing
if (margins_.top())
{
lg.setStart(qreal(0), qreal(0));
lg.setFinalStop(0, margins_.top());
p.fillRect(QRectF(0, 0, width, margins_.top()), lg);
}
// else do nothing
}
painter->drawPixmap(offset, pixmap);
}