Opacity with custom QGraphicsItem

[mkuettler]

Hello.

I wrote a custom graphics item that paints an image with the following code:

@
void ImageItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
Q_UNUSED(option);
Q_UNUSED(widget);
painter->drawImage(QRectF(QPointF(0,0), m_size), m_image,
m_image.rect());
}
@

That work very well, but when the item's opacity is less then 1 the colors in the image do not fade to the background color, but to the respective inverse color: It seems that instead of
@
(1 - opaticy) * backgroundColor + opacity * color
@

the new color is
@
(1 - opaticy) * (backgroundColor - color) + opacity * color
@

For other items such as QGraphicsTextItem the opacity works as expected. Can somebody point me to what the problem might be?