QPainter::setCompositingMode without effect

[olidem]

Hmm, ok, I do not think that there is a connection to antialiasing.

The only idea I have is about the the format of the QImages. In the referred example, all images are kept in a premultiplied format (see the loadImage function), but I think this is mainly for rescaling reasons.

The problem I have is that the first step of there rendering process always at first fills the whole image with transparent color

painter.setCompositionMode(QPainter::CompositionMode_Source);
painter.fillRect(resultImage.rect(), Qt::transparent);

painter.setCompositionMode(QPainter::CompositionMode_Source);
painter.fillRect(resultImage.rect(), Qt::transparent);

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which I can't do because I would delete other things.

What about the attached file? Do you see errors there?