setting QPixmap's alpha channel

[ugluk]

Did you try method #1 without the fill? Perhaps this would work?

QImage image(pixmap.size(), QImage::Format_ARGB32_Premultiplied);
 
{
  QPainter painter(&image);
 
  painter.setCompositionMode(QPainter::CompositionMode_Source);
 
  painter.setOpacity(.2);
  painter.drawPixmap(0, 0, pixmap);
}
 
pixmap = QPixmap::fromImage(image);

QImage image(pixmap.size(), QImage::Format_ARGB32_Premultiplied);

{
  QPainter painter(&image);

  painter.setCompositionMode(QPainter::CompositionMode_Source);

  painter.setOpacity(.2);
  painter.drawPixmap(0, 0, pixmap);
}

pixmap = QPixmap::fromImage(image);

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