wet floor problem

Why doesn't this code work? I'd like to create the wet floor effect with Qt and I am painting on a QWidget.

Code:

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painter.save();
 
  reflectionGradient.setColorAt(.0f, QColor(0, 0, 0, 0));
  reflectionGradient.setColorAt(1.0f, QColor(0, 0, 0, 255));
 
  reflectionGradient.setFinalStop(0, pixmap.height());
 
  painter.setCompositionMode(QPainter::CompositionMode_DestinationIn);
 
  painter.fillRect(pixmap.rect(), reflectionGradient);
 
  painter.setCompositionMode(QPainter::CompositionMode_DestinationOver);
 
  painter.drawPixmap(zero, pixmap);
 
  painter.restore();

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Added after 16 minutes:

I think I have the answer:

int QPaintDevice::depth () const

returns 24, hence no destination alpha. Is it possible to force destination alpha to 32 somehow, or work around this limitation, without creating a temporary QPixmap or QImage.