Black colour instead of transparent during image composition

[AsAndrew]

I can't understand why when m_backgoundTransparency is 255 area which supposed to be transparent becomes black. As I Alpha is 255 means totally transparent?

QPixmap     tempPixmap(m_backroundPixmap->size());
 
    tempPixmap.fill(Qt::transparent);
    QPainter    p(&tempPixmap);
 
    p.setCompositionMode(QPainter::CompositionMode_Source);
    p.drawPixmap(0,0,*m_backroundPixmap);
    p.setCompositionMode(QPainter::CompositionMode_DestinationIn);
    p.fillRect(tempPixmap.rect(),QColor(0,0,0,m_backgoundTransparency));
    p.end();
    /* */
    *m_backroundPixmap = tempPixmap;

QPixmap     tempPixmap(m_backroundPixmap->size());

    tempPixmap.fill(Qt::transparent);
    QPainter    p(&tempPixmap);

    p.setCompositionMode(QPainter::CompositionMode_Source);
    p.drawPixmap(0,0,*m_backroundPixmap);
    p.setCompositionMode(QPainter::CompositionMode_DestinationIn);
    p.fillRect(tempPixmap.rect(),QColor(0,0,0,m_backgoundTransparency));
    p.end();
    /* */
    *m_backroundPixmap = tempPixmap;

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[Lykurg]

As I Alpha is 255 means totally transparent?

No, for full transparency alpha must be 0.