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QPixmap function::drawBackground(const QString& fileName)
{
    QPixmap pix1_(fileName);

    QPixmap temp(pix1_.size());
    temp.fill(Qt::transparent);

    QPainter p1(&temp);
    p1.setCompositionMode(QPainter::CompositionMode_Source);
    p1.drawPixmap(0, 0, pix1_);
    p1.setCompositionMode(QPainter::CompositionMode_DestinationIn);

    //根据QColor中第四个参数设置透明度，0～255
    p1.fillRect(temp.rect(), QColor(0, 0, 0, 125));
    p1.end();

    pix1_ = temp;
    return pix1_;
}