前景图片alpha通道值的设置

//---画背景
QPixmap pixmapBg(":bg.jpg");
QPainter painter(this);
painter.drawPixmap(0, 0, pixmapBg);

QPixmap pixmapTop(":top.jpg");
int alpha = 125;
//---前景图片透明度处理
QPixmap temp(pixmapTop.size());
temp.fill(Qt::transparent);
QPainter p(&temp);
p.setCompositionMode(QPainter::CompositionMode_Source);
p.drawPixmap(0, 0, pixmapTop);
p.setCompositionMode(QPainter::CompositionMode_DestinationIn);
p.fillRect(temp.rect(), QColor(0, 0, 0, alpha));
//--lable显示前景图片
ui->label->setScaledContents(true);
ui->label->setPixmap(temp);