qt – 窗口及其控件透明

void setWindowOpacity(qreal level)

windowOpacity : double
This property holds the level of opacity for the window.
The valid range of opacity is from 1.0 (completely opaque) to 0.0 (completely transparent).
By default the value of this property is 1.0.
This feature is available on Embedded Linux, macOS, Windows, and X11 platforms that support the Composite extension.
Note: On X11 you need to have a composite manager running, and the X11 specific 
_NET_WM_WINDOW_OPACITY atom needs to be supported by the window manager you are using.
Warning: Changing this property from opaque to transparent might issue a paint event that needs to be 
processed before the window is displayed correctly. This affects mainly the use of QPixmap::grabWindow(). 
Also note that semi-transparent windows update and resize significantly slower than opaque windows.

qreal windowOpacity() const

void MainWindow::on_horizontalScrollBar_sliderMoved(int position)
{
    setWindowOpacity(position*1.0/10);
    ui->label_level->setText("level = "+QString::number(position*1.0/10));
}