Transparent Widgets

[CodeFreak]

Hi everyone,
I want to make a transparent window like:
trpr.png

I tried following codes:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    setAutoFillBackground( false );
    setWindowFlags(Qt::FramelessWindowHint);
//    setAttribute(Qt::WA_TranslucentBackground);
//    setAttribute(Qt::WA_TransparentForMouseEvents);
 
    QLinearGradient alphaGradient(rect().topLeft(), rect().bottomLeft());
    alphaGradient.setColorAt(0.0, Qt::transparent);
    alphaGradient.setColorAt(0.5, Qt::black);
    alphaGradient.setColorAt(1.0, Qt::transparent);
    QGraphicsOpacityEffect *effect = new QGraphicsOpacityEffect(this);
    effect->setOpacityMask(alphaGradient);
    effect->setOpacity(0.8);
    setGraphicsEffect(effect);
}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    setAutoFillBackground( false );
    setWindowFlags(Qt::FramelessWindowHint);
//    setAttribute(Qt::WA_TranslucentBackground);
//    setAttribute(Qt::WA_TransparentForMouseEvents);

    QLinearGradient alphaGradient(rect().topLeft(), rect().bottomLeft());
    alphaGradient.setColorAt(0.0, Qt::transparent);
    alphaGradient.setColorAt(0.5, Qt::black);
    alphaGradient.setColorAt(1.0, Qt::transparent);
    QGraphicsOpacityEffect *effect = new QGraphicsOpacityEffect(this);
    effect->setOpacityMask(alphaGradient);
    effect->setOpacity(0.8);
    setGraphicsEffect(effect);
}

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If I use "WA_TranslucentBackground" background will be omitted from the widget (you can select anything behind it !) but, I need a semi transparent window which its opacity/transparency can be adjusted.
Thanks.