How to set up QWidget only background transparency

[Qt_crazyer]

I want to set up QWidget only background transparency, and I will draw some text and some others by qpainter on the widget. Using the setWindowOpacity(0.2) will make all things, including the text, etc, transparent. How should I modify it?

#include "widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    setWindowOpacity(0.2);                       
    resize(200, 200);
    setWindowFlag(Qt::FramelessWindowHint);     
    QBitmap bmp(this->size());
    bmp.fill();
    QPainter p(&bmp);
    p.setRenderHint(QPainter::Antialiasing);
    p.setPen(Qt::NoPen);
    p.setBrush(Qt::black);
    p.drawRoundedRect(bmp.rect(),20,20);
    setMask(bmp);
}

Widget::~Widget()
{

}

void Widget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.setPen(Qt::blue);
    painter.setFont(QFont("Arial", 30));
    painter.drawText(rect(), Qt::AlignCenter, "Qt");

    update();
}

Qt 5.9.1 on windows 10
Thanks in advance.

[raven-worx]

@Qt_crazyer
try to set one (or all) of the following attributes (in conjunction to the already set Qt::FramelessWindowHint window flag):

setAttribute(Qt::WA_NoSystemBackground);
setAttribute(Qt::WA_TranslucentBackground);
setAttribute(Qt::WA_PaintOnScreen);