How QT sets some areas to be transparent in the background image

Client development , You may encounter the need to use some “ Irregular figure ” To fill the background . Or you need to set a certain area of the background to “ Hollowing out ”, That's transparency ！ Basically follow these two steps , You can achieve the effect you want ：

1、 Remove the title bar 、 Set the window property to transparent

setWindowFlags(windowFlags() | Qt::FramelessWindowHint ); //  no border 
setAttribute(Qt::WA_TranslucentBackground);

2、 rewrite paintEvent(QPaintEvent* event)

void paintHollowRect::paintEvent(QPaintEvent* event)
{
    
    QPainter painter(this);
    painter.setRenderHint(QPainter::HighQualityAntialiasing);

	// Make an area of the mask transparent 
#if 0
	// Draw a shadow mask 
	painter.fillRect(0, 0, rect().width(), rect().height(), QColor(0, 0, 0, 88));

	//
	int w = rect().width() / 2;
	int h = rect().height() / 2;
	int l = rect().left() + (w / 2);
	int t = rect().top() + (h / 2);

	// The most common type is SourceOver（ Often referred to as alpha blend ）
	painter.setCompositionMode(QPainter::CompositionMode_Source);
	painter.fillRect(l, t, w, h, QColor(255, 255, 255, 10));
	painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
#endif

	QString img = ":/res/bg_main.png";
	QImageReader reader(img);
	QSize size = reader.size();
	resize(size);

	// Draw background picture 
	painter.drawImage(this->rect(), QImage(img));
	//
	painter.setCompositionMode(QPainter::CompositionMode_DestinationIn);
	// Overlay transparent rectangular areas , To display the image 
	painter.fillRect(QRect(10, 10, 300, 300), QColor(0, 0, 0, 0));

    QWidget::paintEvent(event);
}

3、 design sketch

Be careful ： As long as the dialog box has a border , There is no way to achieve transparency , So the border must be removed , And set the attribute to transparent , To get through paintEvent To fulfill your purpose .