How to create a translucent/transparent `QOpenGLWindow` - c++

Question

As is widely known that, to make a QWidget/QOpenGLWidget translucent/transparent, one only needs to:
widget->setAttribute(Qt::WA_TranslucentBackground)
However, since QWindow/QOpenGLWindow is not a widget and doesn't have setAttribute, I don't know how to do the same for a QOpenGLWindow. I guess this is theoretically possible since QWidget is backed by a QWindow according to Qt's source code.
I searched on Google but there's not much information about the transparency of QWindow

Answer 1

I found out that this will happen in QOpenGLWindow by QSurfaceFormat and setAlphaBufferSize(8);
Look at this example:
in mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QOpenGLWindow>
class MainWindow: public QOpenGLWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
// QOpenGLWindow interface
protected:
void initializeGL();
void resizeGL(int w, int h);
void paintGL();
// QWindow interface
void resizeEvent(QResizeEvent *);
// QPaintDeviceWindow interface
void paintEvent(QPaintEvent *event);
};
#endif // MAINWINDOW_H
in mainwindow.cpp
#include "mainwindow.h"
MainWindow::MainWindow(QWidget *parent)
{
}
MainWindow::~MainWindow()
{
}
void MainWindow::initializeGL()
{
// Set the transparency to the scene to use the transparency of the fragment shader
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
// set the background color = clear color
glClearColor(0.0f, 0.0f, 0.0f, .0f);
glClear(GL_COLOR_BUFFER_BIT);
}
void MainWindow::resizeGL(int w, int h)
{
}
void MainWindow::paintGL()
{
}
void MainWindow::resizeEvent(QResizeEvent *)
{
}
void MainWindow::paintEvent(QPaintEvent *event)
{
paintGL();
}
and finally in main.cpp
#include "mainwindow.h"
#include <QGuiApplication>
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QSurfaceFormat format;
format.setRenderableType(QSurfaceFormat::OpenGL);
format.setProfile(QSurfaceFormat::CoreProfile);
format.setVersion(3, 3);
format.setAlphaBufferSize(8);
MainWindow w;
w.setFormat(format);
w.resize(640, 480);
w.show();
return app.exec();
}
I w.setFormat(format); which means that QOpenGLWindow or MainWindow not QOpenGLContext.
This will be the Result: