@
glWidget->setAutoFillBackground(false);
@
I think this is the problem.
You have disabled any background painting on the viewport (which is then overwritten by you via reimplementing the drawBackground of the DeclarativeView).
What he's then doing is painting transparent ... which is then composed with the "background".
If you try a half-transparent red as your Rectangle color, I would guess you would see red garbage :)

... the only thing which is strange is that:
@
viewer->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
@
should clear the color buffer for every frame ...

2- initialise your qmlApplicationViewer as follows:
@#include <QtGui/QApplication>
#include "qmlapplicationviewer.h"
#include <QtOpenGL>

int main(int argc, char *argv[])
{

QScopedPointer<QApplication> app(createApplication(argc, argv));
QScopedPointer<QmlApplicationViewer> viewer(QmlApplicationViewer::create());

viewer->setWindowFlags(Qt::FramelessWindowHint);
viewer->setAttribute(Qt::WA_NoSystemBackground);
viewer->setStyleSheet("background: transparent;");
QPalette palette;
palette.setColor(QPalette::Base, Qt::transparent);
viewer->setPalette(palette);

viewer->setOrientation(QmlApplicationViewer::ScreenOrientationAuto);
viewer->setMainQmlFile&#40;QLatin1String("qml/SimpleOpenGLWithTransparency/main.qml"&#41;&#41;;

QGLFormat format = QGLFormat::defaultFormat();
format.setSampleBuffers(false);
format.setSwapInterval(1);
format.setAlpha(true);

QGLWidget *glWidget = new QGLWidget(format);
glWidget->setAutoFillBackground(false);
viewer->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
viewer->setViewport(glWidget);

viewer->showExpanded();


return app->exec&#40;&#41;;

}
@

Thank you Mathias for your suggestions, thank you Thomas also, even if your tricks did not work for me.

Best of luck for all!
Bill

[Edited for code syntax highlighting. -Bradley]

"The only way i found to circumvent this issue is to overwrite QGraphicsView::drawBackground() in the QMLApplicationViewer:

void QmlApplicationViewer::drawBackground(QPainter* painter, const QRectF&)
{
painter->beginNativePainting();
glClearDepthf(1.f);
glClearColor(0.f, 0.f, 0.f, 0.f);
glClear(GL_COLOR_BUFFER_BIT| GL_DEPTH_BUFFER_BIT);

painter->endNativePainting();
}

Maybe there is another way.

"

Do you know if it's a good solution? or if there is something else I could try without changing the qmlApplicationViewer itself?
Thank you,
Bill

could also improve things?
Bill

... this should do magic
(before and/or after setViewport ... there is something going on when setting the viewport .. moving this line around in the code can improve things a lot ;) )

... if this doesn't help, try to add:
glWidget->qglClearColor(Qt::transparent);

I got no MS-Windows machine here, but it should be ... somewhat similar on linux and window ;)

Q_DECL_EXPORT int main(int argc, char *argv[])
{
#ifdef OPENGL_GRAPHICSSYSTEM
QApplication::setGraphicsSystem("opengl");
#endif

QScopedPointer<QApplication> app(createApplication(argc, argv));
QScopedPointer<QmlApplicationViewer> viewer(QmlApplicationViewer::create());

#ifndef OPENGL_GRAPHICSSYSTEM
QGLFormat format = QGLFormat::defaultFormat();
format.setSampleBuffers(false);
format.setSwapInterval(1);

QGLWidget* glWidget = new QGLWidget(format);
glWidget->setAutoFillBackground(false);
viewer->setViewport(glWidget);

#endif

viewer->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);

// viewer.setAttribute(Qt::WA_OpaquePaintEvent);
//viewer->setWindowFlags(Qt::FramelessWindowHint|Qt::SplashScreen);
//viewer->setAttribute(Qt::WA_TranslucentBackground);
viewer->setAttribute(Qt::WA_NoSystemBackground);
viewer->setStyleSheet("background: transparent;");
viewer->setFrameStyle(QFrame::NoFrame);
viewer->setOrientation(QmlApplicationViewer::ScreenOrientationAuto);

viewer->setMainQmlFile(QLatin1String("qml/SimpleOpenGLWithTransparency/main.qml"));
viewer->showExpanded();

return app->exec();

}

@

And on our target it displays white color instead of transparent. We have two different hardware framebuffers, my app is running in the FB on top, and i can not see through. If I remove the openGL initialization, I can see through the image without any problem.

Do you think it can improve if I set the :
format.setAlpha(true);
instruction, or it has nothing to do with this?

Regards,
Bill

!http://i40.tinypic.com/29n9jxi.png!