Alpha channel weirdness with QGLContext

[renaissanz]

Solved!

OK, this is for any programmer who comes along with a similar problem. If you need the alpha channel for any purpose, you need to enable it when you create the application by using a special X11 (this was done on Linux) version of the QApplication constructor.

Code as follows:

static int attrListDbl[] = 
{ 
	GLX_RGBA, GLX_DOUBLEBUFFER, 
    GLX_RED_SIZE, 4, 
    GLX_GREEN_SIZE, 4, 
    GLX_BLUE_SIZE, 4, 
    GLX_ALPHA_SIZE, 4,
	GLX_DEPTH_SIZE, 16,
    None 
};
 
int main( int argc, char ** argv )
{
	Display * dpy = XOpenDisplay(0);
	int scr = DefaultScreen(dpy);
	XVisualInfo * vis = glXChooseVisual(dpy, scr, attrListDbl);
 
    //	QApplication a( argc, argv );
	QApplication a(dpy, argc, argv, (long unsigned int)vis);
 
	CFormVolumeViewer w;
	a.setMainWidget((QWidget *)&w);
    w.show();
    a.connect( &a, SIGNAL( lastWindowClosed() ), &a, SLOT( quit() ) );
    return a.exec();
}

static int attrListDbl[] = 
{ 
	GLX_RGBA, GLX_DOUBLEBUFFER, 
    GLX_RED_SIZE, 4, 
    GLX_GREEN_SIZE, 4, 
    GLX_BLUE_SIZE, 4, 
    GLX_ALPHA_SIZE, 4,
	GLX_DEPTH_SIZE, 16,
    None 
};

int main( int argc, char ** argv )
{
	Display * dpy = XOpenDisplay(0);
	int scr = DefaultScreen(dpy);
	XVisualInfo * vis = glXChooseVisual(dpy, scr, attrListDbl);
	
    //	QApplication a( argc, argv );
	QApplication a(dpy, argc, argv, (long unsigned int)vis);
    
	CFormVolumeViewer w;
	a.setMainWidget((QWidget *)&w);
    w.show();
    a.connect( &a, SIGNAL( lastWindowClosed() ), &a, SLOT( quit() ) );
    return a.exec();
}

To copy to clipboard, switch view to plain text mode 

This enables the alpha channel for any subsequent QGLContext that you create. Remember that you still need to call QGLFormat::setAlpha(TRUE) before it will work.

Unfortunately, it doesn't look like there's a WIN32 equivalent to this. I know from programming on WIN32 with OpenGL that you DO need to set the alpha bits using the Pixel Format Descriptor.