Alpha channel weirdness with QGLContext

[renaissanz]

Hi, I'm using QGLContext to render a small viewport with some triangles and rectangles in it. I have problems when trying to read the alpha channel back out using glReadPixels. Can this have something to do with using QGLContext?

Please see below:

I initialize like so:

QGLContext * context = 0;
QGLFormat format;
context = new QGLContext(format, (QPaintDevice *)glPanel);
context->create();
context->makeCurrent();
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glClearDepth(1.0f);

QGLContext * context = 0;
QGLFormat format;
context = new QGLContext(format, (QPaintDevice *)glPanel);
context->create();
context->makeCurrent();
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glClearDepth(1.0f);

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I draw like so:

context->makeCurrent();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_CULL_FACE);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.0);
 
for(int i = 0; i < numWidgets; i++)
{
	if(Widget[i]->type == TriangleWidget)
	{
		Widget[i]->DrawTriangle(mode);
	}
	else
	{
		Widget[i]->DrawRectangle(mode);
	}
}
 
glFlush();
 
if(context->format().doubleBuffer())
	context->swapBuffers();

context->makeCurrent();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_CULL_FACE);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.0);
	
for(int i = 0; i < numWidgets; i++)
{
	if(Widget[i]->type == TriangleWidget)
	{
		Widget[i]->DrawTriangle(mode);
	}
	else
	{
		Widget[i]->DrawRectangle(mode);
	}
}

glFlush();

if(context->format().doubleBuffer())
	context->swapBuffers();

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I use glReadPixels like so:

unsigned char * LUT2D = new unsigned char[256 * 256 * 4];
context->makeCurrent();
glReadBuffer(GL_FRONT); 
glReadPixels(0, 0, 256, 256, GL_RGBA, GL_UNSIGNED_BYTE, LUT2D);
 
//	debug
FILE * fp = fopen("/tmp/LUT2D.txt", "w+");
if(fp)
{
	unsigned int * tmp = (unsigned int *)LUT2D;
	for(int h = 0; h < 256; h++)
	{
		for(int w = 0; w < 256; w++)
		{
			fprintf(fp, "0x%08X, ", tmp[w * h]);
		}
		fprintf(fp, "\n");
	}
	fclose(fp);
}
//	debug

unsigned char * LUT2D = new unsigned char[256 * 256 * 4];
context->makeCurrent();
glReadBuffer(GL_FRONT); 
glReadPixels(0, 0, 256, 256, GL_RGBA, GL_UNSIGNED_BYTE, LUT2D);
	
//	debug
FILE * fp = fopen("/tmp/LUT2D.txt", "w+");
if(fp)
{
	unsigned int * tmp = (unsigned int *)LUT2D;
	for(int h = 0; h < 256; h++)
	{
		for(int w = 0; w < 256; w++)
		{
			fprintf(fp, "0x%08X, ", tmp[w * h]);
		}
		fprintf(fp, "\n");
	}
	fclose(fp);
}
//	debug

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As you can see, I'm writing the buffer to a file to see what's in the alpha channel. It's 0xFF (or 1.0 or 255 depending on your preference) for every value.

Once again, is this some weirdness with using QGLContext?

[renaissanz]

OK, I also tried

format->setAlpha(TRUE);

And that gave me an invalid context. Help anyone?

[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();
}

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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.