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);


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();


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


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();
}


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.