Efficiently read the average color of the screen c

I want to get the average color of the screen content when running XBMC to change the color of a TV ambient light. XBMC is running on a small HTPC with OpenGL ES 2.0 hardware (Raspberry Pi) running a Debian-derived distribution. I guess I have to read from the screen frame buffer in which XBMC draws using OpenGL. (At least, I think and hope that XBMC renders everything using OpenGL.)

Is it possible to read the OpenGL frame buffer representing the whole screen output? What am I going to need to access it? Do I also need an own render context to access the frame buffer of the screen? (I don't render anything by myself to the screen, I only want to read).

Efficiently calculating the average color is the next task. I'm thinking about reading every 8th or 16th pixel per row/column (would be enough, we are talking about 1080p HD movies) and then calculating the average on the CPU. Any ideas for a better solution are welcome.

You should take a look at the source code of Boblight.

Extending Boblight seems like a viable alternative (if it does not support what you need already!).

If not, look at src/clients/ folder. boblight-getpixel.c (for MS Windows) and boblight-X11.c are 'pixel grabbers'; standalone programs that do exactly what you need, and then communicate the grabbed color to the boblight server.

In boblight-X11.c you have examples of using XShmGetImage or the slower XGetImage to read portions of the screen using X11/extensions/XShm.h, a portion of that code does:

[...]

if(!XShmGetImage(dpy, root_win, xim, 0, 0, AllPlanes))
{
sleep(1);
pthread_mutex_unlock(&grabmutex);
return;
}

XSync(dpy, True);

for (x = 0; x < width; x += xadd)
{
    for (y = 0; y < height; y += yadd)
    {
       pixel = XGetPixel(xim, x, y);
       RGB[0] = (pixel >> 16) & 0xff;
       RGB[1] = (pixel >>  8) & 0xff;
       RGB[2] = (pixel >>  0) & 0xff;

       /*add it to the RGB array*/
       boblight_add_pixel (&config, RGB, x, y);
    }
}

[...]