GS function

surface greyscale_image(const surface &surf, bool optimize)
{
    if(surf == NULL)
        return NULL;

    surface nsurf(make_neutral_surface(surf));
    if(nsurf == NULL) {
        std::cerr << "failed to make neutral surface\n";
        return NULL;
    }

    {
        surface_lock lock(nsurf);
        Uint32* beg = lock.pixels();
        Uint32* end = beg + nsurf->w*surf->h;

        while(beg != end) {
            Uint8 alpha = (*beg) >> 24;

            if(alpha) {
                Uint8 r, g, b;
                r = (*beg) >> 16;
                g = (*beg) >> 8;
                b = (*beg);
                //const Uint8 avg = (red+green+blue)/3;

                // Use the correct formula for RGB to grayscale conversion.
                // Ok, this is no big deal :)
                // The correct formula being:
                // gray=0.299red+0.587green+0.114blue
                const Uint8 avg = static_cast<Uint8>((
                    77  * static_cast<Uint16>(r) +
                    150 * static_cast<Uint16>(g) +
                    29  * static_cast<Uint16>(b)  ) / 256);

                *beg = (alpha << 24) | (avg << 16) | (avg << 8) | avg;
            }

            ++beg;
        }
    }

    return optimize ? create_optimized_surface(nsurf) : nsurf;
}