shader
test_luminance_v_dcolor (output color Cout = 0)
{
    color c1 = color(u, v, u+v);
    
    color c2 = color(u+v, u, v);

    float val1 = luminance(c1);
    float val2 = 0.0;
    if (int(P[0]*64)%2==0) {
        val2 = luminance(c2);
    }
    
    Cout = color(Dx(val1), Dy(val1),  Dx(val2) + Dy(val2));
}
