eights
                wr = dr - wy - wm,
                wg = dg - wy - wc,
                wb = db - wc - wm,

                // final luminance
                l = m + kR*wr + kY*wy + kG*wg + kC*wc + kB*wb + kM*wm;

            return half4(l, l, l, c.a);
        }