18basisGradient[13] = 0.5 * (1.0 - rr) * (1.0 + rr) * (-1.0 + 2.0 * ss);
21basisGradient[15] = 0.5 * (1.0 - ss) * (1.0 + ss) * (1.0 + 2.0 * rr);
26basisGradient[19] = 0.5 * (1.0 - rr) * (1.0 + rr) * (1.0 + 2.0 * ss);
29basisGradient[21] = 0.5 * (1.0 - ss) * (1.0 + ss) * (-1.0 + 2.0 * rr);