private void increment(float p[], float dp[]) {
    if (interpX) p[X] += dp[X];
    if (interpZ) p[Z] += dp[Z];

    if (interpARGB) {
      p[R] += dp[R];
      p[G] += dp[G];
      p[B] += dp[B];
      p[A] += dp[A];
    }

    if (interpUV) {
      if (FRY) System.out.println("increment() " + p[V] + " " + dp[V]);
      p[U] += dp[U];
      p[V] += dp[V];
    }
  }