title: simple math formula to code interpretation
body: <p>Im making a shader from a technical paper. But I ran into this formula. Which is hard to understand.
  H=H-2(H,U)U
  where H and U are vectors (x,y,z).
  what operation means a(H,U) my understanding is something like:
  H=H-(2*(H*U)*U);
  But doing this, give me odd results.
  thanks in advance.</p>