title: How to convert quaternion to matrix4 form of real number, what`s the principle?
body: <p>Suppose <code>Quaternion q=a+bi+cj+dk</code>, the matrix4 of q is:</p>
  
  <pre><code>| a -b  d -c|
  | b  a -c -d|
  |-d  c  a -b|
  | c  d  b  a|
  </code></pre>
  
  <p>Where does this matrix come from?</p>