title: What is half-vector of light in glsl?
body: <p><em>I'm playing with per pixel lighting shaders and i don't know one thing:</em> What is half vector of light source ?</p>
  
  <pre><code>vec3 halfVector = normalize(gl_LightSource[1].halfVector.xyz);
  </code></pre>
  
  <p>I would like i you can explain it in math rows, i understand math better than words :)</p>