title: OpenGL - GL_FRONT versus GL_FRONT_AND_BACK
body: <p>I'm tinkering with an open source project that uses OpenGL for rendering in 3D.  In the construction of the materials I see code like this:</p>
  
  <pre><code>// set ambient material reflectance
  glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT, mAmbient);
  </code></pre>
  
  <p>In other examples, this is used:</p>
  
  <pre><code>glMaterialfv(GL_FRONT, GL_AMBIENT, mAmbient);
  </code></pre>
  
  <p>So my question is, what is the difference here?  Under what circumstances would it look different and, if my volume is enclosed with all normals pointing outwards, is there any performance difference?</p>