title: Transformations in Eigen::Affine3f
body: <p>I am not too familiar with Eigen library and I am stuck at this piece of code</p>
  
  <pre><code>motor1_to_motor2 =
            Translation3f(BigApeLeg::distanceJoint1ToJoint3(),0,0)
          * AngleAxisf(M_PI_2, Vector3f::UnitX())
          * AngleAxisf(M_PI_2, Vector3f::UnitY());
  </code></pre>
  
  <p>motor1_to_motor2 is Eigen::Affine3f
  Big::ApeLeg::distance... must return a float number</p>
  
  <p>But my issue is what is Translation3f and what is AngleAxisf. What do they do? And what do they return?</p>
  
  <p>I am familiar with some basic transformations. I would really appreciate if someone can give me any pointers? Thanks</p>