title: Calculate the normal for the 'facet normal' in STL file format
body: <p>So I have my STL code and I have been putting the normal as 0 0 0 for each triangle, as you can imagine this means that only certain software can utilise the code.</p>
  
  <p>What is the method for working out the normal</p>
  
  <p>i.e. the vector maths</p>