title: Angle between 3 points?
body: <p>Given points ABC, how could I find angle ABC? I'm making a feehand tool for a vector drawing application and to minimize the number of points it generates, I wont add a points unless the angle of the mouse position and the last 2 points is greater than a certain threshold.
  Thanks</p>
  
  <p>what I had:</p>
  
  <pre><code>int CGlEngineFunctions::GetAngleABC( POINTFLOAT a, POINTFLOAT b, POINTFLOAT c )
  {
      POINTFLOAT ab;
      POINTFLOAT ac;
  
      ab.x = b.x - a.x;
      ab.y = b.y - a.y;
  
      ac.x = b.x - c.x;
      ac.y = b.y - c.y;
  
      float dotabac = (ab.x * ab.y + ac.x * ac.y);
      float lenab = sqrt(ab.x * ab.x + ab.y * ab.y);
      float lenac = sqrt(ac.x * ac.x + ac.y * ac.y);
  
      float dacos = dotabac / lenab / lenac;
  
      float rslt = acos(dacos);
      float rs = (rslt * 180) / 3.141592;
       RoundNumber(rs);
       return (int)rs;
  
  
  }
  </code></pre>