title: When drawing an arc using CGContextAddArcToPoint(), what does (x1,y1) and (x2,y2) mean?
body: <p>You can use the following code to draw an arc using Quartz:</p>
  
  <pre><code>CGContextMoveToPoint(context2, x, y);
  CGContextAddArcToPoint(context2, x1, y1, x2, y2, r);
  </code></pre>
  
  <p>In these functions, <code>(x,y)</code> is the starting point and <code>r</code> is the arc radius but what are <code>(x1,y1)</code> and <code>(x2,y2)</code>?</p>