title: how to use box2d b2Distance function
body: <p>i know b2Distance is used to compute two shape's distance,but how to use it? i don't know how to set the parameters of b2Distance(),e.g this is the structure of b2DistanceInput ,
  public class b2DistanceInput </p>
  
  <pre><code>{
                  public var proxyA:b2DistanceProxy;
                  public var proxyB:b2DistanceProxy;
                  public var transformA:b2Transform;
                  public var transformB:b2Transform;
                  public var useRadii:Boolean;
  }
  </code></pre>
  
  <p>does anybody can tell me what's b2DistanceProxy?what's useRadii,if you can give me an example,it will help to a lot,thanks</p>