I can make a obj to use the canvas to draw like this:
MyObj.myDiv = new Canvas($("effectDiv"), Setting.width,  Setting.height);
Then, I use this to draw a rectangle on the canvas:
 var c =  new Rectangle(80, 80,
  {
   fill: [220, 40, 90]
  }
 );
 var move = new Timeline;
 move.addKeyframe(0,
  {
   x: 0,
   y: 0
  }
 );
 c.addTimeline(move);
 MyObj.myDiv.append(c);
But after I draw the rectangle, I want clear the canvas, but I don't know which method and how to do this... ...
O...one more thing: it is the CAKE's web site: Link