canvas阴影

<canvas id="canvas" width="200px" height="200px" ></canvas>
<script type="text/javascript">
  var drawing = document.getElementById("canvas");
if(drawing.getContext){
var context=drawing.getContext("2d");
context.shadowOffsetX=5;
context.shadowOffsetY=5;
context.shadowBlur=4;
context.shadowColor="rgba(0,0,0,0.5)";
context.fillStyle="#ff0000";
context.fillRect(10,10,50,50);

}