html5 遮罩 有什么用

shp.mask = new egret.Rectangle(20,20,30,50);

class Test extends egret.DisplayObjectContainer{
    public constructor()
    {
        super();
        this.addEventListener(egret.Event.ADDED_TO_STAGE,this.onAddToStage,this);
    }
    private onAddToStage(event:egret.Event)
    {
        var shp:egret.Shape = new egret.Shape();
        shp.graphics.beginFill( 0xff0000 );
        shp.graphics.drawRect( 0,0,100,100);
        shp.graphics.endFill();
        this.addChild( shp );
        var shp2:egret.Shape = new egret.Shape();
        shp2.graphics.beginFill( 0x00ff00 );
        shp2.graphics.drawCircle( 0,0, 20);
        shp2.graphics.endFill();
        this.addChild( shp2 );
        shp2.x = 20;
        shp2.y = 20;
    }}

var rect:egret.Rectangle = new egret.Rectangle(20,20,30,50);  shp.mask = rect;

//将maskSprite设置为mySprite的遮罩
mySprite.mask = maskSprite;

//画一个红色的正方形
 var square:egret.Shape = new egret.Shape();
 square.graphics.beginFill(0xff0000);
 square.graphics.drawRect(0,0,100,100);
 square.graphics.endFill();
 this.addChild(square);//画一个蓝色的圆形var circle:egret.Shape = new egret.Shape();circle.graphics.beginFill(0x0000ff);circle.graphics.drawCircle(25,25,25);circle.graphics.endFill();this.addChild(circle);square.mask = circle;

mySprite.mask = null;