BitmapData透過の確認

/**
 * Copyright yarhalla1 ( http://wonderfl.net/user/yarhalla1 )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/pEjY
 */

package {
    import flash.geom.*;
    import flash.display.Sprite;
    import flash.display.Bitmap;
    import flash.display.BitmapData;
    //とうめい描画の確認
    public class FlashTest extends Sprite {
        public function FlashTest() {
            var charaData:BitmapData = new BitmapData(32,32,true,0x00ffffff);
            var sprite:Sprite = new Sprite();
            sprite.graphics.beginFill(0x556677);
            sprite.graphics.drawCircle(16, 16, 16);            
            sprite.graphics.endFill();
            charaData.draw(sprite);
           
            var w:Number = stage.stageWidth;
            var h:Number = stage.stageHeight;
            var bmp:BitmapData=new BitmapData(w, h, true, 0x00ffffff);
            
           
           //nullでいいのかよ！
            bmp.copyPixels( charaData, new Rectangle(0, 0, 32,32), new Point(16, 16), null, null, true);
           bmp.copyPixels( charaData, new Rectangle(0, 0, 32,32), new Point(0, 0), null, null, true);

            this.addChild(new Bitmap(bmp));
        }
    }
}