Alpha mask using Actionscript 3.0

Have you ever tried to create an alpha mask using Actionscript 3.0?
The thing is reachable with the method copyChannel of the BitmapData class.

1. Load an external image
2. Create a new instance of BitmapData (bitmap_data) and assign it to bitmapData property of the external bitmap
3. Create a new instance of BitmapData (bitmap_data_copy) with same size of external bitmap
4. Copy the pixels of external bitmap in the new instance bitmap_data_copy
5. Use the dispose method on bitmap_data to free memory
6. Create a mask form using a Shape
7. Create a new instance of BitmapData (bitmap_data_shape) the same size of external image and use draw method over shape
8. bitmap_data_copy uses the method copyChannel on bitmap_data_shape
9. Give bitmap_data_copy to property bitmapData of external image

package
{
    import flash.display.*;
    import flash.events.*;
    import flash.net.*;
    import flash.geom.*;
    import flash.filters.*;

    public class Main extends MovieClip
    {
        private var loaded_bitmap:Bitmap;
        private var bitmap_data:BitmapData;
        private var bitmap_data_copy:BitmapData;
        private var bitmap_data_shape:BitmapData;

        private var shape:Shape;

        public function Main()
        {
            addEventListener(Event.ADDED_TO_STAGE,init);
            addEventListener(Event.REMOVED_FROM_STAGE,destroy);
        }

        private function init(evt:Event):void
        {
            removeEventListener(Event.ADDED_TO_STAGE,init);

            loadImage();
        }

        private function loadImage():void
        {
            var request:URLRequest=new URLRequest("tiger.jpg");
            var loader:Loader=new Loader();
            loader.contentLoaderInfo.addEventListener(Event.COMPLETE,onImageLoaded);
            loader.load(request);
        }

        private function onImageLoaded(evt:Event):void
        {
            evt.target.removeEventListener(Event.COMPLETE,onImageLoaded);

            loaded_bitmap=evt.target.loader.content as Bitmap;

            createAlphaMask();
        }

        private function createAlphaMask():void
        {
            bitmap_data=loaded_bitmap.bitmapData;
            var w:Number=bitmap_data.width;
            var h:Number=bitmap_data.height;

            bitmap_data_copy=new BitmapData(w,h,true,0x00FFFFFF);
            bitmap_data_copy.copyPixels(bitmap_data,bitmap_data.rect,new Point());
            bitmap_data.dispose();

            shape=createMask(w,h);
            bitmap_data_shape=new BitmapData(w,h,true,0x00FFFFFF);
            bitmap_data_shape.draw(shape);

            bitmap_data_copy.copyChannel(bitmap_data_shape,bitmap_data_shape.rect,new Point(),BitmapDataChannel.ALPHA,BitmapDataChannel.ALPHA);
            loaded_bitmap.bitmapData=bitmap_data_copy;
            addChild(loaded_bitmap);
        }

        private function createMask(w:Number,h:Number):Shape
        {
            var new_shape:Shape=new Shape();
            new_shape.graphics.beginFill(0xFFFFFF);
            new_shape.graphics.drawEllipse(50,50,w-100,h-100);
            new_shape.graphics.endFill();

            var blur_filter:BlurFilter=new BlurFilter(50,50,BitmapFilterQuality.HIGH);
            new_shape.filters=[blur_filter];
            addChild(new_shape);

            return new_shape;
        }

        private function destroy(evt:Event):void
        {
            removeEventListener(Event.REMOVED_FROM_STAGE,destroy);

            bitmap_data_copy.dispose();
            bitmap_data_shape.dispose();

            removeChild(loaded_bitmap);
            removeChild(shape);

            loaded_bitmap=null;
            shape=null;
        }
    }
}