Novice as3 bitmap question

programming flash
1

Hello,
I have this tutorial code from AS3 Cookbook, fog like effect on Bitmap data

 import flash.display.Sprite;
    import flash.display.Bitmap;
    import flash.display.BitmapData;
    import flash.events.Event;
    import flash.geom.Point;

 var offset:int=0;
 var bitmap:BitmapData = new BitmapData(330, 100,true,0x0);

            var image:Bitmap = new Bitmap(bitmap);
            addChild(image);
            image.x=0;
            image.y=190;
            
            addEventListener(Event.ENTER_FRAME, onEnter);
       function onEnter(event:Event):void {
            offset++;
            var point:Point = new Point(0, offset);

           
            bitmap.perlinNoise(150, 100, 5, 500, false, true,
                            8, true, [point, point]);
        }

and interested in blending it on some background.It needs to be on the bottom of movie clip, but without that bitmaps border showing around the noise effect…Threshold? Gradient?How is that achieved?
Heres fla.