come faccio a utilizzare un oggetto creato dentro una funzione?

[Aran Benjo]

ciao ragazzi!
ho creato una funzione che mi crea un oggetto bitmap al suo interno.

vorrei utilizzare l'oggetto creato per applicare dei filtri
solo che nn so come fare a richiamare l'oggetto appena creato

carica_img = function (dest_mc:MovieClip, file_name):BitmapData {
	//create a movie clip to hold the external file
	this.createEmptyMovieClip("holder", this.getNextHighestDepth());
	/*
	Use the MovieClipLoader class so we can be told
	when the external file loads or fails to load
	*/
	loader = new MovieClipLoader();
	//Give us status updates by firing events
	loader.addListener(this);
	//Load the external file into the holding movie clip
	loader.loadClip(file_name, holder);
	//this function is called by the 'loader' object when the file is loaded and ready to use
	loader.onLoadInit = function() {
		//Create a new bitmap object in memory that is the same size as
		//the loaded file and fill it with transparent pixels
		myBitmap = new BitmapData(holder._width, holder._height, true, 0x00FFFFFF);
		//Snapshot the movie clip that contains the external file we loaded
		myBitmap.draw(holder);
		//
		//we don't need this movie clip anymore, so remove it
		//we already have the bmp in myBitmap istance
		//NOTE that bmp doesn't appear since we attach it on a clip
		holder.removeMovieClip();
		//
		//
		_root.test.attachBitmap(myBitmap, 1);
	};
	//this function is called by the 'loader' object if the file couldn't be loadd
	function onLoadError() {
		/*		
		    The external file failed to load so we don't need 
		    the holding movieclip anymore
		*/
		holder.removeMovieClip();
	}
	return myBitmap;
};

dopo aver chiamato la func vorrei poter utilizzare myBitmap...
ma come valore è undefined

sapete dirmi perchè?

grazieeeee