import flash.display.BitmapData;
var linkageId:String = "libraryBitmap";
var myBitmapData:BitmapData = BitmapData.loadBitmap(linkageId);
trace(myBitmapData instanceof BitmapData);

var mc:MovieClip = this.createEmptyMovieClip("mc", this.getNextHighestDepth());
mc.attachBitmap(myBitmapData, this.getNextHighestDepth());

mc.onPress = function():Void  {
	var alpha:String = (myBitmapData.getPixel32(this._xmouse, this._ymouse) >> 24 & 0xFF).toString(16);
	trace(">> alpha: "+alpha);
	if (alpha == "ff") {
		mc.startDrag();
	}
};
mc.onRelease = mc.onReleaseOutside=mc.stopDrag;

mc.addEventListener(MouseEvent.MOUSE_DOWN,onMouseDownListener,false,0,false);
mc.addEventListener(MouseEvent.MOUSE_UP,onMouseUPListener,false,0,false);
mc.addEventListener(MouseEvent.MOUSE_OUT,onMouseUPListener,false,0,false);

function onMouseDownListener(e:MouseEvent):void {
	trace(e.target.name);
	if (e.target.name=="alpha_mc") {
		mc.startDrag();
	}
}

function onMouseUPListener(e:MouseEvent):void {
	mc.stopDrag();
}