请帮我详细解说这段代码（最好能给我讲讲bitmapdata类的阈值问题）

package {
import flash.display.Bitmap;
import flash.display.BitmapData;
import flash.display.Loader;
import flash.display.Sprite;
import flash.events.*;
import flash.geom.Point;
import flash.geom.Rectangle;
import flash.net.URLRequest;

public class BitmapDataExample extends Sprite {
private var url:String = "Image.gif";
private var size:uint = 80;

public function BitmapDataExample() {
configureAssets();
}

private function configureAssets():void {
var loader:Loader = new Loader();
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, completeHandler);
loader.contentLoaderInfo.addEventListener(IOErrorEvent.IO_ERROR, ioErrorHandler);

var request:URLRequest = new URLRequest(url);
loader.x = size * numChildren;
loader.load(request);
addChild(loader);
}

private function duplicateImage(original:Bitmap):Bitmap {
var image:Bitmap = new Bitmap(original.bitmapData.clone());
image.x = size * numChildren;
addChild(image);
return image;
}

private function completeHandler(event:Event):void {
var loader:Loader = Loader(event.target.loader);
var image:Bitmap = Bitmap(loader.content);

var duplicate:Bitmap = duplicateImage(image);
var bitmapData:BitmapData = duplicate.bitmapData;
var sourceRect:Rectangle = new Rectangle(0, 0, bitmapData.width, bitmapData.height);
var destPoint:Point = new Point();
var operation:String = ">=";
var threshold:uint = 0xCCCCCCCC;
var color:uint = 0xFFFFFF00;
var mask:uint = 0x000000FF;
var copySource:Boolean = true;

bitmapData.threshold(bitmapData,
sourceRect,
destPoint,
operation,
threshold,
color,
mask,
copySource);
}

private function ioErrorHandler(event:IOErrorEvent):void {
trace("Unable to load image: " + url);
}
}
}

package {
import flash.display.Bitmap;//引入包
import flash.display.BitmapData;
import flash.display.Loader;
import flash.display.Sprite;
import flash.events.*;
import flash.geom.Point;
import flash.geom.Rectangle;
import flash.net.URLRequest;

public class BitmapDataExample extends Sprite {//类BitmapDataExample 继承了Sprite
private var url:String = "Image.gif";//定义字符串
private var size:uint = 80;//定义样式大小

public function BitmapDataExample() //定义函数BitmapDataExample

{
configureAssets();//调用函数configureAssets
}

private function configureAssets()://定义函数configureAssets

void {
var loader:Loader = new Loader();//实例化类Loader

loader.contentLoaderInfo.addEventListener(Event.COMPLETE, completeHandler);//调用方法
loader.contentLoaderInfo.addEventListener(IOErrorEvent.IO_ERROR, ioErrorHandler);//调用方法

var request:URLRequest = new URLRequest(url);//实例化类URLRequest
loader.x = size * numChildren;
loader.load(request);
addChild(loader);
}

private function duplicateImage(original:Bitmap):Bitmap {
var image:Bitmap = new Bitmap(original.bitmapData.clone());
image.x = size * numChildren;
addChild(image);
return image;
}

private function completeHandler(event:Event):void {
var loader:Loader = Loader(event.target.loader);
var image:Bitmap = Bitmap(loader.content);

var duplicate:Bitmap = duplicateImage(image);
var bitmapData:BitmapData = duplicate.bitmapData;
var sourceRect:Rectangle = new Rectangle(0, 0, bitmapData.width, bitmapData.height);
var destPoint:Point = new Point();
var operation:String = ">=";
var threshold:uint = 0xCCCCCCCC;
var color:uint = 0xFFFFFF00;
var mask:uint = 0x000000FF;
var copySource:Boolean = true;

bitmapData.threshold(bitmapData,
sourceRect,
destPoint,
operation,
threshold,
color,
mask,
copySource);
}

private function ioErrorHandler(event:IOErrorEvent):void {
trace("Unable to load image: " + url);
}
}
}

朋友 这个好象是在做验证码