获取按指定尺寸等比例缩放+居中的截图

		/**
		 * 获取按指定尺寸等比例缩放+居中的截图
		 * @param	target        目标对象
		 * @param	tarW          目标尺寸(宽)
		 * @param	tarH          目标尺寸(高)
		 * @param	full          是否显示图片全貌
		 * @return
		 */
		private static function getZoomDraw(target:BitmapData, tarW:int, tarH:int,full:Boolean=true):BitmapData
		{
			//获取显示对象矩形范围
			var rect:Rectangle = new Rectangle(0,0,target.width,target.height);
			//计算出应当缩放的比例
			var perw:Number = tarW / rect.width;
			var perh:Number = tarH / rect.height;
			var scale:Number = full?((perw <= perh)?perw:perh):((perw <= perh)?perh:perw);
			//计算缩放后与规定尺寸之间的偏移量
			var offerW:Number = (tarW - rect.width * scale) / 2;
			var offerH:Number = (tarH - rect.height * scale) / 2;
			//开始绘制快照（这里透明参数是false,是方便观察效果，实际应用可改为true)
			var bmd:BitmapData = new BitmapData(tarW, tarH, false, 0);
			var matrix:Matrix = new Matrix();
			matrix.scale(scale, scale);
			matrix.translate( offerW, offerH);
			bmd.draw(target, matrix);
			//如果是bitmap对象，释放位图资源
			if (target is Bitmap)   (target as Bitmap).bitmapData.dispose();
			//返回截图数据
			return bmd;
		}