[ActionScript 3.0] AS3.0 获取文本的明暗度

/**
 * 获取文字的明暗值
 * @param t 文字
 * @return Number
 */
function getDensity(t:String):Number
{
    var ttf:TextField = new TextField  ;
    ttf.autoSize = TextFieldAutoSize.LEFT;
    ttf.text = t;

    var bmd:BitmapData = new BitmapData(ttf.width,ttf.height,false,0xffffff);
    bmd.draw(ttf);
    var c:int = 0;
    for (var x:int = 0; x < bmd.width; x++)
    {
        for (var y:int = 0; y < bmd.height; y++)
        {
            if (bmd.getPixel(x,y) != 0xffffff)
            {
                c++;
            }
        }
    }
    return c / ttf.width / ttf.height;
}