Device fonts are get ugly when converted to Bitmap in OS X

Dear All,

I'm not sure this will happen in other platforms, but in OS X, either Flash Professional or AS3, if I was trying to make a TextField render to a Bitmap using device fonts, than the texts will become ugly, regardless of what fonts.

In Flash Professional:

When I create a text with "Use device fonts":

Then right click and convert to bitmap:

As you can see, they are get ugly and I don't know why!

In AS3:

import flash.text.TextField;
import flash.display.BitmapData;
import flash.display.Bitmap;


var text : TextField = new TextField();
text.defaultTextFormat = new TextFormat(null, 13, 0xffffff);
text.text = "Some texts";
text.x = 10;
text.y = 10;
addChild (text);


var bd : BitmapData = new BitmapData(512, 512, true, 0);
bd.draw(text);
var bitmap : Bitmap = new Bitmap(bd);
bitmap.x = 10;
bitmap.y = 30;
addChild(bitmap);

Result:

I had tried use drawWithQuality instead and still can not solve the problem.

Is this is a bug? Did I just missing something steps?

I was stuck for a whole day in this problem, I am really looking for help, thanks!