how to set a bitmap as a mask - actionscript-3

Question

i want to apply a picture which contains transparent areas as a mask to a display object.
the mask only show the area which the mask has color.but in fact,the display object still show the whole area.so i convert bitmap to a vector image,that's solve the problem,but the convert method is horrible.
public static function createVectorImage(bd:BitmapData,colorKey:uint = 0):Shape{
if(bd==null){
return null;
}
var sh:Shape = new Shape();
var g:Graphics = sh.graphics;
g.beginBitmapFill(bd);
var beginPixel:int = -1;
var i:int,il:int,j:int,jl:int;
var value:uint;
for(i = 0,il=bd.height;i<il;i++){
for(j = 0,jl = bd.width;j<jl;j++){
value = bd.getPixel32(j,i);
if(value!=colorKey&&beginPixel==-1){
beginPixel = j;
}else if(value==colorKey&&beginPixel!=-1){
//draw rect
g.drawRect(beginPixel,i,j-beginPixel,1);
beginPixel = -1;
}
}
if(beginPixel!=-1){
g.drawRect(beginPixel,i,j-beginPixel,1);
beginPixel = -1
}
}
g.endFill();
return sh;
}
is there any way better than this?

Answer 1

You can convert your display object to a bitmap then apply a mask on it.
Convert you display object to a bitmap data:
var rect:Rectangle = displayObject.getRect();
var displayBD:BitmapData = new BitmapData(rect.width, rect.height, true, 0);
displayBD.draw(displayObject);
Apply the mask:
private static const ORIG:Point = new Point(0, 0);
private static function createBitmapDataWithMask(
baseBD:BitmapData, maskBD:BitmapData):BitmapData
{
var bitmapData:BitmapData;
bitmapData = new BitmapData(baseBD.width, baseBD.height, true, 0x000000);
bitmapData.copyPixels(baseBD, baseBD.rect, ORIG, maskBD, ORIG, true);
return bitmapData;
}
Display the bitmap data:
var bitmapData:BitmapData = createBitmapDataWithMask(displayBD, maskBD);
var bitmap:Bitmap = new Bitmap(bitmapData, "auto", true);
addChild(bitmap);
If you remove the bitmap from the stage, don't forget the free the bitmap data!
removeChild(bitmap);
bitmap.bitmapData.dispose();
bitmap = null;

Answer 2

Another option would be to use bitmap caching. It needs to be applied to the mask as well as the maskee:
var bd:BitmapData = new BitmapData(200, 200, true, 0x00000000);
bd.fillRect(new Rectangle(0, 0, 40, 40), 0xff000000);
var mask:Bitmap = new Bitmap(bd);
mask.cacheAsBitmap = true;
var maskee:Sprite = new Sprite();
maskee.cacheAsBitmap = true;
maskee.graphics.beginFill(0xff0000, 1);
maskee.graphics.drawRect(0, 0, 200, 200);
maskee.graphics.endFill();
maskee.mask = mask;
addChild(mask);
addChild(maskee);