DisplayObject

Code

var stage = new Stage("canvas", 420, 500, 30);

//add a bitmap
var maskee = new Loader();
maskee.load(new URLRequest("./img/photo01.jpg"));
stage.addChild(maskee);

var mask = new Sprite();
mask.x = 100;
mask.y = 100;
mask.graphics.lineStyle(0);
mask.graphics.beginFill(0xFF0000);
mask.graphics.drawCircle(0, 0, 100);
mask.graphics.endFill();
stage.addChild(mask);

//add a child to the mask
var mask_child = new Sprite();
mask_child.x = 50;
mask_child.y = 50;
mask_child.graphics.lineStyle(0);
mask_child.graphics.beginFill(0x000FF);
mask_child.graphics.drawRect(0, 0, 150, 150);
mask_child.graphics.endFill();
mask.addChild(mask_child);

//add a transparent bitmap to the mask
var mask_child2 = new Loader();
mask_child2.x = 100;
mask_child2.y = -100;
mask_child2.load(new URLRequest("./img/Flap.png"));
mask.addChild(mask_child2);

//add a child that animates
var mask_child3 = new Sprite();
mask_child3.x = 50;
mask_child3.y = 200;
mask_child3.graphics.lineStyle(0);
mask_child3.graphics.beginFill(0x000FF);
mask_child3.graphics.drawRect(0, 0, 200, 200);
mask_child3.graphics.endFill();
mask.addChild(mask_child3);

//set the mask
maskee.mask = mask;

mask_child3.addEventListener(Event.ENTER_FRAME, function(e)
{
    this.rotation += 1;
});

Canvas

Output