EaselJS: Using updateCache() with AlphaMaskFilter When Dragging Mask - html5-canvas

Question

I'm using an imported png with an alpha gradient that I'm setting as a mask that reveals the bitmap it is assigned to. The mask object is draggable (kind of like a flashlight). I know I'm supposed to use an AlphaMaskFilter as one of the filters, and I know I'm supposed to use .updateCache()... I'm just not sure I'm using them correctly?
var stage;
var assetQueue;
var bg;
var bgMask;
var container;
var amf;
$(document).ready(function(){
loadImages();
});
function loadImages()
{
// Set up preload queue
assetQueue = new createjs.LoadQueue();
assetQueue.addEventListener("complete", preloadComplete);
assetQueue.loadManifest([{id:"img_bg",src:"images/Nintendo-logo-red.jpg"}, {id:"img_bg_mask",src:"images/background_mask.png"}]);
}
function preloadComplete()
{
assetQueue.removeEventListener("complete", preloadComplete);
init();
}
function init()
{
stage = new createjs.Stage("stage_canvas");
setBackgrounds();
sizeStage();
$(document).mousemove(function(evt){
trackMouse(evt);
});
}
function trackMouse(evt)
{
var mouseX = evt.pageX;
var mouseY = evt.pageY;
// Move the containing clip around
container.x = mouseX - (bgMask.image.width / 2);
container.y = mouseY - (bgMask.image.height / 2);
// Offset the position of the masked image.
bg.x = -container.x;
bg.y = -container.y;
container.updateCache();
stage.update();
}
function setBackgrounds()
{
bg = new createjs.Bitmap(assetQueue.getResult("img_bg"));
bgMask = new createjs.Bitmap(assetQueue.getResult("img_bg_mask"));
container = new createjs.Container();
container.addChild(bg);
amf = new createjs.AlphaMaskFilter(bgMask.image)
container.filters = [amf];
container.cache(0, 0, bg.image.width, bg.image.height);
stage.addChild(container);
stage.update();
}
function sizeStage()
{
var windowW = 600;
var windowH = 600;
stage.canvas.width = windowW;
stage.canvas.height = windowH;
stage.update();
}

Answer 1

Solution found (for anyone interested). The key is to add the image you want to mask to a container. Move the container to any position you want, then offset the contained image within the container. The code has been updated to reflect this.