EaselJS spritesheet animation makes background disappear - animation

Question

I am working on a game using EaselJS, and I am still trying the functions provided by this great library.
What I may need is Alphamaskfilter class & Spritesheet class.
Currently, I have a canvas like this:
<canvas id = 'container3' width = 320px; height = 480px;
style = 'outline: 2px solid; margin-right: 10px;float:left;'></canvas>
and in my script, I have draw a rect with blue color:
var ctx3 = document.getElementById('container3').getContext('2d');
ctx3.beginPath();
ctx3.rect(0,0,320,480);
ctx3.fillStyle = 'blue';
ctx3.fill();
So now I have a blue color 320*480 canvas. And now I have a sprite sheet to animate on it,
here is the sprite and code I wrote:
http://i.imgur.com/XumDvic.png
PS: the sprite frame is 200*200 FYI.
stage = new createjs.Stage(document.getElementById('container3'));
var test = new Image();
test.src = 'trans_blackball.png';
test.onload = function(){
var sprite = new createjs.SpriteSheet({
images: [test],
frames: {width: 200, height: 200},
animations: {
born: [0,3,0]
}
});
var animation = new createjs.BitmapAnimation(sprite);
animation.gotoAndPlay("born");
animation.x = 10;
animation.y = 10;
animation.currentFrame = 0;
stage.addChild(animation);
createjs.Ticker.addListener(window);
createjs.Ticker.useRAF = true;
createjs.Ticker.setFPS(10);
}
function tick(){
stage.update();
}
Okay, my question is: I expect an animation of an enlarging black circle on a blue background(rect), but what I got is firstly the canvas is blue, but for a certain mini-second after, the whole canvas became white, and the animation is running on the white background, what is the reason of this and how can I solve it?
Here is two points that may help:
I am working with chrome, and I check with F12 & console, the fillStyle of the canvas is still blue even it appears white after the sprite animation start
If I set the canvas background property to blue instead of drawing a blue rect on it, everything's fine!!
<canvas id = 'container3' width = 320px; height = 480px;
style = 'background: blue; outline: 2px solid; margin-right:
10px;float:left;'></canvas>
but clearly this is not what I want...
The problem can be solved as well by not using native canvas rect but use EaselJS's Shape to draw the rect:
var shape = new createjs.Shape();
shape.graphics.beginFill("#ff0000").drawRect(0, 0, 320, 480);
stage.addChild(shape);
But I still wanna know why the native rect drawing code is not working...
Please bare my bad english and thanks for reading and any attempt to help...I can elaborate more if there is anything unclear in the situation..thanks!

Answer 1

createjs.Stage.update() will clear the canvas-contents by default before rendering the new frame.
The reason for this is one the one hand the technical behaviour of canvas, the canvas is basically just an image of pixels, you cannot have real 'layers' and say "I just want to remove/update this one object" - you just can modify the flattened pixels of the frame or remove everything and redraw it(which is safer, easier to achieve, and actually fast enough performance-whise. And on the other hand, it makes sense to make consistent use of a framework like EaselJS, and not do some parts with the framework and some parts without it, this would result in a big mess at the end.
So, the right solution is to go with 3.
You can set myStage.autoClear = false; to prevent the Stage from autoClearing the canvas on every update() but I don't think this will suit your needs in this case.

Answer 2

I would recommend instead using the EaselJS Shape class to make your blue box. If it part of the EaselJS display list, it will be drawn with the other content.
javascript
var shape = new createjs.Shape();
shape.graphics.beginFill("blue").drawRect(0,0,320,480);
stage.addChild(shape);