why is my canvas zoom/pan slowing down. - html

Question

I'm a new programmer just trying to learn how to make webpages. I found a code to zoom and pan canvas elements but when I implemented it into an extJS window It started becoming sluggish. It doesn't become sluggish if the image I render is just a shape, only if It's from a file image. I thought at first I was creating instances of objects over and over but I tried deleting the objects after use and it didn't change anything. Why is my zooming slowing down?
Ext.onReady(function(){
Ext.define("w",{
width: 1000,
height: 750,
extend: "Ext.Window",
html: '<canvas id="myCanvas" width="1000" height="750">'
+ 'alternate content'
+ '</canvas>'
,afterRender: function() {
this.callParent(arguments);
var canvas= document.getElementById("myCanvas");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var stage = new createjs.Stage("myCanvas");
/*function addCircle(r,x,y){
var g=new createjs.Graphics().beginFill("#ff0000").drawCircle(0,0,r);
var s=new createjs.Shape(g)
s.x=x;
s.y=y;
stage.addChild(s);
stage.update();
}*///// If I use this function instead of loading an img there's no slowdown.
function setBG(){
var myImage = new Image();
myImage.src = "dbz.jpg";
myImage.onload = setBG;
var bgrd = new createjs.Bitmap(myImage);
stage.addChild(bgrd);
stage.update();
delete myImage;
delete bgrd;
};
setBG();
//addCircle(40,200,100);
//addCircle(50,400,400);
canvas.addEventListener("mousewheel", MouseWheelHandler, false);
canvas.addEventListener("DOMMouseScroll", MouseWheelHandler, false);
var zoom;
function MouseWheelHandler(e) {
if(Math.max(-1, Math.min(1, (e.wheelDelta || -e.detail)))>0)
zoom=1.1;
else
zoom=1/1.1;
stage.regX=stage.mouseX;
stage.regY=stage.mouseY;
stage.x=stage.mouseX;
stage.y=stage.mouseY;
stage.scaleX=stage.scaleY*=zoom;
stage.update();
delete zoom;
}
stage.addEventListener("stagemousedown", function(e) {
var offset={x:stage.x-e.stageX,y:stage.y-e.stageY};
stage.addEventListener("stagemousemove",function(ev) {
stage.x = ev.stageX+offset.x;
stage.y = ev.stageY+offset.y;
stage.update();
delete offset;
});
stage.addEventListener("stagemouseup", function(){
stage.removeAllEventListeners("stagemousemove");
});
});
} //end aferrender
}); //end define
Ext.create("w", {
autoShow: true });
}); //end onready

Answer 1

It looks like you are infinitely re-loading the BG image. After your BG image finishes loading, your onload function callback just makes it call getBG again which will just repeat the same process forever.
function setBG() {
...
myImage.onload = setBG;
...
}
I'm not sure exactly what you expect by doing this.

Answer 2

You really shouldn't need to delete the image. Off the top of my head, this is how I would generally load an image for use in canvas, (based on how your train of thought is working).
function setBG(){
var myImage = new Image();
myImage.src = "dbz.jpg";
myImage.onload = function(){
var bgrd = new createjs.Bitmap(this);
stage.addChild(bgrd);
stage.update();
}
};
setBG();