How to fit canvas to wrap image size

Hello I have this canvas and image in it but I need to make it so the canvas wraps the image linked to its size. Right now i have put a test image and put size to be fixed. I want when i put and image the canvas to wrap it 100% width 100% height of the image.

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

var boxWidth=100;
var boxHeight=100;
var boxRows=Math.ceil(865/boxHeight);
var boxCols=Math.ceil(1152/boxWidth);
var boxes=new Array(boxRows*boxCols);
for(var i=0;i<boxes.length;i++){boxes[i]=false;}

var img=new Image();
img.onload=start;
img.crossOrigin='anonymous';
img.src="http://ift.tt/1Hs5yKZ";
function start(){

ctx.drawImage(img,0,0);

var d=ctx.getImageData(0,0,cw,ch).data;

for(var i=0;i<d.length;i+=4){
if(d[i+3]>200){
  var px=parseInt(i/4);
  var pixelY=parseInt(px/cw);
  var pixelX=px-pixelY*cw;
  var boxX=parseInt(pixelX/boxWidth);
  var boxY=parseInt(pixelY/boxHeight); 
  boxes[boxY*boxCols+boxX]=true;
 }
  }

 ctx.globalAlpha=0.25;
 ctx.fillStyle='red';

 for(var i=0;i<boxes.length;i++){
 var y=parseInt(i/boxCols);
 var x=i-y*boxCols;
 if(boxes[i]==true){
  ctx.fillRect(x*boxWidth,y*boxHeight,boxWidth,boxHeight);        
 }
 }

ctx.globalAlpha=1.00;
ctx.fillStyle='black';

}

http://ift.tt/1EYdqxS