var can1;

var can2;

var ctx1;

var ctx2;

var lastTime;  //上一帧执行的时间

var deltaTime; //两帧时间差

var canWidth;

var canHeight;

var bgPic=new Image();

document.body.onload = game;

function game(){

init ();

lastTime = Date.now();

deltaTime = 0;

gameloop();

}

function init(){ //初始化

//获得canvas context

can1 = document.getElementById("canvas1");//fishes,dust,UI(score),circle

ctx1 = can1.getContext('2d');

can2 = document.getElementById("canvas2");//background,haikui,fruits

ctx2 = can2.getContext('2d');

bgPic.src="./src/background.jpg";

canWidth=can1.width;

canHeight=can1.height;

}

function gameloop(){

window.requestAnimationFrame(gameloop);

var now = Date.now();

deltaTime = now -lastTime;

lastTime = now;

drawBackground();

}