function drawBackground()

{

ctx2.drawImage(bgPic,0,0,canWidth,canHeight);

}

这是background.js

var can1;

var can2;

var ctx1;

var ctx2;

var canWidth;

var canHeight;

var lastTime;

var deltaTime;

var bgPic =new Image();

document.body.onload =game;

function game()

{

    init;

lastTime =Date.now();

deltaTime =0;

gameloop();

}

function init()

{

//获得canvas context

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

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

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

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

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

canWidth =can1.width;

canHeight =can1.height;

}

function gameloop()

{

requestAnimFrame(gameloop);//setInterval,setTimeout,frame per second

var now =Date.now();

deltaTime =now - lastTime;

lastTime =now;

drawBackground();

}

这是main.js

调用这个函数的对像不对吧，把你的代码贴上来吧