能帮我看下那出错了吗？想做出globalCompositeOperation的demo的效果做不出来
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1,user-scalable=no" />
</head>
<body>
<canvas id="canvas" style="border:1px solid #aaa;display: block;margin:50px auto;">
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</canvas>
<script>
var balls = [];
window.onload = function(){
var canvas = document.getElementById('canvas');
canvas.width = 1200;
canvas.height = 800;
var context = canvas.getContext("2d");
for(var i=0;i<100;i++){
var R = Math.floor(Math.random()*255);
var G = Math.floor(Math.random()*255);
var B = Math.floor(Math.random()*255);
var radius = Math.random()*50+20;
aBall = {
color : "rgb("+R+","+G+","+B+")",
radius : radius,
x : Math.random()*(canvas.width - 2*radius)+radius,
y : Math.random()*(canvas.height - 2*radius)+radius,
vy : (Math.random()*5 + 5)*Math.pow(-1,Math.floor(Math.random()*100)),
vy : (Math.random()*5 + 5)*Math.pow(-1,Math.floor(Math.random()*100))
};
balls[i] = aBall;
};
setInterval(function(){
draw(context);
update(canvas.width,canvas.height);
},50);
};
function draw(cxt){
var canvas = cxt.canvas;
cxt.clearRect(0,0,canvas.width,canvas.height);
for(var i=0; i<balls.length;i++){
cxt.fillStyle = balls[i].color;
cxt.beginPath();
cxt.arc(balls[i].x,balls[i].y,balls[i].radius,0,Math.PI*2);
cxt.closePath();
cxt.fill();
}
}
function update(canvasWidth,canvasHeight){
for(var i=0;i<balls.length;i++){
balls[i].x += balls[i].vx;
balls[i].y += balls[i].vy;
if(balls[i].x - balls[i].radius <= 0){
balls[i].vx = -balls[i].vx;
balls[i].x = balls[i].radius;
}
if(balls[i].x + balls[i].radius >= canvasWidth){
balls[i].vx = -balls[i].vx;
balls[i].x = canvasWidth - balls[i].radius;
}
if(balls[i].y - balls[i].radius <= 0){
balls[i].vy = -balls[i].vy;
balls[i].y = balls[i].radius;
}
if(balls[i].y + balls[i].radius >= canvasHeight){
balls[i].vy = -balls[i].vy;
balls[i].y = canvasHeight - balls[i].radius;
}
}
}
</script>
</body>
</html>
