为何我的不能清除之前的图形

var ball = {x:512, y:100, r:20, g:2, vx:-4, vy:0, color:"#005588"};

window.onload = function(){

var canvas = document.getElementById('canvas');

var context = canvas.getContext('2d');

canvas.width = 1200;

canvas.height = 800;

setInterval(function(){

render(context);

update();

},50)

}

function update(){

ball.x += ball.vx;

ball.y += ball.vy;

ball.vy += ball.g;

}

function render(cxt){

cxt.clearRect(0,0,cxt.canvas.width,cxt.canvas.height);

cxt.fillStyle = ball.color;

cxt.arc(ball.x, ball.y, ball.r, 0, 2*Math.PI);

cxt.closePath();

cxt.fill();

}