有个要注意的点，就是canvas标签本身自带width与height属性，设置此属性可以直接更改canvas坐标系的宽与高，而在css里设置宽高，只是把canvas坐标系拉伸了，坐标值不变，显示出来的图像也都被拉伸了。
<html>
<body>
<canvas id = "canvas" width = "800px" height="600px"></canvas>
<script type="text/javascript">
var canvas = document.getElementById("canvas");
var parti = [];
function loop()
{
CreateParticles();
UpdateParticles();
KillParticles();
DrawParticles();
}
function CreateParticles()
{
var partis = {x:Math.random()*canvas.clientWidth, y:0 , color:"white", radius:2 + Math.random()* 5 ,speed:2+Math.random()*5};
if(parti.length < 80)
{
parti.push(partis);
}
}
function UpdateParticles()
{
for(var i in parti)
{
parti[i].y += parti[i].speed;
}
}
function KillParticles()
{
for(var i in parti)
{
if(parti[i].y >= canvas.clientHeight)
{
parti[i].y = 0;
}
}
}
function DrawParticles()
{
var contex = canvas.getContext("2d");
contex.fillStyle = "black";
contex.fillRect(0,0,canvas.clientWidth,canvas.clientHeight);
for(var i in parti)
{
contex.beginPath();
contex.arc(parti[i].x, parti[i].y,parti[i].radius,0,Math.PI * 2);
contex.closePath();
contex.fillStyle = parti[i].color;
contex.fill();
}
}
setInterval(loop,30);
</script>
</body>
</html>