canvas做的一个写字板

<!DOCTYPE html>
<html>
<head>
<title>画板实验</title>
<meta charset="UTF-8">
<meta name="viewport"content="width=device-width, initial-scale=1.0">
<!-- <script type="text/javascript"
src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
</script>-->
<style type="text/css">
</style>
</head>
<body >
<canvas id="myCanvas"></canvas>

<div>
<button onclick="clean();">清 空</button>
<button onclick="save();">生成图片</button>
</div>

<img id='img' alt='请涂鸦……'/>

<textarea id="log"cols="30"rows="5"></textarea>
<script type="text/javascript">
var canvas,board,img;
canvas = document.getElementById('myCanvas');
img= document.getElementById('img');

canvas.height = 300;
canvas.width = 300;

board = canvas.getContext('2d');

var mousePress = false;
var last = null;

function beginDraw(){
mousePress = true;
}

function drawing(event){
event.preventDefault();
if(!mousePress)return;
var xy = pos(event);
if(last!=null){
board.beginPath();
board.moveTo(last.x,last.y);
board.lineTo(xy.x,xy.y);
board.stroke();
}
last = xy;

}

function endDraw(event){
mousePress = false;
event.preventDefault();
last = null;
}

function pos(event){
var x,y;
if(isTouch(event)){
x = event.touches[0].pageX;
y = event.touches[0].pageY;
}else{
x = event.offsetX+event.target.offsetLeft;
y = event.offsetY+event.target.offsetTop;
}
// log('x='+x+' y='+y);
return {x:x,y:y};
}

function log(msg){
var log = document.getElementById('log');
var val = log.value;
log.value = msg+'n'+val;
}

function isTouch(event){
var type = event.type;
if(type.indexOf('touch')>=0){
return true;
}else{
return false;
}
}

function save(){
//base64
var dataUrl = canvas.toDataURL();
// dataUrl = dataUrl.replace("image/png","image/octet-stream");
img.src = dataUrl;
}

function clean(){
board.clearRect(0,0,canvas.width,canvas.height);

}

board.lineWidth = 1;
board.strokeStyle="#0000ff";

canvas.onmousedown = beginDraw;
canvas.onmousemove = drawing;
canvas.onmouseup = endDraw;
canvas.addEventListener('touchstart',beginDraw,false);
canvas.addEventListener('touchmove',drawing,false);
canvas.addEventListener('touchend',endDraw,false);
</script>

[
lim_{x to 0}
]

</body>
</html>