elastic

<!DOCTYPE html>
<html>
<head>
<title>橡皮筋</title>
<script type="text/javascript">
var ctx;
var linew=10;
var color="rgb(255,0,0)";
var isClick;
var mx=50;
var my=200;
var tid;
function setstyle(w,c){
ctx.lineWidth=w;
ctx.strokeStyle=c;
}

function getpoint(ev){
if(ev.layerX||ev.layerX==0){
mx=ev.layerX;
my=ev.layerY;
}else if(ev.offsetX||ev.offsetX==0){
mx=ev.offsetX;
my=ev.offsetY;
}
}

function drawall(){
ctx.clearRect(0,0,400,400);
setstyle(linew,color);
ctx.beginPath();
ctx.moveTo(50,200);
ctx.lineTo(mx,my);
ctx.lineTo(350,200);
ctx.stroke();
setstyle(2,"rbg(0,0,0");
ctx.strokeRect(0,0,400,400);
}

function init(){
isClick=false;
ctx=document.getElementById('canvas').getContext('2d');
drawall();
canvasl=document.getElementById('canvas');
canvasl.addEventListener("mousedown",findball,false);//
canvasl.addEventListener("mousemove",moveit,false);
canvasl.addEventListener("mouseup",finish,false);
}

function findball(ev){
isClick=true;
document.f.lg.value="mousedown";
getpoint(ev);
drawall();
}

function moveit(ev){
if(isClick){
document.f.lg.value="mousemove";
getpoint(ev);
drawall();
}
}

function finish(){
if(isClick){
isClick=false;
document.f.lg.value="mouseup";
tid=setInterval(change,100);
}
}