在客户端实现画线

单击开始画线，单击结束，鼠标按下和抬起事件不是很懂，请高手完善这段代码。
脚本说明:
把如下代码加入<body>区域中
<script>
var winW= document.body.clientWidth;
var winH= document.body.clientHeight;
var MyStr="<object id=DDraw classid=\"clsid:369303C2-D7AC-11d0-89D5-00A0C90833E6\"";
MyStr+=" style=\"position:absolute;left:0;top:0;width:";
MyStr+=winW;
MyStr+=";height:";
MyStr+=winH;
MyStr+="\"></object>";
document.write(MyStr);
</script>
<script language=javascript>
var IsClick=0;
var MouseX0=0;
var MouseY0=0;
function MyDraw(){
if (IsClick) {
if ((MouseX0==0)&&(MouseY0==0)){
MouseX0=event.clientX-winW/2;
MouseY0=event.clientY-winH/2;
}
MouseX=event.clientX-winW/2;
MouseY=event.clientY-winH/2;
var Canvas=DDraw.DrawSurface;
var Lib=DDraw.Library;
Canvas.FillColor(Lib.red);
Canvas.Line(MouseX0,MouseY0,MouseX,MouseY);
DDraw.DrawSurface=Canvas;
MouseX0=MouseX;
MouseY0=MouseY;
}
}function MyClick(){
IsClick=!IsClick;
MouseX0=event.clientX-winW/2;
MouseY0=event.clientY-winH/2;
}document.onclick=MyClick;
document.onmousemove=MyDraw;
</script>