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ballmove.html
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<html>
<head>
<script type="text/javascript" src="jquery-1.2.6.js"></script>
<script type="application/x-javascript">
var x = 150;
var y = 150;
var dx = 0;
var dy = 0;
var intervalID = 0;
var canvas, ctx;
function init() {
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
dx = parseFloat($("#dx").val());
dy = parseFloat($("#dy").val());
x = 150;
y = 150;
draw();
}
function startdraw() {
init();
clearInterval(intervalID);
intervalID = setInterval(draw, 20);
}
function abort() {
clearInterval(intervalID);
ctx.clearRect(0,0,300,300);
init();
}
function draw() {
ctx.clearRect(0,0,300,300);
ctx.beginPath();
ctx.arc(x,y,10,0,Math.PI*2,true);
ctx.fill();
x += dx;
y += dy;
}
</script>
</head>
<body onload="init()">
<canvas id="canvas" width="300" height="300" style="border:1px solid;"></canvas>
<br>
<form>
dx: <input type="text" id="dx" size="4" value="1"/>
dy: <input type="text" id="dy" size="4" value="2"/>
<input type="submit" onclick="startdraw(); return false;" value="start">
<input type="submit" onclick="abort(); return false;" value="clear">
</body>
</html>