使用 canvas 繪圖時，指定的 div 大小一定不要超過該 div 所能獲得的最大範圍，否則繪製的點會跟實際位置發生偏離。
例如
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled 1</title>
<style type="text/css">
.style1 {
font-size: x-small;
}
</style>
</head>
<body >
<div style="margin:2%">
<div id="test" style="width:800px;height:800px;background-color:#cbdad0d9">
<canvas id="container" onmousemove="mousemove(event)" onmousedown="mousedown()" onmouseup="mouseup()"></canvas>
</div>
</div>
<script type="text/javascript">
var paint = false;
var start = false;
var canvas = document.getElementById("container");
canvas.width = 800;
canvas.height = 800;
var offsetY = canvas.offsetTop;
var offsetX = canvas.offsetLeft;
var y;
var x;
var context = canvas.getContext("2d");
function mousemove(e) {
if (paint == true){
if (start == false){
context.moveTo(0, 0);
start = true;
} else {
context.moveTo(x, y);
}
x = e.pageX - offsetX;
y = e.pageY - offsetY;
context.lineTo(x, y);
context.stroke();
}
}
function mousedown(event) {
paint = true;
console.log("down")
}
function mouseup(event) {
paint = false;
console.log("up")
}
</script>
</body>
</html>
上例中可以正確的繪製線圖。
如果更改爲：
<div style="margin:20%">
<div id="test" style="width:800px;height:800px;background-color:#cbdad0d9">
<canvas id="container" onmousemove="mousemove(event)" onmousedown="mousedown()" onmouseup="mouseup()"></canvas>
</div>
</div>
由於 canvas 所需 width 800px 無法滿足，因此繪製線圖時，與實際鼠標位置發生偏離。