我有以下代码，可以让我在画布上绘制图片，类似于MS Paint。因此，一旦用户按下鼠标左键，他/她就可以在画布上移动鼠标以绘制图片。用户将手指从鼠标按钮上抬起后，绘图将停止。
var colors = document.querySelectorAll('#colors div');
var link = document.getElementById('download');
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var drawing = false;
var prev;
document.getElementById('current').style.backgroundColor = context.strokeStyle;
for (var i = 0; i < colors.length; i++) {
colors[i].addEventListener('click', function(e) {
var src = e.srcElement;
var color = src.style.backgroundColor;
context.strokeStyle = color;
document.getElementById('current').style.backgroundColor = color;
});
colors[i].style.height = 100 / colors.length + '%';
colors[i].style.backgroundColor = colors[i].id;
}
function clearCanvas() {
context.clearRect(0, 0, canvas.width, canvas.height);
}
link.addEventListener('click', function() {
link.href = canvas.toDataURL();
link.download = 'image.png';
});
canvas.addEventListener('mousedown', function(e) {
drawing = true;
});
canvas.addEventListener('mousemove', function(e) {
if (drawing) {
var coord = {
'x': e.clientX - this.offsetLeft,
'y': e.clientY - this.offsetTop
};
if (prev !== undefined) {
context.beginPath();
context.moveTo(prev.x, prev.y);
context.lineTo(coord.x, coord.y);
context.stroke();
}
prev = coord;
}
});
canvas.addEventListener('mouseup', function() {
drawing = false;
prev = undefined;
});#draw {
border: 2px outset black;
border-radius: 5px;
display: inline-block;
margin: 5px;
padding: 10px;
}
#container {
border: 2px solid black;
width: 300px;
height: 300px;
position: relative;
}
#canvas {
background-color: lightgrey;
}
#colors {
float: right;
position: relative;
width: 20%;
height: 100%;
}
#colors div {
width: 100%
}
#buttons {
width: 100%;
height: auto;
}
#buttons * {
width: 33%;
background-color: lightgrey;
margin-top: 5px;
margin-left: 5px;
display: inline;
}
#download {
text-align: center;
border-radius: 2px;
padding-left: 5px;
padding-right: 5px;
border: 2px outset lightgrey;
font-family: 'Arial';
background-color: lightgrey;
font-size: 10pt;
cursor: context-menu;
}
#download:link {
text-decoration: none;
color: black;
}
#download:active {
border: 2px inset #00acff;
border-radius: 2px;
}
#current {
height: 100%;
width: 10%;
border: 1px solid black;
display: inline;
position: relative;
margin-left: 25px;
}<div id='draw'>
<div id='container'>
<canvas id='canvas' width='240' height='300'></canvas>
<div id='colors'>
<div id='red'></div>
<div id='blue'></div>
<div id='green'></div>
<div id='black'></div>
<div id='yellow'></div>
<div id='cyan'></div>
<div id='magenta'></div>
</div>
<div id='currentColor'></div>
</div>
<div id='buttons'>
<input type='button' id='clear' value='Clear Canvas' onclick='clearCanvas()' />
<a id='download'>Download Image</a>
<input type='button' id='current' />
</div>
</div>http://jsfiddle.net/SirToadstool/eywraw8t/341261/
现在，虽然此代码在 JSFiddle 中工作，但在外部测试它会使用户绘制的行与鼠标指针的坐标不同。
我有一种感觉，我在代码中包含的 CSS 可能会弄乱 Javascript 中坐标的计算方式，但我不确定要更改什么来补偿，并且尝试使用clientX/Y和screenX/Y作为坐标而不仅仅是event.x/event.y。
关于如何在绘图时正确计算坐标的任何建议？
this.offsetLeft等于0。这就是为什么从中减去什么也不做，因为在函数的范围内，"this"就是画布本身。您希望使用鼠标事件 offsetX\Y 而不是 clientX\Y，因此简单地说：
var coord = { 'x': e.offsetX, 'y': e.offsetY };
工作示例
canvas.width 是绘图画布的宽度，这不一定是 canvas 元素的宽度。这意味着画布上下文的分辨率可以与屏幕不同。因此，您需要相应地调整坐标。即
var coord = {
'x': (e.clientX - this.offsetLeft)*this.width/this.clientWidth,
'y': (e.clientY - this.offsetTop)*this.height/this.clientHeight
};