向canvas元素添加一个单击事件处理程序，返回单击的x和y坐标(相对于canvas元素)，最简单的方法是什么?
不需要传统浏览器兼容性，Safari、Opera和Firefox就可以了。
当前回答
我推荐这个链接 http://miloq.blogspot.in/2011/05/coordinates-mouse-click-canvas.html
<style type="text/css">
#canvas{background-color: #000;}
</style>
<script type="text/javascript">
document.addEventListener("DOMContentLoaded", init, false);
function init()
{
var canvas = document.getElementById("canvas");
canvas.addEventListener("mousedown", getPosition, false);
}
function getPosition(event)
{
var x = new Number();
var y = new Number();
var canvas = document.getElementById("canvas");
if (event.x != undefined && event.y != undefined)
{
x = event.x;
y = event.y;
}
else // Firefox method to get the position
{
x = event.clientX + document.body.scrollLeft +
document.documentElement.scrollLeft;
y = event.clientY + document.body.scrollTop +
document.documentElement.scrollTop;
}
x -= canvas.offsetLeft;
y -= canvas.offsetTop;
alert("x: " + x + " y: " + y);
}
</script>
其他回答
这是一个非常好的教程-
http://www.html5canvastutorials.com/advanced/html5-canvas-mouse-coordinates/
<canvas id="myCanvas" width="578" height="200"></canvas>
<script>
function writeMessage(canvas, message) {
var context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
context.font = '18pt Calibri';
context.fillStyle = 'black';
context.fillText(message, 10, 25);
}
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
canvas.addEventListener('mousemove', function(evt) {
var mousePos = getMousePos(canvas, evt);
var message = 'Mouse position: ' + mousePos.x + ',' + mousePos.y;
writeMessage(canvas, message);
}, false);
希望这能有所帮助!
以下是上述Ryan Artecona解决方案的一些修改。
function myGetPxStyle(e,p)
{
var r=window.getComputedStyle?window.getComputedStyle(e,null)[p]:"";
return parseFloat(r);
}
function myGetClick=function(ev)
{
// {x:ev.layerX,y:ev.layerY} doesn't work when zooming with mac chrome 27
// {x:ev.clientX,y:ev.clientY} not supported by mac firefox 21
// document.body.scrollLeft and document.body.scrollTop seem required when scrolling on iPad
// html is not an offsetParent of body but can have non null offsetX or offsetY (case of wordpress 3.5.1 admin pages for instance)
// html.offsetX and html.offsetY don't work with mac firefox 21
var offsetX=0,offsetY=0,e=this,x,y;
var htmls=document.getElementsByTagName("html"),html=(htmls?htmls[0]:0);
do
{
offsetX+=e.offsetLeft-e.scrollLeft;
offsetY+=e.offsetTop-e.scrollTop;
} while (e=e.offsetParent);
if (html)
{
offsetX+=myGetPxStyle(html,"marginLeft");
offsetY+=myGetPxStyle(html,"marginTop");
}
x=ev.pageX-offsetX-document.body.scrollLeft;
y=ev.pageY-offsetY-document.body.scrollTop;
return {x:x,y:y};
}
我不确定所有这些遍历父元素和做各种奇怪事情的答案的意义是什么。
HTMLElement。getBoundingClientRect方法被设计用来处理任何元素的实际屏幕位置。这包括滚动，所以不需要像scrollTop这样的东西:
(来自MDN)已经完成的视口区域(或 的时候，任何其他可滚动的元素)都被考虑在内 边界矩形
正常的图片
最简单的方法已经贴在这里了。只要不涉及宽泛的CSS规则，这是正确的。
处理拉伸的画布/图像
当图像像素宽度与它的CSS宽度不匹配时，你需要对像素值应用一些比率:
/* Returns pixel coordinates according to the pixel that's under the mouse cursor**/
HTMLCanvasElement.prototype.relativeCoords = function(event) {
var x,y;
//This is the current screen rectangle of canvas
var rect = this.getBoundingClientRect();
var top = rect.top;
var bottom = rect.bottom;
var left = rect.left;
var right = rect.right;
//Recalculate mouse offsets to relative offsets
x = event.clientX - left;
y = event.clientY - top;
//Also recalculate offsets of canvas is stretched
var width = right - left;
//I use this to reduce number of calculations for images that have normal size
if(this.width!=width) {
var height = bottom - top;
//changes coordinates by ratio
x = x*(this.width/width);
y = y*(this.height/height);
}
//Return as an array
return [x,y];
}
只要画布没有边界，它就适用于拉伸图像(jsFiddle)。
处理CSS边框
如果画布的边框很厚，事情就会变得有点复杂。你需要从边界矩形中减去边框。这可以使用. getcomputedstyle来完成。这个答案描述了这个过程。
然后函数增大一点:
/* Returns pixel coordinates according to the pixel that's under the mouse cursor**/
HTMLCanvasElement.prototype.relativeCoords = function(event) {
var x,y;
//This is the current screen rectangle of canvas
var rect = this.getBoundingClientRect();
var top = rect.top;
var bottom = rect.bottom;
var left = rect.left;
var right = rect.right;
//Subtract border size
// Get computed style
var styling=getComputedStyle(this,null);
// Turn the border widths in integers
var topBorder=parseInt(styling.getPropertyValue('border-top-width'),10);
var rightBorder=parseInt(styling.getPropertyValue('border-right-width'),10);
var bottomBorder=parseInt(styling.getPropertyValue('border-bottom-width'),10);
var leftBorder=parseInt(styling.getPropertyValue('border-left-width'),10);
//Subtract border from rectangle
left+=leftBorder;
right-=rightBorder;
top+=topBorder;
bottom-=bottomBorder;
//Proceed as usual
...
}
我想不出有什么会混淆最后这个函数。在JsFiddle见。
笔记
如果您不喜欢修改本机原型，只需更改函数并使用(canvas, event)调用它(并将任何此替换为canvas)。
我推荐这个链接 http://miloq.blogspot.in/2011/05/coordinates-mouse-click-canvas.html
<style type="text/css">
#canvas{background-color: #000;}
</style>
<script type="text/javascript">
document.addEventListener("DOMContentLoaded", init, false);
function init()
{
var canvas = document.getElementById("canvas");
canvas.addEventListener("mousedown", getPosition, false);
}
function getPosition(event)
{
var x = new Number();
var y = new Number();
var canvas = document.getElementById("canvas");
if (event.x != undefined && event.y != undefined)
{
x = event.x;
y = event.y;
}
else // Firefox method to get the position
{
x = event.clientX + document.body.scrollLeft +
document.documentElement.scrollLeft;
y = event.clientY + document.body.scrollTop +
document.documentElement.scrollTop;
}
x -= canvas.offsetLeft;
y -= canvas.offsetTop;
alert("x: " + x + " y: " + y);
}
</script>
现代浏览器现在可以帮你处理这个。Chrome, IE9和Firefox支持这样的offsetX/Y，从点击处理程序传入事件。
function getRelativeCoords(event) {
return { x: event.offsetX, y: event.offsetY };
}
大多数现代浏览器也支持layerX/Y，但是Chrome和IE使用layerX/Y作为页面点击的绝对偏移量，包括边距、填充等。在Firefox中，layerX/Y和offsetX/Y是等价的，但是offset之前并不存在。所以，为了兼容稍微老一点的浏览器，你可以使用:
function getRelativeCoords(event) {
return { x: event.offsetX || event.layerX, y: event.offsetY || event.layerY };
}
