获取鼠标点击相对于Canva位置的2种方法

如果给Canvas添加 onmousedown事件，获取到的鼠标位置都是相对于当前文档的位置（x,y）：

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" alt="" />

第一种转换：

(x-x1,y-y1)

x,y为鼠标点击位置，getBoundingClientRect方法是canvas自带的获取可绘画区域的位置信息的函数

function windowToCanvas(x, y) {
   var bbox = canvas.getBoundingClientRect();
   return { x: x - bbox.left * (canvas.width  / bbox.width),
            y: y - bbox.top  * (canvas.height / bbox.height) };
}

第二种更加简洁：

canvas.onmousedown = function (e) {
   //var loc = windowToCanvas(e.clientX||e.x, e.clientY||e.y);
   var x=e.layerX||e.offsetX;
   var y=e.layerY||e.offsetY;
   e.preventDefault(); // prevent cursor change

   saveDrawingSurface();
   mousedown.x = loc.x;
   mousedown.y = loc.y;
   dragging = true;
};

只有firefox支持layerX,其他浏览器支持标准的offsetX