网页相对定位

function click_canvas(e){
    var scrollTop, scrollLeft;

    if (document.documentElement && document.documentElement.scrollTop) {
        scrollTop = document.documentElement.scrollTop;
        scrollLeft = document.documentElement.scrollLeft;
    } else if (document.body) {
        scrollTop = document.body.scrollTop;
        scrollLeft = document.body.scrollLeft;
    }

    var imgX = e.clientX + scrollLeft
    var imgY = e.clientY + scrollTop - 60//这个60估计是地址栏的高度


　　 //在canvas上定位
    press(img, imgX, imgY)

　　//计算相对位置
    var x = imgX - $(‘#canvas‘).offset().left
    var y = imgY - $(‘#canvas‘).offset().top
　　
　　//计算比率

　　var px = Math.min(Math.max((x / canvas.width),0), 1)

　　　var py = Math.min(Math.max((y / canvas.height),0), 1)

}