javascript - 居中对齐位于 javascript 中的图像

Question

我正在为一个类(class)元素制作网页，我已经设置好框架，但是我遇到了一个问题。我无法让这张图片在我的生活中居中对齐。它目前位于屏幕右侧，我就是无法移动它。您很可能需要将其粘贴到浏览器中才能准确了解正在发生的事情。

initComparisons();
function initComparisons() {
  var x, i;
  /*find all elements with an "overlay" class:*/
  x = document.getElementsByClassName("img-comp-overlay center");
  for (i = 0; i < x.length; i++) {
    /*once for each "overlay" element:
    pass the "overlay" element as a parameter when executing the compareImages function:*/
    compareImages(x[i]);
  }
  function compareImages(img) {
    var slider, img, clicked = 0, w, h;
    /*get the width and height of the img element*/
    w = img.offsetWidth;
    h = img.offsetHeight;
    /*set the width of the img element to 50%:*/
    img.style.width = (w / 2) + "px";
    /*create slider:*/
    slider = document.createElement("DIV");
    slider.setAttribute("class", "img-comp-slider");
    /*insert slider*/
    img.parentElement.insertBefore(slider, img);
    /*position the slider in the middle:*/
    slider.style.top = (h / 2) - (slider.offsetHeight / 2) + "px";
    slider.style.left = (w / 2) - (slider.offsetWidth / 2) + "px";
    /*execute a function when the mouse button is pressed:*/
    slider.addEventListener("mousedown", slideReady);
    /*and another function when the mouse button is released:*/
    window.addEventListener("mouseup", slideFinish);
    /*or touched (for touch screens:*/
    slider.addEventListener("touchstart", slideReady);
     /*and released (for touch screens:*/
    window.addEventListener("touchstop", slideFinish);
    function slideReady(e) {
      /*prevent any other actions that may occur when moving over the image:*/
      e.preventDefault();
      /*the slider is now clicked and ready to move:*/
      clicked = 1;
      /*execute a function when the slider is moved:*/
      window.addEventListener("mousemove", slideMove);
      window.addEventListener("touchmove", slideMove);
    }
    function slideFinish() {
      /*the slider is no longer clicked:*/
      clicked = 0;
    }
    function slideMove(e) {
      var pos;
      /*if the slider is no longer clicked, exit this function:*/
      if (clicked == 0) return false;
      /*get the cursor's x position:*/
      pos = getCursorPos(e)
      /*prevent the slider from being positioned outside the image:*/
      if (pos < 0) pos = 0;
      if (pos > w) pos = w;
      /*execute a function that will resize the overlay image according to the cursor:*/
      slide(pos);
    }
    function getCursorPos(e) {
      var a, x = 0;
      e = e || window.event;
      /*get the x positions of the image:*/
      a = img.getBoundingClientRect();
      /*calculate the cursor's x coordinate, relative to the image:*/
      x = e.pageX - a.left;
      /*consider any page scrolling:*/
      x = x - window.pageXOffset;
      return x;
    }
    function slide(x) {
      /*resize the image:*/
      img.style.width = x + "px";
      /*position the slider:*/
      slider.style.left = img.offsetWidth - (slider.offsetWidth / 2) + "px";
    }
  }
}

body {
  margin: 0;}

  #bg {
  position: fixed; 
  top: 0; 
  left: 0; 

  /* Preserve aspet ratio */
  min-width: 100%;
  min-height: 100%;
}

ul.topnav {
    list-style-type: none;
    margin: 0;
    padding: 0;
    overflow: hidden;
    background-color: #333;
      background-color: rgba(0,255,255, 0.3);
  backdrop-filter: blur(5px);  
  -webkit-backdrop-filter: blur(5px);
  max-width: 100%;
  max-height: 100%;
  overflow: auto;
}

.center {
  display: flex;
  justify-content: center;
  flex-direction: column;

}

ul.topnav li {float: left;}

ul.topnav li a {
    display: block;
    color: white;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
}

ul.topnav li a:hover:not(.active) { background-color: rgba(47,79,79, 0.6);
  backdrop-filter: blur(5px);  
  -webkit-backdrop-filter: blur(5px);}

ul.topnav li a.active {background-color: rgba(112,128,144, 0.6);
  backdrop-filter: blur(5px);  
  -webkit-backdrop-filter: blur(5px);}

ul.topnav li.right {float: right;}

@media screen and (max-width: 600px){
    ul.topnav li.right, 
    ul.topnav li {float: none;}
}

.tanddblur {
    list-style-type: none;
    margin: 0;
    padding: 0;
    overflow: hidden;
    align-content: center;
    align-self:center;
    background-color: #333;
      background-color: rgba(47,79,79, 0.5);
  backdrop-filter: blur(5px);  
  -webkit-backdrop-filter: blur(5px);
  overflow: auto;
  padding-left:10px;
  padding-right:10px;
  border-style: solid;
  border-color: darkslategray;
  border-radius:15px;
  padding-top:10px;
  padding-bottom:10px;
}

.parent {
  position: relative;
}
.child {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

* {box-sizing: border-box;}

.img-comp-container {
  position: absolute;
  height: 200px; /*should be the same height as the images*/
     top: 50%;
  left: 50%;
  right:50%;
  transform: translate(-50%, -50%);
}

.img-comp-img {
  position: absolute;
  width: auto;
  height: auto;
  overflow: hidden;
}

.img-comp-img img {
  display: block;
  vertical-align: middle;
}

.img-comp-slider {
  position:absolute;
https://www.w3schools.com/howto/img_fjords.jpg  z-index: 9;
  cursor: ew-resize;
  /*set the appearance of the slider:*/
  width: 40px;
  height: 40px;
  background-color: #2196F3;
  opacity: 0.7;
  border-radius: 50%;
}

#display_image
{
align-self:center;
}

  <img src="webpagepic.jpg" id="bg" alt="">

<ul class="topnav">
    <li><a class= "active" href="#news">Reenacted Photos</a></li>
  <li><a href="navpage.html">Terms & Definitions</a></li>
  <li><a href="#contact">Video</a></li>
  <li><a href="#about">Review Links</a></li>
  <li class="right"><a href="index.html">Return Home</a></li>
</ul>


<div align= 'center' class="img-comp-container center">
  <div align= 'center' class="img-comp-img">
    <img src="https://www.w3schools.com/howto/img_forest.jpg" width="600" height="300">
  </div>
  <div align= 'center' class="img-comp-img img-comp-overlay center">
    <img src="https://www.w3schools.com/howto/img_fjords.jpg" width="600" height="300">
  </div>
</div>
<div>

Answer 1

对您的 CSS 进行以下更改。基本上，为 left 分配一个百分比，相对于容器的宽度移动图像，为 top 分配一个百分比，使图像相对于父容器的高度移动。我使用 calc 来完美地居中图像。例如，容器的宽度为 600 像素，因此水平中心为 50% - 300 像素(图像宽度的一半)。

最后，我为 html 和 body 分配了宽度和高度，以便 body 的长度和宽度与浏览器窗口相同。

.img-comp-container {
  position: absolute;
  left: calc(50% - 300px);
  top: calc(50% - 23px);
  height: 200px; /*should be the same height as the images*/
  width: 600px;
}

html,body{
  width:100%;
  height:100%;
  margin:0;
}

Here is a fiddle.