How To Compare Two Images


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How TO - Image Comparison Slider

Learn how to create a slider that compares two images.

Image Comparison Slider

Move the blue slider to compare images:

Try it Yourself »

Create an Image Comparison Slider

Step 1) Add HTML:


<div class="img-comp-container">
  <div class="img-comp-img">
    <img src="img_fjords.jpg" width="300" height="200">
  <div class="img-comp-img img-comp-overlay">
    <img src="img_forest.jpg" width="300" height="200">

Step 2) Add CSS:

The container must have a "relative" positioning.


* {box-sizing: border-box;}

.img-comp-container {
  position: relative;
  height: 200px; /*should be the same height as the images*/

.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;
  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%;

Step 3) Add JavaScript:


function initComparisons() {
  var x, i;
  /*find all elements with an "overlay" class:*/
  x = document.getElementsByClassName("img-comp-overlay");
  for (i = 0; i < x.length; i++) {
    /*once for each "overlay" element:
    pass the "overlay" element as a parameter when executing the compareImages function:*/
  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%:*/ = (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:*/ = (h / 2) - (slider.offsetHeight / 2) + "px"; = (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:*/
      /*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:*/
    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:*/ = x + "px";
      /*position the slider:*/ = img.offsetWidth - (slider.offsetWidth / 2) + "px";

Step 4) Execute Script:


Try it Yourself »