Revision:
click and slide the blue slider to compare two images:
<div class="img-comp-container">
<div class="img-comp-img">
<img src="../images/2018-Sh-01.jpg" width="800" height="auto">
</div>
<div class="img-comp-img img-comp-overlay">
<img src="../images/2018-Sh-02.jpg" width="800" height="auto">
</div>
</div>
<style>
.img-comp-container img{margin-left: 1vw; border: 0.5vw outset burlywood; }
.img-comp-container {position: relative; height: 40vw; margin-left: 15vw;/*should be the same height as the images*/}
.img-comp-img {position: absolute; width: auto; height: auto; overflow:hidden;}
.img-comp-img img {display:inline; 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%;}
</style>
<script>
function initComparisons() {
var x, i;
x = document.getElementsByClassName("img-comp-overlay");
for (i = 0; i < x.length; i++) {
compareImages(x[i]);
}
function compareImages(img) {
var slider, img, clicked = 0, w, h;
w = img.offsetWidth;
h = img.offsetHeight;
img.style.width = (w / 2) + "px";
/*create and insert slider:*/
slider = document.createElement("DIV");
slider.setAttribute("class", "img-comp-slider");
img.parentElement.insertBefore(slider, img);
slider.style.top = (h / 2) - (slider.offsetHeight / 2) + "px";
slider.style.left = (w / 2) - (slider.offsetWidth / 2) + "px";
slider.addEventListener("mousedown", slideReady);
window.addEventListener("mouseup", slideFinish);
slider.addEventListener("touchstart", slideReady);
window.addEventListener("touchstop", slideFinish);
function slideReady(e) {
e.preventDefault();
clicked = 1;
window.addEventListener("mousemove", slideMove);
window.addEventListener("touchmove", slideMove);
}
function slideFinish() {
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;
a = img.getBoundingClientRect();
x = e.pageX - a.left;
x = x - window.pageXOffset;
return x;
}
function slide(x) {
img.style.width = x + "px";
slider.style.left = img.offsetWidth - (slider.offsetWidth / 2) + "px";
}
}
}
/*Execute a function that will execute an image compare function for each element with the img-comp-overlay class:*/
initComparisons();
</script>