javascript - How to zoom an image over the mouse position and display result on div

Question

I'm trying to zoom an image and display the result on a div that's around the cursor, something like w3schools does, except they display the result on another image and "I'm not".

The problem is, the zoom is not exactly right with respect to the mouse cursor, it's displaying the closest pixels but not the right ones, and I can't figure out why.

Here's the snippet.

My CSS:

* {
  box-sizing: border-box;
}

.img-zoom-container {
  position: relative;
}

.img-zoom-lens {
  position: absolute;
  border: 1px solid #d4d4d4;
  border-radius: 40px;
  width: 70px;
  height: 70px;
}

.img-zoom-result {
  border: 1px solid #d4d4d4;
  width: 300px;
  height: 300px;
}

.crosshair {
  cursor: crosshair;
}

HTML:

<h1>Image Zoom</h1>

<p>Mouse over the image:</p>

<div class="img-zoom-container crosshair">
  <img id="myimage" src="https://www.pasionfutbol.com/__export/1506450064123/sites/pasionlibertadores/img/2017/09/26/puyol3c.jpg_715985292.jpg">
  <div id="myresult" class="img-zoom-result"></div>
</div>

<p>The image must be placed inside a container with relative positioning.</p>
<p>The result can be put anywhere on the page, but must have the class name "img-zoom-result".</p>
<p>Make sure both the image and the result have IDs. These IDs are used when a javaScript initiates the zoom effect.</p>

The JS:

function imageZoom(imgID, resultID) {
  var img, lens, result, cx, cy;
  img = document.getElementById(imgID);
  result = document.getElementById(resultID);
  /*create lens:*/
  lens = document.createElement("DIV");
  lens.setAttribute("class", "img-zoom-lens");
  /*insert lens:*/
  img.parentElement.insertBefore(lens, img);
  /*calculate the ratio between result DIV and lens:*/
  cx = result.offsetWidth / lens.offsetWidth;
  cy = result.offsetHeight / lens.offsetHeight;
  /*set background properties for the result DIV:*/
  lens.style.backgroundImage = "url('" + img.src + "')";
  lens.style.backgroundSize = (img.width * cx) + "px " + (img.height * cy) + "px";
  /*execute a function when someone moves the cursor over the image, or the lens:*/
  lens.addEventListener("mousemove", moveLens);
  img.addEventListener("mousemove", moveLens);
  /*and also for touch screens:*/
  lens.addEventListener("touchmove", moveLens);
  img.addEventListener("touchmove", moveLens);
  function moveLens(e) {
    var pos, x, y;
    /*prevent any other actions that may occur when moving over the image:*/
    e.preventDefault();
    /*get the cursor's x and y positions:*/
    pos = getCursorPos(e);
    /*calculate the position of the lens:*/
    x = pos.x - (lens.offsetWidth / 2);
    y = pos.y - (lens.offsetHeight / 2);
    /*prevent the lens from being positioned outside the image:*/
    if (x > img.width - lens.offsetWidth) {x = img.width - lens.offsetWidth;}
    if (x < 0) {x = 0;}
    if (y > img.height - lens.offsetHeight) {y = img.height - lens.offsetHeight;}
    if (y < 0) {y = 0;}
    /*set the position of the lens:*/
    lens.style.left = x + "px";
    lens.style.top = y + "px";
    /*display what the lens "sees":*/
    lens.style.backgroundPosition = "-" + (x * cx) + "px -" + (y * cy) + "px";
  }
  function getCursorPos(e) {
    var a, x = 0, y = 0;
    e = e || window.event;
    /*get the x and y positions of the image:*/
    a = img.getBoundingClientRect();
    /*calculate the cursor's x and y coordinates, relative to the image:*/
    x = e.pageX - a.left;
    y = e.pageY - a.top;
    /*consider any page scrolling:*/
    x = x - window.pageXOffset;
    y = y - window.pageYOffset;
    return {x : x, y : y};
  }
}

imageZoom("myimage", "myresult");

As you see is a copy-paste code from w3schools, but I need to do the changes I've explained above.

Answer 1

You need to adjust the position in the zoomed image.

/*display what the lens "sees":*/
var lw=lens.offsetWidth;
var lh=lens.offsetHeight;
// taking here the pos.x of event and scale it with zoom factor , then subtract the lense width divided by zoom factor;
// then some fine tuning : subtracting the 0.5 lense width in the zoomed image 
// UPDATED
var zx=pos.x*cx -(0.5*lw) ; 
var zy=pos.y*cy -(0.5*lh) ; 
lens.style.backgroundPosition = "-" + (zx) + "px -" + (zy) + "px";

I hope that helps :)