Keeping image centers in the center

If you ever need to keep an image’s center centered on a website no matter what viewport size your browser has, there is a neat little trick I stumbled upon on StackOverflow provided by Amber Yust. The goal is that the image is cropped not only on the right but also on the left side to keep the center of the image also in the center of the screen. The appoach mentioned by Amber is to position the image absolutely with an offset of 50% from the left and then re-adjust this offset using a negative margin of minus half the image’s actual width.

---

#container {
    position: relative;
}
#container img#demoimage {
    display: block;
    margin: auto;
    position: absolute;
    left: 50%;
    margin-left: -600px;
/* If 600px is half of your image's width */
}

This has one caveat, though: You have to know the dimensions of the image in advance or determine them via JavaScript and adjust the negative margin after the fact.

(function(img) {
    // This will get called, once the image has actually been
    // loaded from the server
    function updater() {
        img.style.position = "absolute";
        img.style.left = "50%";
        img.style.marginLeft = "" + -img.width / 2 + "px";
        console.log(img.style.marginLeft);
    }
    img.addEventListener('load', updater, false);
})(document.getElementById("demoimage"));

If JavaScript is disabled, the margin:auto up there will at least center the image if there is enough room for it and prevent a slight jumping if JavaScript is enabled and the repositioning takes place.

For IE you will have to add some extra code to ensure your event is fired even then the image is already available within the browser’s cache, but that’s mostly it.

Another approach to this issue, which I usually prefer, is to have the image itself be situated in the background of the controller and positoned in its center. The upside of this solution is, that you don’t need to position the image using JavaScript if you know its height to make room for it in the container. A downside here is, that without JavaScript this won’t work if there is an imagemap on top of the image (which was the case and the original reason why I was looking for a different solution).