Make images load when they enter visible section of browser?

Question

I was browsing around the web and I saw something I've never seen before. on this site: http://blogof.francescomugnai.com/2009/04/mega-roundup-of-geektool-scripts-inspiration-gallery/

When you navigate down the page, the images only load when they are in the visible portion of the browser. I have never seen this before and was wondering if anyone else has and how exactly one would do it.

I'm guessing this is some sort of Wordpress plugin (that's what he's using) but I'm not sure. Is it javascript? Are they actually loading on page load but just become visible later for a "snazzy" effect or is this actually useful for quicker page load times?

Answer 1

If you look at the source of the page you referenced, it contains this bit of code:

jQuery(document).ready(function($){
  jQuery(".SC img").lazyload({
    effect:"fadeIn",
    placeholder: "http://blogof.francescomugnai.com/wp-content/plugins/jquery-image-lazy-loading/images/grey.gif"
  });
});

I suspect that's how they're accomplishing the effect. It uses the jQuery LazyLoad plugin, which can be found here:

http://www.appelsiini.net/projects/lazyload

Answer 2

"wp-content/plugins/jquery-image-lazy-loading"

Lazy loader is a jQuery plugin written in JavaScript. It delays loading of images in (long) web pages. Images outside of viewport (visible part of web page) wont be loaded before user scrolls to them. This is opposite of image preloading.

Using lazy load on long web pages containing many large images makes the page load faster. Browser will be in ready state after loading visible images. In some cases it can also help to reduce server load.

http://www.appelsiini.net/projects/lazyload

So it seems it goes through every image specified or inside of the context of an element and replaces the src with a placeholder gif before the images fully load, saves the original URI and when the image is "visible" it replaces the placeholder with the real image.