progressive-image.js

A lazy-loading progressive image system similar to those seen on Facebook and Medium. A very small, blurred image is replaced with the full-resolution equivalent when the element is scrolled into view.

View demonstration on codepen.io...

Benefits:

• fast loading, high performance, images loaded on view
• supports any image type (JPEG photographs are most appropriate)
• supports responsive images (srcset and sizes attributes)
• small: 1,007 bytes of JavaScript, 463 bytes of CSS (minified)
• any CSS reveal effect can be applied
• no external dependencies - works with any framework
• works in all modern browsers (IE10+)
• progressively-enhanced to work in older browsers
• easy to use

Usage instructions

Include the minified CSS and JavaScript on your page:

<link rel="stylesheet" href="css/progressive-image.min.css">
<script src="js/progressive-image.min.js"></script>

Basic example:

<a href="full.jpg" class="progressive replace">
  <img src="tiny.jpg" class="preview" alt="image" />
</a>

Where:

• full.jpg is the large image.
• tiny.jpg is a very small preview image - typically 20px in width saved with high JPEG compression. It be added to the page directly or inlined as a data URI as necessary.

Both images must have the same aspect ratio.

If image loading or JavaScript fails, a blurred version of the preview image can be clicked to view the full image.

When JavaScript runs successfully, the large image is loaded and revealed when the preview is scrolled into view. The link click is disabled and resulting HTML will be:

<a href="full.jpg" class="progressive">
  <img src="full.jpg" alt="image" />
</a>

Responsive images of differing sizes and resolutions can be defined in the link using the data-srcset and data-sizes attributes which map to the standard srcset and sizes attributes, e.g.

<a href="small.jpg" data-srcset="small.jpg 800w, large.jpg 1200w" data-sizes="100vw" class="progressive replace">
  <img src="tiny.jpg" class="preview" alt="image" />
</a>

On replacement, the image code becomes:

<img src="small.jpg" srcset="small.jpg 800w, large.jpg 1200w" sizes="100vw" alt="image" />

Modern browsers will load large.jpg on screens of 800px width or greater.

Usage notes

1. The preview and full-size images must have identical aspect ratios, e.g. 20x10 and 1200x600.
2. Only vertical scrolling is checked. All images in the horizontal plane will be loaded.
3. Progressive images dynamically added to the page using JavaScript will only be replaced when a scroll or resize event occurs.
4. You may be able to improve actual or perceived performance using data URIs to inline images or intrinsic placeholders.
5. Firefox can struggle when replacing large images and show a noticeable flicker.