BluePil.js is a (vanilla) javascript library allowing to easily make the images of a document load progressively, with a smooth animation. It actually mimics how the images are loaded on Medium.

DEMO

Features

• Supports css background-images and img tags
• Dependency free
• Custom builds: bluepil can be built into different versions according to the needed functionalities.
• Supports all the relevant web browsers
• Customisable animations: the transition between the miniature image (placeholder) and the full image is completely customisable.

I wrote an article describing how it works. You can find it here and on Medium.

Download

The library is available on github, along with a “quick start guide” and the complete documentation.