MediaElement.js Plugin for Preview Thumbnails

html5 video, mediaelement.js
bower install mep_feature_time_rail_thumbnails


MediaElement.js Plugin for Preview Thumbnails

Hover over the time rail on a MediaElement.js player and see video thumbnails.

See a video of this in action

See it in action on the NCSU Libraries Rare & Unique Digital Collection site.


To use this feature you will first need to create an image sprite and a metadata WebVTT file.

Create an Image Sprite

The idea is to take a snapshot of your video every N seconds and then stitch the images together into a sprite. This means that only one image file needs to be requested from the server and switching between thumbnails can be very quick.

Here's one way you could create the image sprite using ffmpeg and montage (from ImageMagick):

ffmpeg -i "video-name.mp4" -f image2 -vf fps=fps=1/5 video-name-%05d.jpg
montage video-name*jpg -tile 5x -geometry 150x video-name-sprite.jpg

First ffmpeg takes a snapshot of your video every 5 seconds. Then montage reduces each image to 150px across and tiles them from left to right 5 across.

Create a WebVTT metadata file

Once you have the image sprite you'll also need to create a WebVTT metadata file that contains the URL to your image sprite. For each time range of 5 seconds the URL is given including a spatial Media Fragment hash. This gives information about where in your sprite to look for the thumbnail for that time range. Here's an example of what one looks like:


00:00:00.000 --> 00:00:05.000,0,150,100

00:00:05.000 --> 00:00:10.000,0,150,100

00:00:10.000 --> 00:00:15.000,0,150,100

00:00:15.000 --> 00:00:20.000,0,150,100

00:00:20.000 --> 00:00:25.000,0,150,100

00:00:25.000 --> 00:00:30.000,100,150,100

Current Sprite Limitations

All the images embedded in the sprite should have exactly the same dimensions. The styling for the thumbnail hover area is calculated based on the dimensions of the first URL in the WebVTT file.

The durations that each thumbnail ought to show should be consistent. The default is 5 seconds, though this is configurable when initializing the player.


Add a new track element to your video element like the following:

<track kind="metadata" class="time-rail-thumbnails" src=""></track>

The kind attribute must be "metadata" and the class must be "time-rail-thumbnails". The WebVTT file should also be accessible via an AJAX request, so either have it on the same domain or allow cross-origin requests.


Add 'timerailsthumbnails' as the last feature when initializing the mediaelement player:

   features: ['playpause','progress','current','duration','tracks','volume', 'timerailthumbnails'],
    timeRailThumbnailsSeconds: 5

Also, either use the default of 5 second intervals for each thumbnail or set timeRailThumbnailsSeconds to the appropriate value.


This JavaScript plugin is made available as both a Rails Engine gem for the asset pipeline (unreleased) and a bower package. Choose your poison.


This plugin relies on the vtt.js library for WebVTT parsing. You'll need to include this before including mep-feature-time-rail-thumbnails.js.


Include it in your Gemfile:

gem 'mep_feature_time_rail_thumbnails'

Add it to your application.js:

//= require mep-feature-time-rail-thumbnails


Install with bower:

bower i mep_feature_time_rail_thumbnails

This will install vtt.js as well.

Include them both in HTML:

<script src="/bower_components/vtt.js/vtt.min.js"></script>
<script src="/bower_components/mep_feature_time_rail_thumbnails/vendor/assets/javascripts/mep-feature-time-rail-thumbnails.js"></script>

Browser Support

The plugin currently uses MutationObserver and is disabled for browsers without support. You may be able to use a polyfill, but I have not tried that yet.


  • Make the interval of thumbnails really use the timestamps in the WebVTT file rather than relying on configuration and regular intervals.


Jason Ronallo


This project rocks and uses MIT-LICENSE.