This gem provides polyfills for 3 wys to render responsive images:

• picturefill
• jquery-picture
• srcset

picturefill is currently the best way for rendering Responsive Images on a web page.

picturefill-rails provides nice view helper methods to render the picturefill.

<div data-picture data-alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
    <div data-src="small.jpg"></div>
    <div data-src="small.jpg"         data-media="(min-device-pixel-ratio: 2.0)"></div>
    <div data-src="medium.jpg"        data-media="(min-width: 400px)"></div>
    <div data-src="medium_x2.jpg"     data-media="(min-width: 400px) and (min-device-pixel-ratio: 2.0)"></div>
    <div data-src="large.jpg"         data-media="(min-width: 800px)"></div>
    <div data-src="large_x2.jpg"      data-media="(min-width: 800px) and (min-device-pixel-ratio: 2.0)"></div>  
    <div data-src="extralarge.jpg"    data-media="(min-width: 1000px)"></div>
    <div data-src="extralarge_x2.jpg" data-media="(min-width: 1000px) and (min-device-pixel-ratio: 2.0)"></div> 

    <!-- Fallback content for non-JS browsers. Same img src as the initial, unqualified source element. -->
    <noscript>
        <img src="external/imgs/small.jpg" alt="A giant stone face at The Bayon temple in Angkor Thom, Cambodia">
    </noscript>
</div>

The above can be rendered in Rails 3+ by writing the following code, using the View helpers provided by the Rails engine included:

= picturefill 'A giant stone face at The Bayon temple in Angkor Thom, Cambodia' do
  = picture_src 'small.jpg'
  = picture_src 'small.jpg',     "(min-device-pixel-ratio: 2.0)"
  = picture_src 'medium.jpg',    "(min-width: 400px)"
  = picture_src 'medium_x2.jpg', "(min-width: 400px) and (min-device-pixel-ratio: 2.0)"
  = picture_src 'largs.jpg',     "(min-width: 800px)"
  = picture_src 'large_x2.jpg',  "(min-width: 800px) and (min-device-pixel-ratio: 2.0)"
  # ...
  = picture_fallback "external/imgs/small.jpg", alt: "A giant stone face at The Bayon temple in Angkor Thom, Cambodia" 

Note: This example uses HAML as the rendering engine.

Using conventions, and an extra ratio: option, the following shorthand is possible:

= picturefill 'A giant stone face at The Bayon temple in Angkor Thom, Cambodia' do
  = picture_src 'small.jpg', ratio: 'x2'
  = picture_src 'medium.jpg', "400", ratio: 'x2'
  = picture_src 'large.jpg',  "800", ratio: 'x2'  
  # ...
  = picture_fallback "external/imgs/small.jpg", alt: "A giant stone face at The Bayon temple in Angkor Thom, Cambodia"

This will ouput exactly the same HTML as the previous example :) See the specs for more details...

In your Gemfile:

gem 'picturefill-rails'

A number of specs are included which all pass and should ensure that the view helpers work as expected.

The #picture_src method works, but could use some heavy refactoring! I don't like methods of more than 10 lines! Reponsibilities should be off-loaded to other methods (or classes)

Currently the gem only supports Ruby 1.9+ and has only been tested on 1.9.3.

The gem includes the picturefill javascript assets that are automatically available for the asset pipeline. In your application.js manifest file require:

• picturefill.js
• picturefill\matchemedia.js

See demo for a full example!

jquery picture is now also partly supported. It is very similar to picturefill but with slightly different tags.

The assets jquery-picture.min.js and jquery-picture.js are included in vendor/assets.

The view helper includes the helper methods:

• picture_tag(alt)
• `source_tag(src, media, options = {})

Usage example:

= picture_tag 'A giant stone face at The Bayon temple in Angkor Thom, Cambodia' do
  = source_tag 'small.jpg', ratio: 'x2'
  = source_tag 'medium.jpg', "400", ratio: 'x2'
  = source_tag 'large.jpg',  "800", ratio: 'x2'  
  # ...
  = picture_fallback "external/imgs/small.jpg", alt: "A giant stone face at The Bayon 

And to enable via jquery:

$(function(){
    $('picture').picture();
});

Arcticles

critique of picturefill

See the specification for the reference algorithm.

See [repo]: https://github.com/borismus/srcset-polyfill

Use the srcset attribute of <img> elements. For example:

<img alt="The Breakfast Combo"
     src="banner.jpeg"
     srcset="banner-HD.jpeg 2x, banner-phone.jpeg 100w,
             banner-phone-HD.jpeg 100w 2x"/>

Include srcset.min.js in your page.

View helper

• imgset_tag src, srcset, options = {} (alias imageset_tag)

= imgset_tag "banner.jpeg", "banner-HD.jpeg 2x, banner-phone.jpeg 100w,banner-phone-HD.jpeg 100w 2x", alt: "The Breakfast Combo"

outputs the HTML code shown above.

If you leave out the srcset argument it will fallback to a single src image

= imgset_tag "banner.jpeg", alt: "The Breakfast Combo"

Same as

= image_tag "banner.jpeg", alt: "The Breakfast Combo"

The gem includes srcset javascript assets that are automatically available for the asset pipeline. In your application.js manifest file require:

• srcset.min.js (prod)
• srcset.js (dev/test)

• How to reliably check for srcset support in the browser (so as to not attempt to polyfill if it's not necessary?)
• Is it safe to use -webkit-transform to scale things?
• Is it worth falling back to -webkit-image-set if available?

• Check out the latest master to make sure the feature hasn't been implemented or the bug hasn't been fixed yet.
• Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it.
• Fork the project.
• Start a feature/bugfix branch.
• Commit and push until you are happy with your contribution.
• Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
• Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.

Copyright (c) 2012 Kristian Mandrup. See LICENSE.txt for further details.