Install gem:

$ gem install light_resizer

And set it in config:

config.middleware.insert_before(Rack::Sendfile, LightResizer::Middleware, Rails.root)

For example we have image in public folder by next url:

example.com/images/kitten.jpg

Url of resized image will be:

example.com/images/light_resize/100x150/kitten.jpg

light_resizer will find /public/images/kitten.jpg and create light_resize with resized to size of 100x150px image.

Default resize doesn't crop image and fill new space with transparent background.

To crop image use 'crop' in url:

example.com/images/light_resize/crop/100x150/kitten.png