Sprite Factory

The sprite factory is a ruby library that can be used to generate CSS sprites. It combines individual image files from a directory into a single unified sprite image and creates an appropriate CSS stylesheet for use in your web application.

The library provides:

• both a ruby API and a command line script
• many customizable options
• support for any stylesheet syntax, including CSS and Sass.
• support for any image library, including RMagick and ChunkyPNG.
• support for pngcrush'n the generated image file

Installation

$ gem install sprite-factory

An image library is also required. SpriteFactory comes with built in support for RMagick or ChunkyPng.

RMagick is the most common image libary to use, installation instructions for ubuntu:

$ sudo aptitude install imageMagick libMagickWand-dev
$ sudo gem install rmagick

ChunkyPng is lighter weight but only supports .png format:

$ gem install chunky_png

SpriteFactory can also be easily extended to use the image library of your choice.

Usage

Use the sf command line script specifying the location of your images.

$ sf images/icons

This will combine the individual image files within that directory and generate:

• images/icons.png
• images/icons.css

You can also use the SpriteFactory class directly from your own code:

require 'sprite_factory'

SpriteFactory.run!('images/icons')

The original image name is used for the CSS class to show that image in HTML:

<img src='s.gif' class='high'>
<img src='s.gif' class='medium'>
<img src='s.gif' class='low'>

When using a framework such as Rails, you would usually DRY this up with a helper method:

def sprite_tag(name)
  image_tag('s.gif', :class => name)
end

Customization

Much of the behavior can be customized by overriding the following options:

• :output - specify output location for generated files
• :layout - specify layout algorithm (horizontal, vertical or packed)
• :style - specify output style (css or sass)
• :library - specify image library to use (rmagick or chunkypng)
• :selector - specify custom css selector (see below)
• :csspath - specify custom path for css image url (see below)
• :padding - add padding to each sprite
• :width - fix width of each sprite to a specific size
• :height - fix height of each sprite to a specific size
• :pngcrush - pngcrush the generated output image (if pngcrush is available)
• :output_css_override - CSS file output is disabled, instead CSS is returned as a string (useful if you want to store the CSS in the database)

Options can be passed as command line arguments to the sf script:

$ sf images/icons --style sass --layout packed

Options can also be passed as the 2nd argument to the #run! method:

SpriteFactory.run!('images/icons', :style => :sass, :layout => :packed)

You can see the results of many of these options by viewing the sample page that comes with the gem in test/images/reference/index.html.

Layout

The generated image can be laid out in a horizontal or a vertical strip by providing a :layout option (defaults to horizontal). A new option in v1.2.0 is to use a :packed layout which will attempt to generate an optimized packed square-ish layout.

For more details on the bin-packing algorithm used:

• You can find a description here
• You can find a demo here

Customizing the CSS Selector

By default, the CSS generated is fairly simple. It assumes you will be using <img> elements for your sprites, and that the basename of each individual file is suitable for use as a CSS classname. For example:

img.high   { width: 16px; height: 16px; background: url(images/icons.png)   0px 0px no-repeat; }
img.medium { width: 16px; height: 16px; background: url(images/icons.png) -16px 0px no-repeat; }
img.low    { width: 16px; height: 16px; background: url(images/icons.png) -32px 0px no-repeat; }

If you want to use different selectors for your rules, you can provide the :selector option. For example:

SpriteFactory.run!('images/icons', :selector => 'span.icon_')

will generate:

span.icon_high   { width: 16px; height: 16px; background: url(images/icons.png)   0px 0px no-repeat; }
span.icon_medium { width: 16px; height: 16px; background: url(images/icons.png) -16px 0px no-repeat; }
span.icon_low    { width: 16px; height: 16px; background: url(images/icons.png) -32px 0px no-repeat; }

Customizing the CSS Image Path

Within the generated CSS file, it can be tricky to get the correct path to your unified sprite image. For example, you might be hosting your images on Amazon S3, or if you are building a Ruby on Rails application you might need to generate URL's using the #image_path helper method to ensure it gets the appopriate cache-busting query parameter.

By default, the SpriteFactory generates simple url's that contain only the basename of the unified sprite image, but you can control the generation of these url's using the :csspath option:

For most CDN's, you can prepend a simple string to the image name:

SpriteFactory.run('images/icons',
                  :csspath => "http://s3.amazonaws.com/")

# generates:  url(http://s3.amazonaws.com/icons.png)

For more control, you can provide a lambda function and generate your own paths:

SpriteFactory.run('images/icons',
                   :csspath => lambda{|image| image_path(image)})

# generates:   url(/images/icons.png?v123456)

Customizing the entire CSS output

If you want complete control over the generated styles, you can pass a block to the run! method.

The block will be provided with information about each image, including the generated css attributes. Whatever content the block returns will be inserted into the generated css file.

SpriteFactory.run!('images/timer') do |images|
  rules = []
  rules << "div.running img.button { cursor: pointer; #{images[:running][:style]} }"
  rules << "div.stopped img.button { cursor: pointer; #{images[:stopped][:style]} }"
  rules.join("\n")
end

The images argument is a hash, where each key is the basename of an image file, and the value is a hash of image metadata that includes the following:

• :style - the default generated style
• :cssx - the css sprite x position
• :cssy - the css sprite y position
• :cssw - the css sprite width
• :cssh - the css sprite height
• :x - the image x position
• :y - the image y position
• :width - the image width
• :height - the image height

(NOTE: the image coords can differ form the css sprite coords when padding or fixed width/height options are specified)

Extending the Library

The sprite factory library can also be extended in a number of other ways.

• provide a custom layout algorithm in the SpriteFactory::Layout module.
• provide a custom style generator in the SpriteFactory::Style module.
• provide a custom image library in the SpriteFactory::Library module.

(see existing code for examples of each).

License

See LICENSE file.

Credits

Thanks to my employer, LiquidPlanner for allowing me to take this idea from our online project management web application and release it into the wild.

Contact

If you have any ideas, feedback, requests or bug reports, you can reach me at [email protected], or via my website: Code inComplete.