Retinator 
Retinator resizes the source image and generates three images in @1x, @2x and @3x iOS resolutions.
Usage
retinator source_image destination_resolution
source_image: path to the source imagedestination_resolution: a number indicating the resolution of the smaller dimension of the destination @1x image
Example
For an input image of 3000x1500, retinator test.jpg 200 will output:
test.png (400x200)
test@2x.png (800x400)
test@3x.png (1200x600)
Installation
This gem needs ImageMagick or GraphicksMagick in order to run. It has to be installed using a brew formula. Run this command in your terminal:
brew install graphicsmagick
After that you can install the tool the usual way:
gem install retinator
Or, in dev mode:
bundle install
gem build retinator.gemspec
gem install retinator-[VERSION].gem
TODO
- Allow batch processing