Dynamically defining retina images size. Based on: http://stackoverflow.com/questions/16651900/conditional-image-resizing-with-carrierwave?rq=1

account_logo_uploader.rb

# -*- encoding : utf-8 -*-
class AccountLogoUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick

  if Rails.env.development? || Rails.env.test?
    storage :file
  else
    storage :s3
    s3_access_policy 'public-read'
  end

  max_file_size = 5.megabytes

  version :original do
    process
  end

  version :thumbnail do
    process :resize_to_limit => [100, 75]
  end

  version :logo do
    process :resize_to_fit => [160, 160]
  end

  version :retina do
    process :resize_to_fill => [160, 160] ,:if => :is_not_retina_resolution?
    process :resize_to_fill => [160*2, 160*2] ,:if => :is_retina_resolution?
  end

  def store_dir
    "account_logos"
  end

  def extension_white_list
    %w(jpg jpeg gif png)
  end

  # Override the filename of the uploaded files
  def filename
    "#{model.account_id}.#{file.extension}" if original_filename && model && model.account_id
  end

private

  def is_retina_resolution?(new_file)
    image = MiniMagick::Image.open(self.file.file)
    true if image[:height] >= 160*2 and image[:width] >= 160*2
  end

  def is_not_retina_resolution?(new_file)
    image = MiniMagick::Image.open(self.file.file)
    true if image[:height] < 160*2 and image[:width] < 160*2
  end
end