Simple watermarking with Thumbkit and CarrierWave

my_uploader.rb

class MyUploader < CarrierWave::Uploader::Base
  include Thumbkit::Adapters::CarrierWave


  # Watermark should always be processed after thumbkit, ensuring that we always
  # have a valid image and we don't need to change the extension
  def watermark(watermark_image, options = {})
    cache_stored_file! if !cached?
    Watermarker.new(current_path, watermark_image).watermark!(options)
  end

  version :preview, :if => :is_image do
    process :thumbkit => [200, 200]
    process :watermark => [Rails.root.join('app/assets/images/watermark.gif')]

    def full_filename(for_file = model.file.file)
      [version_name, thumbkit_filename(for_file)].join('_')
    end
  end

watermarker.rb

require 'mini_magick'

class Watermarker
  def initialize(original_path, watermark_path)
    @original_path = original_path.to_s
    @watermark_path = watermark_path.to_s
  end

  # Accepted options: :gravity
  #
  #   Gravity options: None, Center, East, Forget, NorthEast, North, NorthWest,
  #     SouthEast, South, SouthWest, West, Static
  def watermark!(options = {})
    options[:gravity] ||= 'center'

    image = MiniMagick::Image.open(@original_path)
    result = image.composite(MiniMagick::Image.open(@watermark_path)) do |c|
      c.gravity options[:gravity]
    end
    result.write @original_path
  end

end