carrier.rb

# lib/carrier.rb
module Carrier
  def send_to_browser(file, options = {})
    extension = File.extname(file.path)[1..-1].downcase # '.jpg' => 'jpg' for Mime::Type
    options[:type] ||= Mime::Type.lookup_by_extension(extension)

    send_file(file.path, options)
  end

  # TODO: it changes parameter value!!!
  # This hack is only for the IE error in a filename encoding with headers
  # Using URI.escape
  def escape_filename_if_msie(filename)
    if request.env['HTTP_USER_AGENT'] =~ /MSIE/
      filename = URI.escape(filename, Regexp.new("[^#{URI::PATTERN::UNRESERVED}]"))
    end

    filename
  end
end

pages_controller.rb

# app/controllers/pages_controller.rb
class PagesController < ApplicationController
  def show
    authorize :view, document

    page_image = preview_page || watermarked_page
    send_to_browser(page_image, disposition: 'inline')
  end

  private
    def document
      @document ||= Document.find(params[:id])
    end

    def preview_page
      document_page.image.preview if params[:style] == 'preview'
    end

    def watermarked_page
      Watermarker.new(watermark).mark(document_page.image)
    end

    def document_page
      @document_page ||=
        document.current.pages.where(number: params[:page_number]).first!
    end

    def watermark
      ' ' + [ Time.now.to_s(:long), current_user.presents.fullname, document.title ].join(' - ')
    end
end

watermarker.rb

# lib/watermarker.rb
class Watermarker
  @@ramdisk = "#{Rails.root}/data/ramdisk/"

  attr_accessor :watermark, :border_size, :output_path

  def initialize(watermark)
    self.watermark = watermark
    self.border_size = 1
    self.output_path = generate_output_path('.png')
  end

  def mark(image_file)
    image_path = image_file.path
    change_output_values unless png?(image_path)

    system "convert -size 900x900 xc:none -fill '#CCCCCC70' \
            -gravity NorthWest -draw \"font-size 34 rotate 45 text 10,10 '#{watermark}'\" miff:- |\
            composite -tile - \"#{image_path}\" miff:- |\
            convert -bordercolor '#808080' -border #{border_size} - \"#{output_path}\" >/dev/null 2>&1"

    File.new(output_path)
  end

  private
    def change_output_values(path)
      self.border_size = 10
      self.output_path = generate_output_path('.jpg')
    end

    def generate_output_path(extension)
      @@ramdisk + SecureRandom.hex(6) + extension
    end

    def png?(path)
      File.extname(path) == '.png'
    end
end