Generate jpg image from pdf files

convert_pdf_to_png.rb

require 'grim'
require 'thread'

class ThreadPool
  def initialize(size = 5)
    @jobs = Queue.new
    @size = size
    @pool = Array.new(@size) do |i|
      Thread.new do
        Thread.current[:id] = i
        catch(:exit) do
          loop do
            job, args = @jobs.pop
            job.call *args
          end
        end
      end
    end
  end

  def spawn(*args, &block)
    @jobs << [block, args]
  end

  def shutdown
    @size.times do
      spawn { throw :exit }
    end

    @pool.map(&:join)
  end
end

class Converter
  def initialize filepath, concurent = 10
    @filepath = filepath
    @concurent = concurent.to_i
    @count = Grim.reap(@filepath).count
  end

  def run
    @pool = ThreadPool.new @concurent
    block = @count / @concurent
    (0...@concurent).to_a.each do |index|
      first_page = block * index + 1
      last_page = first_page + block-1
      @pool.spawn do
        `gs -dNOPAUSE -sDEVICE=jpeg -dFirstPage=#{first_page} -dLastPage=#{last_page} -r100x100 -sOutputFile=image-#{index}-%d.jpg -q #{@filepath} -c quit`
      end
    end
  end

  def exit
    @pool.shutdown
  end
end

if $0 == __FILE__
  unless ARGV[0].nil?
    converter = Converter.new(ARGV[0], ARGV[1])
    converter.run
    at_exit { converter.exit }
  end
end

This implementation use ghostscript library and grim gems
Actually grim was used for only calculate page count of pdf file. We can us ghostscript to do it

We using Thread Pool to run some process of ghostscript concurently.
Full implementation should upload files right after generating image
Upload should be implemented by evented loop for high performance