watermarker.rb

#!/usr/bin/env ruby

require 'prawn'
require 'prawn/templates'

module Watermarker

  def apply(document, image_path, page_idx = 1, image_opts = {}, text_data = nil, text_opts = {})
    document.go_to_page page_idx
    document.image image_path, image_opts if image_path
    document.draw_text text_data, text_opts if text_data
  end

  def apply_operations(input, output, operations = {})
    Prawn::Document.generate(output, :template => StringIO.new(File.read input)) do |d|
      operations.each do |idx, operation|
        apply(d, operation[:image_path], idx, operation[:image_opts], operation[:text_data], operation[:text_opts])
      end
    end
  end

  extend self
end

=begin
Prawn::Document.generate(output_filename, :template => StringIO.new(File.read input_filename)) do |d|
 d.page_count.times do |i|
   d.go_to_page i
   #d.stroke_line [d.bounds.left,d.bounds.bottom], [d.bounds.right,d.bounds.top]
   d.image "a.png", scale: 0.1, at: [420,350]
 end
end
=end