gistfile1.rb

require 'imgkit'

class InvalidInputFileError < Exception; end

class Converter
  def initialize(input, output = nil)
    raise InvalidInputFileError unless html_file?(input)
    @input = input
    if @output
      @ouput = output
    else
      @output = output_filename_from @input
    end
  end

  def convert_file
    kit = IMGKit.new(File.new(@input))
    kit.to_img(:png)
    kit.to_file(@output)
  end

  protected

  def html_file?(filename)
    filename.match(/\.html$/)
  end

  def output_filename_from(filename)
    File.basename(filename, File.extname(filename))
  end
end

conv = Converter.new("faktura.html")
conv.convert_file