test.rb

require 'rubygems'
require 'sinatra'
class Jaxer
  class << self
    def json_to_png(json)
      # producing image data
      
      File.open('foo', 'w+') do |f|
        cmd = "echo" 
        raise "Failed to convert canvas to PNG." unless system(cmd)
        yield(f)
      end
    end
  
  end
end

get '/' do
  Jaxer.json_to_png(params[:data]) do |img|
    @str = 'hello'
  end
  
  @str
end