on-the-fly making gnuplot movie

main.rb

#!/usr/bin/ruby
# coding: utf-8

require 'open3'
Open3.popen3('gnuplot') do |gp_in, gp_out, gp_err|

  gp_in.puts "set term png"
  gp_in.puts "set output \"| ffmpeg -y -f image2pipe -i - -vcodec copy out.mp4 >ffmpeg.log 2>&1"

  500.times do |i|
    delta = 0.1*i
    gp_in.puts "plot sin(x+%g)" % delta
    gp_in.puts "print \"%d\"" % i
    puts gp_err.gets
  end

  gp_in.puts "set output"
  gp_in.puts "print \"end\""
  puts gp_err.gets
  
  gp_in.puts "exit"

end