コマンドラインで画像のPSNRを測定する ref: http://qiita.com/items/ca9ffa66bd1703c745dd

file0.txt

compare -metric PSNR ref.jpg target.jpg diffpng

file1.rb

#!/bin/env ruby
# -*- coding: utf-8 -*-

#
# Usage
#   $ ruby comp.rb TARGET_FOLDER
#

require 'rubygems'
require 'systemu'

# http://www.imagemagick.org/script/command-line-options.php#metric
COMPARE_METRIC = "PSNR"

REF_FOLDER = "RefFolder"
REF_FORMAT_EXT = ".bmp"
TARGET_FOLDER = ARGV[0]

Dir.glob(File.join(TARGET_FOLDER, "*")).sort.each do |target_file|
    ref_file = File.join(REF_FOLDER, File.basename(target_file, ".jpg")) + REF_FORMAT_EXT
    command = "compare -metric #{COMPARE_METRIC} #{ref_file} #{target_file} diff.png"

    # Metrics value is outputed to stderr somehow
    status, stdout, stderr = systemu command
    puts "#{target_file}\t#{stderr}"
end