compose_ring.rb

#!/usr/bin/env ruby

require 'pry'
require 'pathname'

if ARGV.count != 3
  puts "usage: #{__FILE__} <TEMPLATE_IMAGE> <SOURCE_DIR> <DEST_DIR>"
  exit(1)
end

template_image = Pathname.new(File.expand_path(ARGV[0]))
source = Pathname.new(File.expand_path(ARGV[1]))
dest = Pathname.new(File.expand_path(ARGV[2]))

source.each_child do |source_image|
  dest_image = dest.join(source_image.basename)
  `composite -compose Dst_Out -gravity center  #{source_image.to_s} #{template_image.to_s} -alpha Set #{dest_image.to_s}`
end