guard with guard-shell, down size retina image, hd to sd with imagemagick

Guardfile

guard :shell do
  watch(%r{-hd\.png$}) do |m|
    input = m[0]
    output = input.gsub(/-hd.png$/,'.png')
    puts "Downsizing #{input} to #{output}"
    `convert #{input} -resize 50% #{output}`
  end
end