resize.rb

require 'graphicsmagick'

size_strings = %w(
120x240
120x60
120x600
120x90
125x125
160x600
180x150
234x60
240x400
250x250
300x100
300x250
300x600
336x280
468x60
720x300
728x90
88x15
88x31
)

sizes = Hash[size_strings.map do |str| 
  width, height = str.split('x').map{ |number_string| number_string.to_i }
  [width.to_f / height, [width, height]]
end]

files = ARGV
puts files.inspect

files.each_with_index do |img_path, index|
  img = GraphicsMagick::Image.new img_path

  img_ratio = img.width.to_f / img.height
  ratios = sizes.keys.sort
  crop_size = sizes[ratios.select{ |r| r < img_ratio }.sort.last]

  `gm convert -scale #{crop_size.join('x')} #{img_path} #{index+1}artbanner#{crop_size.join('x')}.jpg`
end