Ruby script to resize images using ImageMagick's convert program

resize_image

#!/usr/bin/ruby

require 'optparse'
require 'shellwords'

def die(msg)
  $stderr.puts msg
  exit 1
end

scale = nil
new_height = nil
new_width = nil
quality = nil

o = OptionParser.new
o.banner = "Usage: resize_image [options] OLD_FILE NEW_FILE"
o.on('-h', '--height=HEIGHT', 'Resize height in pixels') {|v| new_height = v.to_i }
o.on('-s', '--scale=SCALE', 'Resize scale as (0.1, 1.0]') {|v| scale = v.to_f }
o.on('-w', '--width=WIDTH', 'Resize width in pixels') {|v| new_width = v.to_i }
o.on('-q', '--quality=QUALITY', 'JPEG quality as [0, 100]') {|v| quality = v.to_i }
args = o.parse(*ARGV)

args.size == 2 || die(o.help)

actions = (scale ? 1 : 0) + (new_width ? 1 : 0) + (new_height ? 1 : 0)
if actions == 0
  scale = 1
elsif actions > 1
  die("Set either --scale, --width, or --height")
end

old_file, new_file = args
die("#{old_file} doesn't exist") if !File.exists?(old_file)

out = `identify  -format "%w %h\n" -ping "#{old_file}"`
parts = out.split(' ')
die("Unable to get dimensions") if parts.size != 2
old_width, old_height = parts.map {|p| p.to_i }

if scale
  new_width = (old_width * scale).round.to_i
  new_height = (old_height * scale).round.to_i
elsif new_width
  scale = new_width.to_f / old_width
  new_height = (old_height * scale).round.to_i
else  # new_height
  scale = new_height.to_f / old_height
  new_width = (old_width * scale).round.to_i
end

puts "#{old_file} (#{old_width}x#{old_height}) -> #{new_file} (#{new_width}x#{new_height})"
cmd = "convert #{Shellwords.escape(old_file)} " +
  "-colorspace RGB " +
  "-resize #{new_width}x#{new_height} " +
  (quality ? "-quality #{quality} " : "") +
  Shellwords.escape(new_file)
system(cmd) || die("Failed to run #{cmd}")
puts "#{File.size(old_file)} -> #{File.size(new_file)} bytes"