Down scale png image from @3x to @2x, @1x

resize-png.rb

#! /usr/bin/env ruby

require 'mini_magick'

# gem install mini_magick

def resize_image(image_name, output_name)
  current_scale = 3
  if image_name.include?("@3x")
    current_scale = 3
  elsif image_name.include?("@2x")
    current_scale = 2
  end

  image = MiniMagick::Image.open(image_name)

  target_scale = current_scale - 1

  while target_scale > 0 do
    target_width = image.width * target_scale / current_scale
    target_height = image.height * target_scale / current_scale

    # puts "width: #{image.width}, height: #{image.height}"
    # puts "target width: #{target_width}, target height: #{target_height}"

    image.resize "#{target_width}x#{target_height}"
    image.format "png"

    if !Dir.exist?('output')
      Dir.mkdir('output')
    end

    name = File.basename(image_name, ".*")
    name = name.split("@").first
    scale_name = "@#{target_scale.to_i}x"
    image.write "output/#{output_name || name}#{scale_name}.png"

    current_scale_name = "@#{current_scale.to_i}x"
    unless image_name.include?(current_scale_name)
      system "cp #{image_name} output/#{output_name || name}#{current_scale_name}.png"
    else
      system "cp #{image_name} output/#{image_name}"
    end

    target_scale -= 1
  end
end

def usage
  puts "Usage: change-size <image_name> [output_name]"
  puts "Usage: change-size <folder_name>"
  exit 1
end


image_or_folder_name = ARGV[0]
output_name = ARGV[1]  # optional

if image_or_folder_name.nil? || (File.directory?(image_or_folder_name) && !Dir.exist?(image_or_folder_name)) || !File.exist?(image_or_folder_name)
  usage
end

if !File.directory?(image_or_folder_name) && output_name && output_name.end_with?(".png")
  output_name = output_name.gsub(/\.png$/, "")
end

if File.directory?(image_or_folder_name)
  Dir.glob(File.join(image_or_folder_name, "**", "*.png")).each do |file|
    resize_image(file, output_name)
  end
else
  resize_image(image_or_folder_name, output_name)
end