#!/usr/bin/env ruby
require 'rmagick'

TWO_PI = 2 * Math::PI

image_path = ARGV[0]  # User input
extname = File.extname image_path
planet_path = image_path + '.planet' + extname

original = Magick::Image.read(image_path).first
width = original.columns
height = original.rows

target_size = height * 2
planet = Magick::Image.new(target_size, target_size)

target_size.times do |x|
 target_size.times do |y|
    r, θ = Complex(height - y, height - x).polar  # Cheat using complex plane
    next if r > height
    x_original = width / 2 - θ * width / TWO_PI
    color = original.pixel_color(x_original, height - r)
    planet.pixel_color(x, y, color)
  end
end

planet.write(planet_path)