See http://randomperspective.com/comic/93/

rpdecode.rb

#!/usr/bin/env ruby

# Snippet to decode images hidden using method described
# in http://randomperspective.com/comic/93/

require 'rubygems'
require 'oily_png'

input_name = ARGV.first

# masks
LOW_RGB_BITS = 0b00000001_00000001_00000001_00000000
LOW_R_BIT    = 0b00000001_00000000_00000000_00000000
ALL_R_BITS   = 0b11111111_00000000_00000000_00000000
LOW_G_BIT    = 0b00000000_00000001_00000000_00000000
ALL_G_BITS   = 0b00000000_11111111_00000000_00000000
LOW_B_BIT    = 0b00000000_00000000_00000001_00000000
ALL_B_BITS   = 0b00000000_00000000_11111111_00000000
ALL_A_BITS   = 0b00000000_00000000_00000000_11111111

ChunkyPNG::Image.from_file(input_name).tap do |input|
  w, h = input.width / 3, input.height / 3

  ChunkyPNG::Image.new(w, h, ChunkyPNG::Color::TRANSPARENT).tap do |output|
    0.upto(w-1) do |x|
      0.upto(h-1) do |y|
        ix, iy = 3*x, 3*y

        new_color = ALL_A_BITS # to create an opaque image

        new_color |= ((input.get_pixel(ix+0, iy+0) & LOW_RGB_BITS) << 7)
        new_color |= ((input.get_pixel(ix+1, iy+0) & LOW_RGB_BITS) << 6)
        new_color |= ((input.get_pixel(ix+2, iy+0) & LOW_RGB_BITS) << 5)
        new_color |= ((input.get_pixel(ix+0, iy+1) & LOW_RGB_BITS) << 4)
        new_color |= ((input.get_pixel(ix+1, iy+1) & LOW_RGB_BITS) << 3)
        new_color |= ((input.get_pixel(ix+2, iy+1) & LOW_RGB_BITS) << 2)
        new_color |= ((input.get_pixel(ix+0, iy+2) & LOW_RGB_BITS) << 1)
        new_color |=  (input.get_pixel(ix+1, iy+2) & LOW_RGB_BITS)

        # flip bits in color if "flip bit" is on
        flip_color = input.get_pixel(ix+2, iy+2)
        new_color ^= ALL_R_BITS if (flip_color & LOW_R_BIT) == LOW_R_BIT
        new_color ^= ALL_G_BITS if (flip_color & LOW_G_BIT) == LOW_G_BIT
        new_color ^= ALL_B_BITS if (flip_color & LOW_B_BIT) == LOW_B_BIT

        output.set_pixel(x, y, new_color)
      end
    end
    output.save("#{input_name}.extract.png", :fast_rgb)
  end
end