gistfile1.rbx

####
# Get the dimensions for a Jpeg
#
class JpegFile
  def initialize file
    @io = File.open(file, 'rb')
  end

  def dimensions
    raise 'Not a jpeg' unless @io.read(2).unpack('CC') == [0xFF, 0xD8]
    loop do
      bytes = @io.read(2).unpack('CC')

      raise unless bytes[0] == 0xFF
      return @io.read(7)[3..-1].unpack('nn') if bytes[1] === 0xC0..0xC3

      @io.read(@io.read(2).unpack('n').first - 2)
    end
  end
end

p JpegFile.new(ARGV[0]).dimensions