rot-gf2.rb

#!/usr/bin/env ruby

$data = IO.read(ARGV[0], 0x2400)

def check_contents_at(offset, contents, description)
  size = contents.bytesize
  raise "#{description} not found at offset #{offset}" unless $data[offset, size] == contents
end

EXIF_ORIENTATION = "\x12\x01\x03\x00\x01\x00\x00\x00"
check_contents_at 0x00c0, "DMC-GF2", "camera model"
check_contents_at 0x002e, EXIF_ORIENTATION, "image orientation" # 0x36
check_contents_at 0x2380, EXIF_ORIENTATION, "thumbnail orientation" # 0x2388

ROTATION = {
  "n" => "\x00",
  "l" => "\x08",
  "r" => "\x06",
}

rotation = ROTATION[ARGV[1]]

File.open(ARGV[0], "r+") do |f|
  f.seek(0x36, IO::SEEK_SET)
  f.write(rotation)

  f.seek(0x2388, IO::SEEK_SET)
  f.write(rotation)
end