MPQ

MPQ
Filename extension .mpq
Developed by Mike O'Brien

MPQ (Mo'PaQ, short for Mike O'Brien Pack, named after its creator), is an archiving file format used in several of Blizzard Entertainment's games.

MPQs used in Blizzard's games generally contain a game's data files, including graphics, sounds, and level data. The format's capabilities include compression, encryption, file segmentation, extensible file metadata, cryptographic signature and the ability to store multiple versions of the same file for internationalization and platform-specific differences. MPQ archives can use a variety of compression algorithms which may also be combined.

File indexing

In order to meet the requirements of speed generally demanded by a computer game, files are indexed in a hash table using a quick, low-collision hashing algorithm. The index of a specific file within the hash table is the hash of the lowercased filename modulo the size of the hash table, allowing for quick verification of a file's existence within the archive. If multiple files within the archive have the same hash, colliding entries will follow each other in increasing index order (forming a colliding hash cluster). In order to identify the exact entry for the requested file within a colliding hash cluster, each hash table entry stores 2 additional hashes of the lowercased filename, each using the same hashing algorithm but with a different seed value, as well as a locale code and platform code. The end of a colliding hash cluster is detected either by encountering an empty hash table entry or by traversing the entire hash table (including the modulo loopback) back to the initial hash table index.

Encryption

Both the block table (which contains information on where the file data is located in the archive) and the hash table used for file indexing are encrypted when stored. The encryption process which is used by default uses a known algorithm.

Revisions

The file header reserves space to contain format version data. Warcraft III ignores format version data of .mpq compliant files it loads and assumes all are version 1.

Archive metadata

MPQ archives do not have specific structures to store metadata beyond what is absolutely necessary to access archived files. Instead, the convention is to use regular files whose filename is enclosed by parentheses.

Below are known metadata files.

Compression

In modern MPQ archives, each segment (or sector) of a file can be compressed using a combination of compression algorithms. A header byte is prepended to every compressed sector to indicate which compressions were used. The order in which those compressors are applied is hardcoded.

The following algorithms are currently in use by Blizzard games:

Since there was only one compression algorithm available when MPQs were first deployed in Diablo, those archives used a different archive file metadata flag to indicate compression and did not use a compression header byte.

Warcraft III cinematics

Cutscene cinematics with the MPQ extension are included with Warcraft III and, despite the file extension, are not actual MPQ files. Rather they are AVI files compressed with Blizzard's renamed MPEG-4 codec, BLZ0 (which actually is DivX). These files are playable in ordinary media players, provided the proper codecs are installed. Blizzard probably used the MPQ extension on those files to hide them from players, who could have spoiled the game by seeing them out of the gameplay. Nevertheless, World of Warcraft cinematics use the AVI extension.

Usage in gaming

Blizzard has utilized the MPQ file format for archiving game files in a number of their games, including:

Replacement: CASC

On April 3, 2014, with the beginning of alpha testing for World of Warcraft: Warlords of Draenor, Blizzard announced that they were testing a new proprietary file format dubbed CASC (Content Addressable Storage Container) to replace MPQ in World of Warcraft. Among the improvements touted for it include a reduction in file corruption by creating a self-maintaining system, improved in-game performance and faster patching. The CASC format was initially tested in the internal alpha for Heroes of the Storm, and will be tested in the alpha and beta tests for Warlords of Draenor before it is implemented within the main game itself prior to the expansion's release. [2]

References

  1. ↑ "Phillip Katz, Computer Software Pioneer, 37". The New York Times. May 1, 2000. Retrieved 2011-07-14.
  2. ↑ Warlords of Draenor Alpha Testing Begins - World of Warcraft
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