||This article possibly contains original research. (November 2009)|
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.
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.
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.
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.
- Version 1 was used before World of Warcraft.
- Version 2 added an extended header to the format which contained data for an extended block table to allow for larger archive sizes.
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.
- (listfile): Contains a list of the archive's files, one filename per line. May or may not be exhaustive.
- (signature): Contains the weak cryptographic signature of the archive. This type of signature is deprecated.
- (attributes): Contains extended file metadata. Currently known attributes are file creation date, CRC32 checksum and MD5 checksum.
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:
- PKZIP (licensed from PKWARE). The first compression algorithm available.
- Huffman tree compression combined with ADPCM 4:1 compression (both introduced in StarCraft). Latter algorithm is lossy and only suitable for raw PCM input data.
- zlib (introduced in Warcraft III).
- bzip2 (introduced in World of Warcraft).
- LZMA (introduced in StarCraft II).
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:
- Diablo: Hellfire developed by Synergistic Software
- Lords of Magic developed by Sierra Entertainment
- Diablo II
- Diablo II: Lord of Destruction
- StarCraft: Brood War
- WarCraft II: Battle.net Edition
- Warcraft III: Reign of Chaos
- Warcraft III: The Frozen Throne
- World of Warcraft and its expansions up to Mists of Pandaria
- StarCraft II: Wings of Liberty
- StarCraft II: Heart of the Swarm
- Diablo III
- Hearthstone: Heroes of Warcraft
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. 
- "Phillip Katz, Computer Software Pioneer, 37". The New York Times. May 1, 2000. Retrieved 2011-07-14.
- Warlords of Draenor Alpha Testing Begins - World of Warcraft