The EGG archive format, short for Evil Genius Game archive, is a proprietary compressed file format developed by Rebellion Developments for packaging game assets. It is primarily used in the Evil Genius video game series. The EGG format allows game developers to efficiently store and access various types of game data, including textures, models, audio files, and configuration files.
EGG archives follow a specific structure to organize the contained files and metadata. The archive begins with a header that contains information about the archive version, the number of files within the archive, and the total size of the archive. Following the header, there is a file allocation table (FAT) that provides an index of all the files stored in the archive, along with their respective offsets and sizes.
One of the key features of the EGG format is its compression mechanism. The game assets within an EGG archive are typically compressed using a custom compression algorithm developed by Rebellion Developments. This compression helps reduce the overall size of the archive, making it more efficient to distribute and load game assets during runtime. The specific details of the compression algorithm are not publicly disclosed, as they are considered proprietary information.
To access the files within an EGG archive, a game or tool must first parse the archive header to obtain the necessary metadata. This includes reading the archive version to ensure compatibility and determining the number of files present in the archive. The file allocation table is then processed to retrieve the offsets and sizes of each file.
Once the file metadata is obtained, the game or tool can seek to the appropriate offset within the archive and read the compressed data for a specific file. The compressed data is then decompressed using the corresponding decompression algorithm, which reverses the compression applied during the archive creation process. After decompression, the game or tool can utilize the extracted file data as needed, such as loading textures, models, or audio files.
The EGG format also supports optional file encryption to provide an additional layer of security for game assets. When encryption is used, the file data within the archive is encrypted using a symmetric encryption algorithm. The encryption key is typically derived from a combination of factors, such as the archive password and other game-specific parameters. Decryption of the file data occurs after decompression, using the appropriate encryption key.
Rebellion Developments provides a software development kit (SDK) for game developers to work with EGG archives. The SDK includes libraries and tools that facilitate the creation, manipulation, and extraction of EGG archives. These tools handle the low-level details of the format, such as compression, decompression, and encryption, allowing developers to focus on integrating the assets into their games.
One advantage of using the EGG format is its ability to efficiently load game assets during runtime. By packaging related assets together in a single archive, the game can minimize disk I/O operations and improve loading times. The EGG format's compression also reduces the memory footprint of the loaded assets, enabling more efficient memory usage.
However, the proprietary nature of the EGG format can pose challenges for modding communities and third-party tools. Without official documentation or reverse-engineering efforts, it can be difficult to create tools that can extract or modify the contents of EGG archives. This limitation can hinder the development of mods, custom content, or asset extraction utilities for games using the EGG format.
Despite its proprietary nature, the EGG archive format has proven to be an effective solution for Rebellion Developments in managing and distributing game assets. Its compression capabilities, file organization, and optional encryption features make it well-suited for the needs of the Evil Genius game series. As the format continues to evolve with new versions and updates, it remains an integral part of Rebellion Developments' game development pipeline.
File compression reduces redundancy so the same information takes fewer bits. The upper bound on how far you can go is governed by information theory: for lossless compression, the limit is the entropy of the source (see Shannon’s source coding theorem and his original 1948 paper “A Mathematical Theory of Communication”). For lossy compression, the trade-off between rate and quality is captured by rate–distortion theory.
Most compressors have two stages. First, a model predicts or exposes structure in the data. Second, a coder turns those predictions into near-optimal bit patterns. A classic modeling family is Lempel–Ziv: LZ77 (1977) and LZ78 (1978) detect repeated substrings and emit references instead of raw bytes. On the coding side, Huffman coding (see the original paper 1952) assigns shorter codes to more likely symbols. Arithmetic coding and range coding are finer-grained alternatives that squeeze closer to the entropy limit, while modern Asymmetric Numeral Systems (ANS) achieves similar compression with fast table-driven implementations.
DEFLATE (used by gzip, zlib, and ZIP) combines LZ77 with Huffman coding. Its specs are public: DEFLATE RFC 1951, zlib wrapper RFC 1950, and gzip file format RFC 1952. Gzip is framed for streaming and explicitly does not attempt to provide random access. PNG images standardize DEFLATE as their only compression method (with a max 32 KiB window), per the PNG spec “Compression method 0… deflate/inflate… at most 32768 bytes” and W3C/ISO PNG 2nd Edition.
Zstandard (zstd): a newer general-purpose compressor designed for high ratios with very fast decompression. The format is documented in RFC 8878 (also HTML mirror) and the reference spec on GitHub. Like gzip, the basic frame doesn’t aim for random access. One of zstd’s superpowers is dictionaries: small samples from your corpus that dramatically improve compression on many tiny or similar files (see python-zstandard dictionary docs and Nigel Tao’s worked example). Implementations accept both “unstructured” and “structured” dictionaries (discussion).
Brotli: optimized for web content (e.g., WOFF2 fonts, HTTP). It mixes a static dictionary with a DEFLATE-like LZ+entropy core. The spec is RFC 7932, which also notes a sliding window of 2WBITS−16 with WBITS in [10, 24] (1 KiB−16 B up to 16 MiB−16 B) and that it does not attempt random access. Brotli often beats gzip on web text while decoding quickly.
ZIP container: ZIP is a file archive that can store entries with various compression methods (deflate, store, zstd, etc.). The de facto standard is PKWARE’s APPNOTE (see APPNOTE portal, a hosted copy, and LC overviews ZIP File Format (PKWARE) / ZIP 6.3.3).
LZ4 targets raw speed with modest ratios. See its project page (“extremely fast compression”) and frame format. It’s ideal for in-memory caches, telemetry, or hot paths where decompression must be near RAM speed.
XZ / LZMA push for density (great ratios) with relatively slow compression. XZ is a container; the heavy lifting is typically LZMA/LZMA2 (LZ77-like modeling + range coding). See .xz file format, the LZMA spec (Pavlov), and Linux kernel notes on XZ Embedded. XZ usually out-compresses gzip and often competes with high-ratio modern codecs, but with slower encode times.
bzip2 applies the Burrows–Wheeler Transform (BWT), move-to-front, RLE, and Huffman coding. It’s typically smaller than gzip but slower; see the official manual and man pages (Linux).
“Window size” matters. DEFLATE references can only look back 32 KiB (RFC 1951 and PNG’s 32 KiB cap noted here). Brotli’s window ranges from about 1 KiB to 16 MiB (RFC 7932). Zstd tunes window and search depth by level (RFC 8878). Basic gzip/zstd/brotli streams are designed for sequential decoding; the base formats don’t promise random access, though containers (e.g., tar indexes, chunked framing, or format-specific indexes) can layer it on.
The formats above are lossless: you can reconstruct exact bytes. Media codecs are often lossy: they discard imperceptible detail to hit lower bitrates. In images, classic JPEG (DCT, quantization, entropy coding) is standardized in ITU-T T.81 / ISO/IEC 10918-1. In audio, MP3 (MPEG-1 Layer III) and AAC (MPEG-2/4) rely on perceptual models and MDCT transforms (see ISO/IEC 11172-3, ISO/IEC 13818-7, and an MDCT overview here). Lossy and lossless can coexist (e.g., PNG for UI assets; Web codecs for images/video/audio).
Theory: Shannon 1948 · Rate–distortion · Coding: Huffman 1952 · Arithmetic coding · Range coding · ANS. Formats: DEFLATE · zlib · gzip · Zstandard · Brotli · LZ4 frame · XZ format. BWT stack: Burrows–Wheeler (1994) · bzip2 manual. Media: JPEG T.81 · MP3 ISO/IEC 11172-3 · AAC ISO/IEC 13818-7 · MDCT.
Bottom line: choose a compressor that matches your data and constraints, measure on real inputs, and don’t forget the gains from dictionaries and smart framing. With the right pairing, you can get smaller files, faster transfers, and snappier apps — without sacrificing correctness or portability.
File compression is a process that reduces the size of a file or files, typically to save storage space or speed up transmission over a network.
File compression works by identifying and removing redundancy in the data. It uses algorithms to encode the original data in a smaller space.
The two primary types of file compression are lossless and lossy compression. Lossless compression allows the original file to be perfectly restored, while lossy compression enables more significant size reduction at the cost of some loss in data quality.
A popular example of a file compression tool is WinZip, which supports multiple compression formats including ZIP and RAR.
With lossless compression, the quality remains unchanged. However, with lossy compression, there can be a noticeable decrease in quality since it eliminates less-important data to reduce file size more significantly.
Yes, file compression is safe in terms of data integrity, especially with lossless compression. However, like any files, compressed files can be targeted by malware or viruses, so it's always important to have reputable security software in place.
Almost all types of files can be compressed, including text files, images, audio, video, and software files. However, the level of compression achievable can significantly vary between file types.
A ZIP file is a type of file format that uses lossless compression to reduce the size of one or more files. Multiple files in a ZIP file are effectively bundled together into a single file, which also makes sharing easier.
Technically, yes, although the additional size reduction might be minimal or even counterproductive. Compressing an already compressed file might sometimes increase its size due to metadata added by the compression algorithm.
To decompress a file, you typically need a decompression or unzipping tool, like WinZip or 7-Zip. These tools can extract the original files from the compressed format.