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Group 3 FAX
The raw compressed data stream used by classic fax machines.
When we talk about the 'FAX' file format, we typically refer to a raw data stream compressed using the CCITT Group 3 (G3) or Group 4 (G4) algorithms. These are specialized compression methods designed for bi-level (black and white) images of text documents. In most modern contexts, you won't see a standalone `.fax` or `.g3` file. Instead, this compression data is wrapped inside a container like TIFF or PDF. A `.fax` file is essentially a headerless chunk of this compressed data, which makes it difficult for modern software to open without knowing the specific dimensions (width/height) beforehand.
Group 3 compression is based on Huffman coding and Run-Length Encoding (RLE). It scans a line of pixels and records the lengths of alternating runs of black and white pixels. Because most documents are largely white space, this results in significant compression. Group 4 is an improvement that uses 2D compression (referencing the previous line to predict the next), offering better ratios but requiring an error-free transmission channel (like a digital network), unlike G3 which had to survive noisy phone lines.
Standardized by the CCITT (Consultative Committee for International Telephony and Telegraphy), now ITU-T, in 1980 (Group 3) and 1984 (Group 4). These standards enabled the global fax boom of the 80s and 90s.
GIF 87a
The original 1987 specification of the Graphics Interchange Format.
GIF 87a is the original version of the GIF format, published by CompuServe in 1987. It introduced the core features that made GIF famous: LZW compression for small file sizes and support for 256 indexed colors. However, unlike the ubiquitous GIF 89a, this original version lacks support for transparency, animation delays, and metadata comments.
A GIF 87a file starts with the header signature `GIF87a`. It defines the Logical Screen Descriptor and the Global Color Table. While it supports multiple image blocks within a single file (which could conceptually form an animation), it lacks the 'Graphic Control Extension' block introduced in 89a. This means there is no standard way to define frame delays, disposal methods, or transparent indices.
Developed by CompuServe in 1987 to provide a hardware-independent, compressed color image format for their online service. It replaced the earlier RLE-based formats and was designed to be efficient over slow modems.
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