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DCX (Multi-page PCX)
A legacy multi-page image format created for PC-based fax software.
DCX is a multi-page bitmap image format that essentially acts as a container for multiple PCX files. It was developed by ZSoft Corporation, the same company that created PC Paintrush and the PCX format. The primary purpose of DCX was to serve as the file format for early digital fax software, allowing a multi-page document to be stored in a single computer file. Technically, a DCX file begins with a header containing a list of offsets (pointers) to the individual PCX images stored within the file. Each 'page' is a fully valid PCX image with its own header and palette. The format relies on the simple RLE (Run-Length Encoding) compression inherited from PCX, which is efficient for simple black-and-white fax documents but poor for complex photographs.
A DCX file consists of a 4-byte signature (987654320) followed by an array of up to 1024 32-bit integer offsets. Each offset points to the start of a PCX image structure within the file. The list ends with a zero (null) terminator. Because it is wrappers around PCX, it shares all the characteristics of that format: support from 1-bit monochrome up to 24-bit RGB color. However, since it was primarily used for faxing, the vast majority of DCX files encountered today are 1-bit black and white.
DCX became popular in the early 1990s alongside the rise of fax modems and software like WinFax. It allowed users to scan or 'print' a document to a fax driver, which would save the pages as a linear .dcx file before transmission. As PDF became the dominant document format and email replaced faxing, DCX faded into obsolescence.
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.
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