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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.
JPEG 2000 Code Stream
Another extension for the raw JPEG 2000 code stream.
J2K is functionality identical to J2C. It represents a raw JPEG 2000 codestream without the JP2 file format wrapper. It is often used interchangeably with J2C and JPC in technical contexts.
Like J2C, a J2K file contains the wavelet-compressed image data (SOP/EPH markers). It relies on the decoder to interpret the stream markers correctly without the 'boxes' found in the JP2 container format.
Part of the ISO/IEC 15444-1 standard. The multiple extensions (.j2k, .j2c, .jpc) arose from different vendor implementations early in the standard's life.
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