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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.
Microsoft Icon
The standard format for icons on Microsoft Windows and the web (favicons).
The ICO format is a container for one or more images at multiple sizes and color depths. It allows a single file to store appropriate icons for different display contexts (e.g., 16x16 for menus, 256x256 for desktop). It supports transparency and, in modern versions, PNG compression.
ICO files contain a header followed by an icon directory that lists each image entry. Each entry can use either BMP (uncompressed) or PNG compression. The format supports 1, 4, 8, 16, 24, and 32-bit color depths with optional alpha channels.
Introduced with Microsoft Windows 1.0. It became the standard for website favicons with the release of Internet Explorer 5.
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