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Portable Gray Map
The standard uncompressed grayscale format.
PGM (Portable Gray Map) is the grayscale member of the Netpbm format family. It sits between PBM (1-bit black and white) and PPM (full color). It is designed to store grayscale images where each pixel has a value ranging from black to white, with typical depths of 8-bit (0-255) or 16-bit (0-65535).
Like other Netpbm formats, PGM has two sub-formats: P2 (ASCII) and P5 (Binary). The header defines the dimensions and the maximum gray value (Maxval). In P2, pixel values are written as plain text numbers separated by spaces. In P5, they are raw bytes. It is strictly a single-channel format.
Created by Jef Poskanzer in the late 80s to handle scanned photos that were better than 1-bit bitmaps but didn't need full color. It became a staple in computer vision research and Unix tools.
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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