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Portable Pixel Map
The standard uncompressed color format of the Netpbm suite.
PPM (Portable Pixel Map) is the color counterpart to PGM and PBM. It is the most widely used format in the Netpbm library for storing full-color RGB images. Like its siblings, it is designed for extreme simplicity and ease of interchange between Unix tools.
PPM files come in two flavors: P3 (ASCII) and P6 (Binary). - **ASCII (P3)**: Pixels are written as readable RGB triplets (e.g., `255 0 0` for red). - **Binary (P6)**: Pixels are raw byte triplets. The header is minimal: Magic Number, Width, Height, Maxval. It does not support alpha channels (use PAM for that) or compression.
Developed by Jef Poskanzer in 1991 to bring color support to the pbmplus (Netpbm) toolkit. It became the lingua franca for Unix graphics utilities.
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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