Dynapik offers a free online tool to change image types - no need to download anything. It's quick and easy to use. You can change your FAX images to PNM format. This tool works for both professionals and casual users. Convert your images to PNM in seconds.
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.
Portable Any Map
A super-class extension for the PBM, PGM, and PPM formats.
PNM (Portable Any Map) is not a distinct file format itself but rather a collective name (and file extension) used to handle files that could be any of the specialized Netpbm formats: PBM (Bitmap), PGM (Graymap), or PPM (Pixmap). A file ending in `.pnm` allows a program to implement a single reader that automatically detects the internal format ('P1' through 'P6') and handles it accordingly.
A `.pnm` file is simply a PBM, PGM, or PPM file renamed. The header magic number (e.g., `P6`) determines the actual content. This abstraction simplifies command-line pipelines, allowing users to pipe `image.pnm` without worrying if the image is black-and-white or full color.
Part of the Netpbm project philosophy to create a unified library for handling raster graphics. It simplifies naming conventions for software that can output any of the subtypes.
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