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Palm Database
The generic database container for Palm OS.
PDB (Palm Database) is the standard file format for all data on Palm OS devices, from address books to e-books. In the context of images, a PDB file acts as a container for 'Palm Pixmap' images, typically storing them as a series of records. It was the only way to get data onto a Palm pilot.
A PDB file consists of a header (name, attributes, creation time) and a list of records. For images, these records contain the bitmap data (often compressed with RLE/PackBits). ImageMagick treats PDB files as a multi-image sequence if multiple bitmaps are stored in the records. The format is strictly structured to map directly to the device's RAM storage model.
Created by Palm Computing in 1996. It was designed to facilitate 'HotSync' between the handheld's RAM and the desktop PC. It became ubiquitous in the early 2000s PDA era.
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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