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DPX (Digital Picture Exchange)
The industry standard for digital intermediate and visual effects work in film.
Digital Picture Exchange (DPX) is a raster image format used primarily in the motion picture industry for visual effects (VFX) and Digital Intermediate (DI) work. It is an ANSI/SMPTE standard (SMPTE 268M-2003) designed to represent the density of film scans without loss of quality. Unlike consumer formats, DPX is usually uncompressed and stores color information in a 'logarithmic' (Log) format to preserve the full dynamic range of motion picture film. A single second of 4K movie footage in DPX format can consume enormous amounts of storage (hundreds of megabytes), making it strictly a production format, not one for distribution.
A DPX file starts with a 'Generic Image Header' (magic number: SDPX) containing core details like file size and image orientation. This is followed by 'Industry Specific Headers' for Motion Picture or Television data (timecodes, frame rates). Finally, the 'Image Data' block contains the raw pixel values. DPX supports a wide variety of bit depths, but 10-bit Log RGB is the most common industry standard. It packs these 10-bit values tightly into 32-bit words (10+10+10+2 padding) for efficient processing. The logarithmic encoding mimics the human eye's response to light and the physical characteristics of film stock.
DPX is the direct successor to Kodak's Cineon (.cin) format, which was developed in the early 1990s for the first digital film scanners. As the industry moved towards digital workflows, SMPTE (Society of Motion Picture and Television Engineers) standardized Cineon into DPX to ensure compatibility between different vendors' scanners, printers, and software.
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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