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DCX (Multi-page PCX)
A legacy multi-page image format created for PC-based fax software.
DCX is a multi-page bitmap image format that essentially acts as a container for multiple PCX files. It was developed by ZSoft Corporation, the same company that created PC Paintrush and the PCX format. The primary purpose of DCX was to serve as the file format for early digital fax software, allowing a multi-page document to be stored in a single computer file. Technically, a DCX file begins with a header containing a list of offsets (pointers) to the individual PCX images stored within the file. Each 'page' is a fully valid PCX image with its own header and palette. The format relies on the simple RLE (Run-Length Encoding) compression inherited from PCX, which is efficient for simple black-and-white fax documents but poor for complex photographs.
A DCX file consists of a 4-byte signature (987654320) followed by an array of up to 1024 32-bit integer offsets. Each offset points to the start of a PCX image structure within the file. The list ends with a zero (null) terminator. Because it is wrappers around PCX, it shares all the characteristics of that format: support from 1-bit monochrome up to 24-bit RGB color. However, since it was primarily used for faxing, the vast majority of DCX files encountered today are 1-bit black and white.
DCX became popular in the early 1990s alongside the rise of fax modems and software like WinFax. It allowed users to scan or 'print' a document to a fax driver, which would save the pages as a linear .dcx file before transmission. As PDF became the dominant document format and email replaced faxing, DCX faded into obsolescence.
JPEG XL
The true next-generation successor designed to replace JPEG, PNG, and GIF entirely.
JPEG XL (JXL) is the most advanced image format currently in existence. Unlike WebP and AVIF, which are derived from video codecs, JPEG XL was designed from the ground up specifically for still images. This gives it unique superpowers, such as the ability to losslessly re-compress existing JPEG files to be 20% smaller without losing a single bit of data. It offers best-in-class lossless compression (beating PNG), best-in-class lossy compression (beating JPEG), and supports features needed for professional workflows like CMYK, layers, and ultra-high resolutions. However, its adoption has been slowed by political battles, most notably Google Chrome's decision to remove support in 2022, though Apple has since added full support in Safari 17.
JPEG XL uses a combination of two coding modes: VarDCT (Variable Block-size DCT) for lossy images and Modular Mode for lossless/artistic images. Its 'killer feature' is legacy JPEG transcoding: it can take the DCT coefficients from an old JPEG file and repackage them into the more efficient JXL container. This process is reversible, meaning you can convert your entire JPEG library to JXL to save 20% space, and convert them back to the exact original byte-for-byte JPEGs later if needed. It also supports 'progressive decoding' far better than any other format, allowing a high-quality preview to appear almost instantly.
The JPEG committee began the call for proposals for a next-gen standard in 2017. The final standard (ISO/IEC 18181) was published in 2021/2022, combining the best parts of Google's PIK and Cloudinary's FUIF proposals. Despite being technically superior, its rollout hit a wall when the Chrome team removed the experimental flag in late 2022, citing lack of ecosystem interest. However, Apple's adoption in 2023 (iOS 17/macOS 14) has breathed new life into the format.
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