Sustainability of Digital Formats: Planning for Library of Congress Collections

PiCture eXchange (PCX) files are raster images files created in the graphics editing software PC Paintbrush, developed by the software company ZSoft Corporation. Initially released in 1984- 1985 for MS-DOS operating systems in response to IBM’s PCPaint, both the company and the format today are considered “obsolete” or “defunct.”

According to Chocolate-Doom.org, last updated March 2022, “Doom originally supported saving images in the PCX format, however, this format is now considered to be obsolete as it only supports very basic run-length encoding, is not supported by the majority of modern software, and was last updated in 1991 - essentially making it a dead format.”

AccuSoft.com describes the format as “One of the oldest PC-based bitmap formats.”

PCX Image Format:

PCX Wikipedia, PCX is designed for IBM-compatible PCs and always uses little endian byte ordering. PCX files begin with a 128-byte header, followed by the image data (encoded graphic data) and (optionally) 256-color palette.

• A 128-byte header.
  • Per Lifewire.com, What Is a PCX File?, “The header of a PCX file holds information such as a version number, image dimensions, identifier byte, compression value (which is always set to 1), 16 palette colors, number color planes, and the bit depth of each plane.”
• Body/bitmap image data.
  • Per Lifewire.com, What Is a PCX File?, “The image data is stored in rows in a top-down fashion, with red data at the top and then the green and blue data. If there are multiple planes in the PCX file, the same format is used within each plane, with the first plane located at the top.”
• 256-color Palette (optional).
  • The color palette information varies depending on the version of the PCX file. According to PCX Wikipedia, “A PCX file has space in its header for a 16 color palette. When 256-color VGA hardware became available there was not enough space for the palette in a PCX file...The solution chosen was to put the palette at the end of the file, along with a marker byte to confirm its existence. If a PCX file has a 256-color palette, it is found 768 bytes from the end of the file.”

The ZSoft PCX File Format Technical Reference Manual (FileFormat.com) defines the fixed length PCX File Header Format, including byte, version number, image dimensions, 16 palette colors, number of color planes, plane bit depth, and compression method value.

PCX Version Information:

• 0 = Version 2.5 of PC Paintbrush
• 2 = Version 2.8 with palette information
• 3 = Version 2.8 without palette information
• 4 = PC Paintbrush for Windows
• 5 = Version 3.0, including 24-bit files

PCX image data is compressed using run-length encoding (RLE), a simple lossless compression algorithm. PaintShopPro.com’s page PCX File: The Ultimate Guide to PCX Files states, PCX uses RLE, “which allows it to maintain the quality and data documents, even when compressed.”

PCX Multi-Page Version:

According to AccuSoft.com’s DCX page, “This file format was designed to allow multiple PCX files to be stored in one file. This is especially desirable for multi-page faxes (for which the PCX format is often used). Up to 1024 PCX images can be stored in one DCX file. The DCX construct begins with a simple header, then the PCX files are simply stored end-to-end, complete with their individual headers and palettes. An array in the header called 'Pagetable' contains offsets to each PCX. The one piece of vital information not stored with the PCX files under a DCX is their original filenames.”

AccuSoft.com’s PCX page states, November 2021, PCX “is popular for fax documents because it allows them to be viewed within many popular paint and image display programs.”

Uses of PCX:

According to Adobe.com’s PCX file page, “Windows operating systems still support PCX files. They’re also compatible with Adobe Illustrator, Adobe Photoshop, and several other applications...Though they are no longer commonplace, PCX files have several uses.”

See Adoption for more information.

Developed by ZSoft Corporation. Partially documented.

According to DrDobbs.com’s PCX Graphics article, “The PCX format has never been completely documented. When ZSoft first created PC Paintbrush, the only video displays they had to contend with were two monochrome adapters (Hercules and Tecmar) and the IBM Color Graphics Adapter (CGA). They have since quietly modified and extended their format on several occasions to support EGA, VGA and SuperVGA displays. Documentation is scarce, incomplete and sometimes contradictory.”

Comments welcome.

No formal specification but informal information is available.

FileFormat.info - PCX File Format Summary:

• ZSoft Technical Reference Manual – describes the PCX format
  (https://www.fileformat.info/format/pcx/spec/a10e75307b3a4cc49c3bbe6db4c41fa2/view.htm)

Comments welcome.

According to Adobe.com’s PCX Files page, November 2021, “The PCX file became a widely accepted standard format for the MS-DOS operating system. It was also one of the first Bitmap image formats used on Windows, but fell out of favor after the release of more advanced image file formats like JPEG, GIF, and PNG...Some software programs still use PCX because the file type has faxing (DCX extension) and scanning applications.”

As stated on PCX Wikipedia, “PCX was designed during the early development of PC display hardware and most of the formats it supported are no longer used...Contemporary image editing programs may not read PCX file that match order software.”

Image processing software that supports PCX (per PCX Wikipedia):

• ACDSee
• FastStone
• GIMP
• ImageWorks
• InfanView
• LView
• Netpbm
• PaintShop Pro
• Photoshop
• Visio
• PMview
• XnViem
• GraphicConverter

Comments welcome.

Little to no information found regarding PCX image format licenses or patents.

Comments welcome.

Depends upon algorithms and tools to read; will require sophistication to build tools.

Comments welcome.

Some information found on PCX metadata inclusion.

According to VeryPDF.com’sHow to Convert RFT to PCX?, “PCX is a raster image format, the file header stores metadata about the image, separately form the actual image bitmap.”

Easy2Convert.com states, “The .PCX metadata stores color palettes, color planes and their bit depth, image dimensions, etc.”

Comments welcome.

As stated on FileFormat.info: PCX File Format Summary, “PCX is hardware-dependent in the sense that it was originally designed to accommodate a specific type of display hardware. Data may be stored either plane- or pixel-oriented.”

• Plane-oriented IBM EGA
• Pixel-oriented IBM VGA

Comments welcome.

None. PC Paintbrush Manual states the program and manual are proprietary, but no mention of proprietary file format PCX.

Comments welcome.

According to Adobe.com’s PCX Files page, November 2021, “PCX supports 24-bit color images, 8-bit grayscale, and 1-bit black and white, as well as indexed color images...Initially, PCXs only supported up to 256 palette-based colors. This was extended to incorporate 24-bit color, too. Currently, PCX can support up to 16 million colors.”

Color palette information varies depending on the version of the PCX file. The header of a PCX file contains information including the color palette up to 16 colors. An optional color palette can be added to the end of a PCX file if the image contains more than 16 colors. See Description above for more information.

ZSoft Corporation was formed in 1980 by Mark Zachmann, based out of Marietta, Georgia. It was a software company known for the PC Paintbrush software and the PCX file format. Over time, ZSoft has been sold and resold. According to Solvusoft.com, “The company merged with Mediagenic in 1989. Mediagenic later went bankrupt, but ZSoft regained its independence just in time. ZSoft was sold to Wordstar, which was later sold to SoftKey, which was later acquired by The Learning Company.”