What is PCX? PiCture eXchange Format Explained

How PCX Works

PCX stores a raster image using a fixed size header followed by compressed pixel data. The header records essential details such as a version indicator, the image dimensions, the number of bits per pixel, and the number of color planes. Because the header layout is fixed, a reader can parse these fields at known offsets before touching the image body.

For compression, PCX uses a simple form of run length encoding. Instead of storing every pixel individually, it collapses consecutive runs of the same value into a short code that says, in effect, repeat this value this many times. This works well on images with large flat areas of a single color, which were common in the icons, screenshots, and drawn graphics of the DOS era.

The pixel data is organized as scanlines, one row at a time. For images with multiple color planes, the planes for a given row are typically stored one after another rather than interleaved pixel by pixel. A decoder walks through the run length codes, expanding them back into full scanlines according to the width and plane count declared in the header.

Color handling depends on bit depth. For low bit depth images the palette is small enough to sit inside the header itself, while for 256 color images a full palette is appended at the end of the file, after the image data, marked so that readers can find it. This split arrangement is a quirk of the format's incremental history.

History and Standardization

PCX was created by ZSoft Corporation for its PC Paintbrush program, which is where the name comes from. It dates to the early to mid 1980s, making it one of the oldest raster image formats to see wide use on personal computers, and it spread alongside the popularity of that paint software.

Because the format evolved over several versions, later revisions added capabilities such as deeper color support, which is why features like the appended 256 color palette were bolted on rather than designed in from the beginning. There was no formal international standards body governing it; PCX was a de facto standard defined by its documentation and by the behavior of the software that read and wrote it.

During the DOS era PCX was extremely common, but as computing moved on it was largely superseded by formats such as GIF, BMP, and eventually PNG. Today it is mainly encountered when dealing with older files or legacy software rather than as a format chosen for new work.

PCX Compared to Other Formats

Compared to BMP, PCX at least attempts compression, since classic BMP files are frequently stored uncompressed and can be larger. BMP is arguably simpler to parse and became more universally supported on later systems, so it often displaced PCX even though PCX files could be smaller on suitable images.

Compared to GIF, PCX generally loses on compression for palette based images. GIF uses LZW compression, which handles a wider range of patterns than plain run length encoding and typically produces smaller files for the limited color graphics both formats target. GIF also added animation, which PCX never had.

Compared to modern PNG, PCX is clearly outmatched: PNG offers much stronger lossless compression, transparency, and broad support across current software. The honest assessment is that PCX run length encoding is weak, effective mainly on images with long stretches of identical pixels and poor on photographs, so PCX survives today chiefly for compatibility with old files rather than for any technical advantage.