PCX, the IBM PC Bitmap

1985

Vaporwave paint loop. ZSoft PC Paintbrush ships the PCX format and teaches DOS to draw. Place: ZSoft Corporation, Marietta, Georgia Figures: ZSoft engineering team · the PC Paintbrush user base ZSoft ships PC Paintbrush for the IBM PC in 1985. The companion file format, PCX (PC Paintbrush eXchange), is purpose-built for the program: header, palette, raster data, RLE-compressed run by run. Simple enough that a competent C programmer can write a decoder in an afternoon. By 1988 PCX is the dominant bitmap format on MS-DOS, supported by every PC paint program, every screen-capture utility, and every fax driver of the era. The compression is run-length encoding: a byte indicating how many consecutive pixels share a color, then the color. On screenshots and line art the ratios are excellent. On photographic or dithered imagery they are catastrophic; a typical PCX of a scanned photograph is LARGER than the raw bitmap. The format never solves this. RLE has a ceiling that LZW (see [[lzw-patent-1985]]) does not. Video: LGR traces the line from ZSoft PC Paintbrush and its PCX format through every Microsoft Paint revision.