PNM to SIXEL Converter

SIXEL (Six Pixel) is a bitmap graphics encoding format created by Digital Equipment Corporation (DEC) in 1983 for rendering images on character-cell printers and video terminals. The name derives from the encoding's fundamental unit: a column of six pixels represented by a single ASCII character. Each printable character in the sixel data stream (ASCII 63-126) encodes a 6-pixel vertical column, with the character's binary value determining which pixels are on or off. Color is specified through register-based palette control: a Select Color Sequence assigns an HLS or RGB color value to a numbered register, and subsequent sixel characters use that color until another register is selected. The encoding supports raster attributes for specifying pixel aspect ratio and image dimensions, repeat sequences (! followed by a count and character) for run-length compression of identical columns, and $ (carriage return) and - (new line) for navigating the sixel grid. DEC implemented SIXEL support in their VT240, VT241, VT330, and VT340 terminals, as well as multiple printer models. One advantage of the SIXEL encoding is its ASCII-clean nature: the data stream consists entirely of printable characters and standard control sequences, meaning SIXEL graphics can be transmitted through any text-based communication channel — serial terminals, SSH sessions, telnet connections — without requiring binary-safe transport or protocol modifications. The format's modern renaissance provides another remarkable dimension: after decades of obscurity, SIXEL support has been implemented in numerous contemporary terminal emulators, enabling inline image display in command-line workflows. SIXEL output can be generated by ImageMagick, libsixel, chafa, and various plotting libraries.