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Sixel
Sixel ("six pixels") does not appear to have any true, up-to-date "standard". It seems to have been introduced on DEC printers, and made its first glass appearance on the VT125, though the VT3xx documentation gets the most play (the VT340 appears to have been the first terminal which could sensibly scroll pixel graphics). A single sixel combines a set of programmable color registers with a vertical stack of six pixels; a sixel sprite (sprixel?) is MxN pixels where N is a multiple of 6. It is probably the most widely implemented method of pixel graphics for terminal emulators (kitty has its own protocol, as does ITerm2). ReGIS is a similar method for vector graphics. A single horizontal strip of sixels is a "band"; a sprixel has one or more bands.
A sprixel must typically be emitted as a single unit to be rendered as a coherent whole (i.e. without gaps between bands). Sprixels do not annihilate entire cells, but only the pixels covered. A glyph emitted atop a sixel will annihilate all pixels contained within the cell.
Color registers
A terminal supports some number of color registers for each sprixel. For xterm, this is controlled by the numColorRegisters resource (set to a (low) value based off decTerminalID if not explicitly provided):
XTerm*numColorRegisters: 1024
Each register is specified with a type ("1" for HSL, "2" for RGB) and three components, each between 0 and 100 inclusive. Assuming 8bpc RGB, every 2.6 bits of component get squashed to 1.
Support
- xterm supports scrolling sprixels when compiled with --enable-sixel-graphics
- and launched with at least a vt340 decGraphicsID (or decTerminalID)
- mlterm works
- alacritty has a patch outstanding to add support, but it is not yet merged as of 0.8.0-dev
- VTE supports sprixels when built with -Dsixel=true
- Microsoft's Windows Terminal is said to support sixel
