2011/04/09

Extended colour support, terminals, and ECMA-48

tl;dr summary: Terminal authors - please accept CSI 38:5:123 m as well as anything else, to set extended colours. NOTE THE COLON. Many terminals these days are starting to support extended colour modes; specifically things like 256 colours. They're all doing it wrong.
CSI 38;5;123 m
No no no no no. That is NOT what you think. It does not select colour 123 from palette 5. CSI arguments are separated by semicolons. This sets three unrelated SGRs; 38, 5, and 123. 38 sets a foreground colour to .. er.. nothing in particular. 5 is blinking mode, 123 has no defined meaning to my knowledge. All the other following are exactly identical:
CSI 5;38;123 m
CSI 38 m CSI 5 m CSI 123 m
ECMA-48 already defines a perfectly good way for parameters to take sub-parameters. It's the colon. The correct way to encode this concept is
CSI 38:5:123 m
Why does this matter? It matters for parsers not to have to understand what is going on. Consider
CSI 3;38:9:5:4:3:2:1;11;1
This is equivalent to
CSI 3 CSI 38:9:5:4:3:2:1 CSI 11; CSI 1
I.e. I have no idea what palette 9 is, but I can definitely parse out the contents of this SGR from the others, knowing exactly where it ends. I don't have to "just know" that palette 9 happens to take 5 parameters. The CSI encodes this. My own terminal emulator library, libvterm, already understands these colon-based arguments. It parses them correctly. This is important for other palettes that don't take just one value. For example, the RGB, RGBA, CMY, and CMYK palettes. It's vital to be able to parse out these sub-arguments from the single SGR 38 or 48. Standards exist for a reason, people. Please use them.
Edit 2012/12/20: actually, I have recently learned that xterm now supports this in its correct form as well, since xterm patch 282.

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