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True Colour (16 million colours) support in various terminal applications and terminals

Colours in terminal

It's a common confusion about terminal colours... Actually we have this:

  • plain ascii
  • ansi escape codes (16 colour codes with bold/italic and background)
  • 256 colour palette (216 colours + 16 ansi + 24 gray) (colors are 24bit)
  • 24bit true colour ("888" colours (aka 16 milion))
printf "\x1b[${bg};2;${red};${green};${blue}m\n"

The 256 colour palete is configured at start, and it's a 666 cube of colours, each of them defined as a 24bit (888 rgb) colour.

This means that current support can only display 256 different colours in the terminal, while truecolour means that you can display 16 milion different colours at the same time.

Truecolour escape codes doesn't use a colour palette. They just specify the colour itself.

Here's a test case:

printf "\x1b[38;2;255;100;0mTRUECOLOR\x1b[0m\n"
awk 'BEGIN{
    s="/\\/\\/\\/\\/\\"; s=s s s s s s s s;
    for (colnum = 0; colnum<77; colnum++) {
        r = 255-(colnum*255/76);
        g = (colnum*510/76);
        b = (colnum*255/76);
        if (g>255) g = 510-g;
        printf "\033[48;2;%d;%d;%dm", r,g,b;
        printf "\033[38;2;%d;%d;%dm", 255-r,255-g,255-b;
        printf "%s\033[0m", substr(s,colnum+1,1);
    }
    printf "\n";
}'

Keep in mind that it is possible to use both ';' and ':' as parameters delimiter.

According to Wikipedia[1], this is only supported by xterm and konsole.

[1] https://en.wikipedia.org/wiki/ANSI_color

Since ncurses-6.0-20180121 terminfo started to support 24-bit truecolor capability under the name of "RGB" - you need to use the "setaf" and "setab" commands to set foreground and background correspondingly.

Detection

There's no reliable way until the new release of terminfo/ncurses. After that the "RGB" flag should be available for detection. S-Lang author added a check for $COLORTERM containing either "truecolor" or "24bit" (case sensitive). In turn, VTE, Konsole and iTerm2 set this variable to "truecolor" (it's been there in VTE for a while, it's relatively new and maybe still git-only in Konsole and iTerm2).

This is obviously not a reliable method, and is not forwarded via sudo, ssh etc. However, whenever it errs, it errs on the safe side: does not advertise support whereas it's actually supported. App developers can freely choose to check for this same variable, or introduce their own method (e.g. an option in their config file), whichever matches better the overall design of the given app. Checking $COLORTERM is recommended though, since that would lead to a more unique desktop experience where the user has to set one variable only and it takes effect across all the apps, rather than something separately for each app.

Here are terminals discussions:

Now supporting truecolour

But there are bunch of libvte-based terminals for GTK2 so they are listed in the another section.

Also, while this one is not exactly a terminal, but a terminal replayer, it still worth mentioning:

Improper support for true colors

Parsing ANSI colour sequences, but approximating them to 256 palette

Note about colour differences: a) RGB axes are not orthogonal, so you cannot use sqrt(R^2+G^2+B^2) formula, b) for colour differences there is more correct (but much more complex) CIEDE2000 formula (which may easily blow up performance if used blindly) [2].

[2] https://github.com/neovim/neovim/issues/793#issuecomment-48106948

Terminal multiplexers

NOT supporting truecolour

Here are another console programs discussions:

Supporting True Colour:

Not supporting True Colour:

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