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nixos/true_colour_tests/TrueColour.md
Edward Litherland-Smith f07b34e790 Add checks for true colour support in terminal (visual only)
2021-02-22 09:05:59 +00:00

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Terminal Colors

There exists common confusion about terminal colors. This is what we have right now:

  • Plain ASCII
  • ANSI escape codes: 16 color codes with bold/italic and background
  • 256 color palette: 216 colors + 16 ANSI + 24 gray (colors are 24-bit)
  • 24-bit true color: "888" colors (aka 16 milion)
printf "\x1b[${bg};2;${red};${green};${blue}m\n"

The 256-color palette is configured at start and is a 666-cube of colors, each of them defined as a 24-bit (888 rgb) color.

This means that current support can only display 256 different colors in the terminal while "true color" means that you can display 16 million different colors at the same time.

Truecolor escape codes do not use a color palette. They just specify the color itself.

This is a good 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 Control Sequence Introducer delimiters.

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

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

Since ncurses-6.0-20180121, terminfo began to support the 24-bit True Color capability under the name of "RGB". You need to use the "setaf" and "setab" commands to set the foreground and background respectively.

True Color Detection

There will be no reliable way to detect the "RGB" flag until the new release of terminfo/ncurses. S-Lang author added a check for $COLORTERM containing either "truecolor" or "24bit" (case sensitive). In addition, VTE, Konsole and iTerm2 set this variable to "truecolor". It has been in VTE for a while and but is relatively new, being 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. It does not advertise support when it is 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). They should use whichever method best matches the overall design of their app. Checking $COLORTERM is recommended though since it will lead to a more seamless desktop experience where only one variable needs to be set. This would be system-wide so that the user would not need to set it separately for each app.

Querying The Terminal

A more reliable method in an interactive program which can read terminal responses, and one that is transparent to things like sudo, SSH, etc.. is to simply try setting a truecolor value and then query the terminal to ask what color it currently has. If the response replies the same color that was set then it indicates truecolor is supported.

$ (echo -e '\e[48:2:1:2:3m\eP$qm\e\\' ; xxd)

^[P1$r48:2:1:2:3m^[\
00000000: 1b50 3124 7234 383a 323a 313a 323a 336d  .P1$r48:2:1:2:3m

Here we ask to set the background color to RGB(1,2,3) - an unlikely default choice - and request the value that we just set. The response comes back that the request was understood (1), and that the color is indeed 48:2:1:2:3. This tells us also that the terminal supports the colon delimiter. If instead, the terminal did not support truecolor we might see a response like

^[P1$r40m^[\
00000000: 1b50 3124 7234 306d 1b5c 0a              .P1$r40m.\.

This terminal replied the color is 40 - it has not accepted our request to set 48:2:1:2:3.

^[P0$r^[\
00000000: 1b50 3024 721b 5c0a                      .P0$r.\.

This terminal did not even understand the DECRQSS request - its response was 0$r. We do not learn if it managed to set the color, but since it doesn't understand how to reply to our request it is unlikely to support truecolor either.

Terminals + True Color

Now Supporting True Color

There are a bunch of libvte-based terminals for GTK2, so they are listed in the another section.

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

Improper Support for True Color

Terminals that parse ANSI color sequences, but approximate them to 256 palette

Note about color differences: a) RGB axes are not orthogonal, so you cannot use sqrt(R^2+G^2+B^2) formula b) for color 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 True Color

Console Programs + True Color

Console Programs Supporting True Color

Console Programs Not Supporting True Color