terminal-light

This crate answers the question "Is the terminal dark or light?".

It provides

• the background color, either as RGB or ANSI
• the background color's luma, which varies from 0 (black) to 1 (white)

A use case in a TUI is to determine what set of colors would be most suitable depending on the terminal's background:

let should_use_light_skin = terminal_light::luma()
    .map_or(false, |luma| luma > 0.6);

If you have very specialized skins, you may choose a more precise switch:

match terminal_light::luma() {
    Ok(luma) if luma > 0.85 => {
        // Use a "light mode" skin.
    }
    Ok(luma) if luma < 0.2 => {
        // Use a "dark mode" skin.
    }
    _ => {
        // Either we couldn't determine the mode or it's kind of medium.
        // We should use an itermediate skin, or one defining the background.
    }
}

Strategies

Here are the various strategies automatically used by terminal-light to determine the background:

$COLORFGBG strategy

This environment variable is set by some terminals, like konsole or the rxvt family. It can also be set by users. Its value is like 15;0 where the second number is the ANSI code for the background color.

Upside:

• querying an env variable is a fast operation

Downsides:

• this env variable isn't always immediately updated when you change the color of the terminal
• the value isn't precise: 0 is "dark" and 15 is "light" but the real RGB color is uncertain as the low ANSI codes are often modified by the user

"Dynamic colors" OSC escape sequence strategy

Modern terminals implement this xterm extension: a query making it possible to know the background color as RGB.

Terminal-light sends the query to stdout, waits for the answer on stdin with a timeout of 20ms, then analyses this answer.

Upsides:

• this works well on all tested unix terminals, eg Linux and MacOS
• the value is precise (RGB)
• the value is up to date when it's available

Downsides:

• waiting for stdin with a timeout isn't implemented on Windows in this crate (help welcome)
• this isn't instant, a delay of 10 ms to get the answer isn't unusual
• if a not compatible terminal doesn't answer at all, we're waiting for 20ms
• it may fail on some terminal multiplexers

Global strategy used by Terminal-light

1. if we're on a unix-like platform, we try the escape sequence strategy
2. if it failed or we're not on unix, we try the $COLORFGBG strategy
3. without a solution, we return a TlError::Unsupported error