True-colour ↔ ANSI terminal palette converter

ansi_colours is a library which converts between 24-bit sRGB colours and 8-bit colour palette used by ANSI terminals such as xterm on rxvt-unicode in 256-colour mode.

The most common use case is when using 24-bit colours in a terminal emulator which only support 8-bit colour palette. This package allows true-colours to be approximated by values supported by the terminal.

When mapping true-colour into available 256-colour palette (of which only 240 are actually usable), this package tries to balance accuracy and performance. It doesn’t implement the fastest algorithm nor is it the most accurate, instead it uses a formula which should be fast enough and accurate enough for most use-cases.

Usage

This library has C, C++ and Rust bindings and can be easily used from any of those languages.

Rust

Using this package with Cargo projects is as simple as adding a single dependency:

[dependencies]
ansi_colours = "^1.0"

and then using one of the two functions that the library provides:

extern crate ansi_colours;

use ansi_colours::*;

fn main() {
    // Colour at given index:
    println!("{:-3}: {:?}", 50, rgb_from_ansi256(50));

    // Approximate true-colour by colour in the palette:
    let rgb = (100, 200, 150);
    let index = ansi256_from_rgb(rgb);
    println!("{:?} ~ {:-3} {:?}", rgb, index, rgb_from_ansi256(index));
}

C and C++

The easiest way to use this package in C or C++ is to copy the ansi_colour.h and ansi256.c files to your project (unfortunately, C nor C++ has any centralised package repository), set up compilation step for the ansi256.c file, add header file to include path and once all that is done use the two functions provided by this library can be used:

#include <stdio.h>

#include "ansi_colours.h"

int main() {
    // Colour at given index:
    printf("%-3u #%06x\n",  50, rgb_from_ansi256(50));

    // Approximate true-colour by colour in the palette:
    uint32_t rgb = 0x64C896;
    uint8_t index = ansi256_from_rgb(rgb);
    printf("#%06x ~ %-3u %06x\n", rgb, index, rgb_from_ansi256(index));

    return 0;
}

C nor C++ ecosystem has a centralised library distribution service which is why at this stage the easiest way is to just copy the two required files.