Convert HEX to closest X11 Color Number

Question

I\'m looking for a Algorithm/Way to convert given HEX (e.g. #111111 R:0x11, G:0x11, B:0x11) to the closest X11 color number (Terminal is either 88 or 256 colors) using either Py

Answer 1

I had the same issue (taking an existing vim colorscheme with gvim hex colors and filling in the terminal color values). Here's the script I used (although the ColorDist function would probably benefit from a better color distance metric like some of the other posters are suggesting)

import re
import math

colors = {
    '000000':  '16', '00005f':  '17', '000087':  '18', '0000af':  '19', '0000d7':  '20',
    '0000ff':  '21', '005f00':  '22', '005f5f':  '23', '005f87':  '24', '005faf':  '25',
    '005fd7':  '26', '005fff':  '27', '008700':  '28', '00875f':  '29', '008787':  '30',
    '0087af':  '31', '0087d7':  '32', '0087ff':  '33', '00af00':  '34', '00af5f':  '35',
    '00af87':  '36', '00afaf':  '37', '00afd7':  '38', '00afff':  '39', '00d700':  '40',
    '00d75f':  '41', '00d787':  '42', '00d7af':  '43', '00d7d7':  '44', '00d7ff':  '45',
    '00ff00':  '46', '00ff5f':  '47', '00ff87':  '48', '00ffaf':  '49', '00ffd7':  '50',
    '00ffff':  '51', '5f0000':  '52', '5f005f':  '53', '5f0087':  '54', '5f00af':  '55',
    '5f00d7':  '56', '5f00ff':  '57', '5f5f00':  '58', '5f5f5f':  '59', '5f5f87':  '60',
    '5f5faf':  '61', '5f5fd7':  '62', '5f5fff':  '63', '5f8700':  '64', '5f875f':  '65',
    '5f8787':  '66', '5f87af':  '67', '5f87d7':  '68', '5f87ff':  '69', '5faf00':  '70',
    '5faf5f':  '71', '5faf87':  '72', '5fafaf':  '73', '5fafd7':  '74', '5fafff':  '75',
    '5fd700':  '76', '5fd75f':  '77', '5fd787':  '78', '5fd7af':  '79', '5fd7d7':  '80',
    '5fd7ff':  '81', '5fff00':  '82', '5fff5f':  '83', '5fff87':  '84', '5fffaf':  '85',
    '5fffd7':  '86', '5fffff':  '87', '870000':  '88', '87005f':  '89', '870087':  '90',
    '8700af':  '91', '8700d7':  '92', '8700ff':  '93', '875f00':  '94', '875f5f':  '95',
    '875f87':  '96', '875faf':  '97', '875fd7':  '98', '875fff':  '99', '878700': '100',
    '87875f': '101', '878787': '102', '8787af': '103', '8787d7': '104', '8787ff': '105',
    '87af00': '106', '87af5f': '107', '87af87': '108', '87afaf': '109', '87afd7': '110',
    '87afff': '111', '87d700': '112', '87d75f': '113', '87d787': '114', '87d7af': '115',
    '87d7d7': '116', '87d7ff': '117', '87ff00': '118', '87ff5f': '119', '87ff87': '120',
    '87ffaf': '121', '87ffd7': '122', '87ffff': '123', 'af0000': '124', 'af005f': '125',
    'af0087': '126', 'af00af': '127', 'af00d7': '128', 'af00ff': '129', 'af5f00': '130',
    'af5f5f': '131', 'af5f87': '132', 'af5faf': '133', 'af5fd7': '134', 'af5fff': '135',
    'af8700': '136', 'af875f': '137', 'af8787': '138', 'af87af': '139', 'af87d7': '140',
    'af87ff': '141', 'afaf00': '142', 'afaf5f': '143', 'afaf87': '144', 'afafaf': '145',
    'afafd7': '146', 'afafff': '147', 'afd700': '148', 'afd75f': '149', 'afd787': '150',
    'afd7af': '151', 'afd7d7': '152', 'afd7ff': '153', 'afff00': '154', 'afff5f': '155',
    'afff87': '156', 'afffaf': '157', 'afffd7': '158', 'afffff': '159', 'd70000': '160',
    'd7005f': '161', 'd70087': '162', 'd700af': '163', 'd700d7': '164', 'd700ff': '165',
    'd75f00': '166', 'd75f5f': '167', 'd75f87': '168', 'd75faf': '169', 'd75fd7': '170',
    'd75fff': '171', 'd78700': '172', 'd7875f': '173', 'd78787': '174', 'd787af': '175',
    'd787d7': '176', 'd787ff': '177', 'd7af00': '178', 'd7af5f': '179', 'd7af87': '180',
    'd7afaf': '181', 'd7afd7': '182', 'd7afff': '183', 'd7d700': '184', 'd7d75f': '185',
    'd7d787': '186', 'd7d7af': '187', 'd7d7d7': '188', 'd7d7ff': '189', 'd7ff00': '190',
    'd7ff5f': '191', 'd7ff87': '192', 'd7ffaf': '193', 'd7ffd7': '194', 'd7ffff': '195',
    'ff0000': '196', 'ff005f': '197', 'ff0087': '198', 'ff00af': '199', 'ff00d7': '200',
    'ff00ff': '201', 'ff5f00': '202', 'ff5f5f': '203', 'ff5f87': '204', 'ff5faf': '205',
    'ff5fd7': '206', 'ff5fff': '207', 'ff8700': '208', 'ff875f': '209', 'ff8787': '210',
    'ff87af': '211', 'ff87d7': '212', 'ff87ff': '213', 'ffaf00': '214', 'ffaf5f': '215',
    'ffaf87': '216', 'ffafaf': '217', 'ffafd7': '218', 'ffafff': '219', 'ffd700': '220',
    'ffd75f': '221', 'ffd787': '222', 'ffd7af': '223', 'ffd7d7': '224', 'ffd7ff': '225',
    'ffff00': '226', 'ffff5f': '227', 'ffff87': '228', 'ffffaf': '229', 'ffffd7': '230',
    'ffffff': '231', '080808': '232', '121212': '233', '1c1c1c': '234', '262626': '235',
    '303030': '236', '3a3a3a': '237', '444444': '238', '4e4e4e': '239', '585858': '240',
    '626262': '241', '6c6c6c': '242', '767676': '243', '808080': '244', '8a8a8a': '245',
    '949494': '246', '9e9e9e': '247', 'a8a8a8': '248', 'b2b2b2': '249', 'bcbcbc': '250',
    'c6c6c6': '251', 'd0d0d0': '252', 'dadada': '253', 'e4e4e4': '254', 'eeeeee': '255',
}

## Example line from a vim colorscheme file
##hi Normal          ctermfg=NONE ctermbg=NONE gui=NONE guifg=#b7af9f guibg=#202020

def Decompose(hexval):
    return float(int(hexval[0:2], 16)), float(int(hexval[2:4], 16)), float(int(hexval[4:6], 16))

def Normalize(r, g, b):
    magsqr = r*r + g*g + b*b
    if magsqr < 0.0001:
        return 0.0, 0.0, 0.0
    n = 1.0 / math.sqrt(magsqr)
    return r*n, g*n, b*n

def ColorDist(c1, c2):
    c1r, c1g, c1b = Decompose(c1)
    c2r, c2g, c2b = Decompose(c2)

    dr = c1r - c2r
    dg = c1g - c2g
    db = c1b - c2b
    return dr*dr + dg*dg + db*db

def BestMatch(hexval):
    best = None
    bestdist = 0.0
    for key in colors.keys():
        dist = ColorDist(hexval, key)
        if best is None or dist < bestdist:
            best = colors[key]
            bestdist = dist
    return best

##                  1   2             3   4        5      6
fg = re.compile(r'^(.*)(ctermfg=)NONE(.*)(guifg=#)([^ ]*)(.*)\n$')
bg = re.compile(r'^(.*)(ctermbg=)NONE(.*)(guibg=#)([^ ]*)(.*)\n$')

with open(r'input_color_scheme.vim', 'r') as f:
    with open(r'output_color_scheme.vim', 'w') as fout:
        for line in f.readlines():
            fgmatch = fg.match(line)
            if fgmatch is not None:
                line = (fgmatch.group(1) +
                        fgmatch.group(2) +
                        BestMatch(fgmatch.group(5)) + 
                        fgmatch.group(3) +
                        fgmatch.group(4) +
                        fgmatch.group(5) +
                        fgmatch.group(6)) + '\n'

            bgmatch = bg.match(line)
            if bgmatch is not None:
                line = (bgmatch.group(1) +
                        bgmatch.group(2) +
                        BestMatch(bgmatch.group(5)) + 
                        bgmatch.group(3) +
                        bgmatch.group(4) +
                        bgmatch.group(5) +
                        bgmatch.group(6) + '\n')

            ## -- print the line
            fout.write(line)