xterm: #xxxxxx into 0-255 color conversion - how?

Question

I'm printing out all the 255 xterm-colors (background) using ANSI extended set background sequences, like so: \033[48;5;%dm (%d is substituted from 0-255). I set the xterm window color using #eee9bf via .Xdefaults with xrdb:

XTerm*background: LemonChiffon2

How is it possible that the ANSI colors don't match the #eee9bf when I inspect visually with a little help from gimp color picker? The xterm color gamut is from 0-255 only, so what is going on here - how is xterm converting #xxxxxx into 0-255? How does ANSI fit into this?

import sys

bgn_marker = '\033[48;5;%dm '
fgn_marker = '\033[%dm'
end_marker = '\033[0m'

text = "11. hello, how are you - today is not a it;s d'ay"
space = ' ' * 40

def burp(bg):
    print '######################## %d ####################################' % bg
    for fg in range(30,38):
        print (bgn_marker + fgn_marker + text + space + end_marker) % (bg, fg)

(What I'm trying to do is pick a decent background for xterm that doesn't hide the ANSI colors that are often displayed when you use vim/dmesg etc.)

score 3 · Answer 1 · edited Apr 13 '17 at 12:36

ANSI has no role in this process. It never did, except for naming convenience. ANSI defined a set of 8 colors which most people number 0-7. When extending the scheme to 16-, 88- and 256-colors, it was convenient to make an array of color values which are addressed by the same type of index.

The X server (and your display configuration) do play a part.

xterm takes the hexadecimal value from your resources or indirectly via a name in rgb.txt and asks the X server for a color "close" to the R/G/B value. The server may (and often does) approximate the colors. In xterm's source-code, a good place to start reading is (appropriately enough) AllocateAnsiColor.

xterm: #xxxxxx into 0-255 color conversion - how?

1 Answers1