[historical] reasoning behind 88 colors

[Nick Black]

[ hey there! this is really more of an xterm question than an
  ncurses question, but the former doesn't appear to have a
  mailing list, and last time i asked a question here, i got great
  answers. so...i hope not to offend ]

While doing some research for a book I'm writing[0], I came
across the support for 88 color palettes in xterm and (later)
at least rxvt. Patch #115 [1] appears to have added the 88-color
support to xterm (the NCURSES NEWS file mentions 88-color
terminfo sources by 1999). The Xterm FAQ mentions:

"88-colors (using the same control sequence) came next, to
 reduce the amount of memory needed. XTerm stores both foreground
 and background color indexes for each cell on the screen. That
 is two bytes, which doubled the amount of memory used by xterm
 for the scrollback. Reducing that to a single byte allowed a
 similar scheme using a 4x4x4 cube and a proportionately shorter
 grayscale ramp."

My question(s) is(are):

 (1) How is this a "single byte"? For 88 colors, you need 7
  bits. Even if this was paired with 8 or 16 colors already being
  stored elsewhere, and only 72/80 "new" ones needed
  representation, that still can't be done in 6 bits. 256 colors
  would be a single byte, but the FAQ specs two, suggesting this
  spec included both front and back. In that case, you still need
  12 bits, i.e. three nibbles, i.e. 75% of two bytes. Unless you
  only had 1 bit devoted to background color, I don't understand
  this.
 
 (2) Even if it is just a net savings of 1 bit per dimension,
  why 88 rather than 128? Was this merely because the 4x4x4 cube
  was the biggest that could fit in 7 bits, assuming the
  preexisting 16 aixterm colors? If so, why not use the other 24
  colors for greys, as opposed to only 8? 8 doesn't even seem to
  be proportional; 4/6 == 2/3, suggesting 16 greyscales (which
  would fit comfortably with 16 + 64 in 7 bits).

i guess my question boils down to: what made 88 more appealing
than either 64 or 128? were 88 colors encodable in fewer than 7
net bits? did this mechanism not stretch to 96 colors?

Stephen P Wall, do you read this list? :)

thanks! apologies if this is unsuiable for bugs-ncurses.

--nick

[0] https://nick-black.com/dankwiki/index.php/Hacking_The_Planet!_with_Notcurses

  (note that this is still very much in draft mode. the relevant
   material is in chapter 6 and appendix a.)

[1] https://invisible-island.net/xterm/xterm.log.html#xterm_115

-- 
nick black -=- https://www.nick-black.com
to make an apple pie from scratch,
you need first invent a universe.