Re: face colors on 256 colors terminals

[David Kastrup <dak@gnu.org>]

"Eli Zaretskii" <eliz@gnu.org> writes:

>> From: Dan Nicolaescu <dann@ics.uci.edu>
>> Date: Wed, 06 Apr 2005 01:17:11 -0700
>> 
>> This comment in tty-colors.el:tty-color-standard-values 
>> 
>>          ;; Translate the string "#XXYYZZ" into a list
>>          ;; of numbers (XX YY ZZ).  If the primary colors
>>          ;; are specified with less than 4 hex digits,
>>          ;; the used digits represent the most significant
>>          ;; bits of the value (e.g. #XYZ = #X000Y000Z000).
>> 
>> does not seem to match the way the `color-name-rgb-alist' seem to have
>> been created from the values in rgb.txt. 
>> A random example:
>> >From color-name-rgb-alist:
>>     ("lavenderblush"    65535   61680   62965)
>>                         ^^^^    ^^^^    ^^^^
>>                         0xffff  0xf0f0  0xf5f5      
>> 
>> >From rgb.txt: lavender blush     255   240   245
>>                                  0xff  0xf0  0xf5
>> So the 8 to 16 bit conversion seems use the same byte value for both
>> the high and low byte value. 
>
> The comment you cited reflects what I found in the X documentation.
> Here, for example, is an excerpt from the X(7) man page on a Debian
> GNU/Linux box (fencepost.gnu.org):
>
>        For backward compatibility, an older syntax for RGB Device
>        is  supported,  but  its  continued use is not encouraged.
>        The syntax is an initial sharp sign character followed  by
>        a numeric specification, in one of the following formats:
>
>            #RGB                      (4 bits each)
>            #RRGGBB                   (8 bits each)
>            #RRRGGGBBB                (12 bits each)
>            #RRRRGGGGBBBB             (16 bits each)
>
>        The R, G, and B represent single hexadecimal digits.  When
>        fewer than 16 bits each are specified, they represent  the
>        most-significant bits of the value (unlike the "rgb:" syn-
>        tax, in which values are scaled).  For  example,  #3a7  is
>        the same as #3000a0007000.
>
> So I think the code in tty-colors.el is correct in this matter.  It
> is, however, possible that the RGB values in color-name-rgb-alist were
> incorrectly scaled from 8-bit variants, and need to be amended.

Actually, it does not make sense to scale in that way.  #3a7 really
should be the same as #3333aaaa7777, so that #fff is the same as
#ffffffffffff, pure white.

Somebody should tell the X people to do that.  I just have no idea who
to tell, and it would appear that there is a bit of previous art that
might rely on the current behavior...

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum