Christian Brabandt wrote:

> On Fr, 10 Jan 2014, Bram Moolenaar wrote:
> 
> > Christian Brabandt wrote:
> > 
> > > On Di, 07 Jan 2014, Matteo Cavalleri wrote:
> > > 
> > > > some easy steps to reproduce the problem:
> > > > 
> > > > :set t_Co?
> > > > t_Co=256
> > > > :set background?
> > > > background=light
> > > > :set background=dark
> > > > :set background?
> > > > background=dark
> > > > :hi Normal ctermfg=grey ctermbg=234
> > > > :set background?
> > > > background=light
> > > > :hi Normal ctermfg=grey ctermbg=0
> > > > :set background?
> > > > background=dark
> > > 
> > > > as you can see, the "hi" command changes background to light when
> > > > ctermbg is set to 234, however, according to
> > > > http://upload.wikimedia.org/wikipedia/commons/9/95/Xterm_color_chart.png
> > > > color 234 is indeed a dark grey
> > > 
> > > I see the problem:
> > > 
> > > 7728           if (color >= 0)
> > > 7729            {
> > > 7730                if (termcap_active)
> > > 7731                    term_bg_color(color);
> > > 7732                if (t_colors < 16)
> > > 7733                    i = (color == 0 || color == 4);
> > > 7734                else
> > > 7735                    i = (color < 7 || color == 8);
> > > 7736                /* Set the 'background' option if the value is
> > > 7737                 * wrong. */
> > > 7738                if (i != (*p_bg == 'd'))
> > > 7739                    set_option_value((char_u *)"bg", 0L,
> > > 7740                            i ?  (char_u *)"dark"
> > > 7741                              : (char_u *)"light", 0);
> > > 7742            }
> > > 
> > > i.e. Vim always sets a light background for all colors except the first 
> > > 7 and color 8 for which it will set a dark background. I don't think Vim 
> > > should try to determine what color is used and depending on it's value 
> > > try to set a sane 'bg' value for the other 256 colors (in a 256 color 
> > > terminal or the 88 rxvt color cube) so for now, I would propose to 
> > > adjust the logic only slightly to something like this:
> > > 
> > > diff --git a/src/syntax.c b/src/syntax.c
> > > --- a/src/syntax.c
> > > +++ b/src/syntax.c
> > > @@ -7731,6 +7731,14 @@
> > >                                 term_bg_color(color);
> > >                             if (t_colors < 16)
> > >                                 i = (color == 0 || color == 4);
> > > +                           else if (t_colors == 256)
> > > +                           {
> > > +                               i = (color < 7 || color == 8 ||
> > > +                                       (color > 231 && color < 244));
> > > +                               if (i > 16 && i < 232)
> > > +                                   i = (*p_bg == 'd');
> > > +                           }
> > > +
> > > 
> > > Which means only for the lower 12 (dark) gray colors (232 until 243) in 
> > > the 256 color cube set the 'bg' to dark, for all other (except the ones 
> > > mentioned above) leave the bg setting alone.
> > 
> > It's a start.  However, if we change this we might as well do it
> > properly.
> > 
> > In my opinion we should take the color chart and compute the brightness
> > of each color.  If it's above 50% 'background' should be set to light.
> > 
> > There at least is this image:
> > http://upload.wikimedia.org/wikipedia/en/thumb/1/15/Xterm_256color_chart.svg/2000px-Xterm_256color_chart.svg.png
> > 
> > A list of #RGB values could be used to compute the brightness.
> 
> How about the attached patch? For colors > 15, it uses the rxvt color 
> cube for 88 color terminals and the xterm color cube for 256 color 
> terminals and computes the luminance. If the luminance is above a 
> certain treshhold, it uses a light background, else it uses a dark 
> background.
> 
> It uses the same logic, that I have been using for my Colorizer plugin 
> (https://github.com/chrisbra/color_highlight) for which I haven't seen 
> any complaints about the luminance yet (of course this doesn't mean this 
> is correct by all means).

I suppose this would be OK.  But I was thinking of a built-in table.
Avoids computing stuff at runtime, just lookup the flag.


-- 
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