bug#39799: 28.0.50; Most emoji sequences don’t render correctly

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Glenn Morris <rgm@gnu.org>,  mfabian@redhat.com,  39799@debbugs.gnu.org
> Date: Fri, 28 Feb 2020 15:14:01 +0100
> 
>     >> It DTRT for me under Cairo if I change my fontset settings to use
>     >> 'Noto Color Emoji' instead of Symbola for:
> 
>     Eli> Is that a free font (it's from Google, AFAIK, so it might not be)?  
> If
>     Eli> it is free, we could modify fontset.el to use this font if available.
>     Eli> (Or maybe there are better free Emoji fonts out there?)
> 
> Its license is Apache 2.0. It seems fairly popular. I have no opinion
> either way.

What about the fact that we still support XFT?

And anyway, the name "Noto Color Emoji" seems to imply it's a font
created to display Emoji, not symbols in general, let alone non-symbol
blocks we currently set up to use Symbola if that is available.

>     >> Weʼd run into the 4-character maximum for that, since some of the
>     >> sequences are 7 or more characters long.
> 
>     Eli> If the sequences are 7 character long, then the forward-matching
>     Eli> pattern will hit the same limitation as well, no?
> 
> C-h v composition-function-table says:
> 
>     PREV-CHARS is a non-negative integer (less than 4) specifying how many
>     characters before C to check the matching with PATTERN.  If it is 0,
>     PATTERN must match C and the following characters.  If it is 1,
>     PATTERN must match a character before C and the following characters.
> 
> which on careful re-reading says that the lookback canʼt be more than
> 3 characters, but that matching forward has no limit.

Depends on the patterns used, I guess.

>     Eli> The ones in 
> http://www.unicode.org/Public/emoji/12.0/emoji-sequences.txt,
>     Eli> and specifically the flag sequences and the skin color sequences.  At
>     Eli> least AFAIU the original report.
> 
> As Mike clarified, you need to change the fontsets in order to get
> them to display in colour (uncomposed, of course).

I don't see how that is relevant.  Fontsets are just means to cause
Emacs use a certain font for a certain range of characters.  Fontsets
do not affect color Emoji support.