Re: Better emoji support

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Mon, 20 Sep 2021 21:54:57 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> From: Robert Pluim <rpluim@gmail.com>
    >> Cc: kevin.legouguec@gmail.com,  emacs-devel@gnu.org
    >> Date: Mon, 20 Sep 2021 19:32:23 +0200
    >> 
    Eli> My idea is to trick the font_range function, which verifies that the
    Eli> sequence can be composed using a single font, to use an Emoji font
    Eli> when it sees something followed by a variation selector, instead of
    Eli> the font for the first character in the sequence.  For that, the rules
    Eli> for Emoji sequences in composition-function-table should be anchored
    Eli> on the VS-n codepoints (which I think is a good idea regardless).
    >> 
    >> Weʼd have to raise the lookback limit for composition-function-table
    >> rules higher than 3 (maybe only to 4).

    Eli> Examples?  Not that it's a catastrophe.

From emoji-zwj-sequences.txt:

1F468 1F3FB 200D 2764 FE0F 200D 1F468 1F3FB ; RGI_Emoji_ZWJ_Sequence
; couple with heart: man, man, light skin tone                   #
E13.1  [1] (👨🏻‍❤️‍👨🏻)

With the current limit you'd get no further than the 1F3FB if you
anchored at FE0F, and miss the 1F468.

    >> I guess it reduces the number of entries in
    >> composition-function-table, but then you end up with a lot of rules
    >> for eg VS-16.

    Eli> Why do you think we need to have a lot of such rules?  What kind of
    Eli> rules did you think about?

For whatever reason, a lot of the sequences in emoji-zwj-sequences.txt
contain codepoints with Emoji_Presentation = No, hence theyʼre
followed by VS-16. As a result, anchoring to VS-16 would produces a
lot of rules for VS-16.

    >> emoji-zwj-sequences.txt would result in about 840 rules,
    >> with a lot of redundancy, which could be reduced, but I think that can
    >> wait until after the zwj sequence stuff goes in.

    Eli> I guess I'm missing something because I don't see a problem there.

Perhaps Iʼm worrying too much. It would be a lot of rules to check,
but only when encountering VS-16.

Anyway, we can measure the difference, if any, once we have the base
implementation and Someone™ implements the VS-16 anchored version (it
would only be a dozen lines of awk, I think).

Robert
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