Commit 5af9e5ba authored by Eli Zaretskii's avatar Eli Zaretskii

; Add an entry to TODO

* etc/TODO: Expand the ligature support entry.  Add a new entry
about better support of Emoji.
parent d4241959
Pipeline #4895 failed with stage
in 54 minutes and 23 seconds
......@@ -244,6 +244,23 @@ populate composition-function-table with those rules. See
composite.el for examples of this, and also grep lisp/language/*.el
for references to composition-function-table.
One problem with character compositions that will need to be solved is
that composition-function-table, the char-table which holds the
composition rules, is a global variable, whereas use of ligatures is
inherently specific to buffer-local stuff like the major mode and the
script or language in use. So there should be a buffer-local variable
to augment/customize/override the global composition rules.
Another problem is that ligatures are frequently composed of ASCII
characters, and some of those ASCII characters are present in the mode
line, for example "--". Since displaying a ligature instead of 2
separate '-' characters on a mode line is not right, there should be a
way of preventing the ligation from happening. One possibility is to
have a ZWNJ character separate these ASCII characters; another
possibility is to introduce a special text property that prevents
character composition, and place that property on the relevant parts
of the mode line.
The prettify-symbols-mode should be deprecated once ligature support
is in place.
......@@ -267,6 +284,59 @@ should invoke the 'shape' method. 'hbfont_shape' should be extended
to pass to 'hb_shape_full' the required array of features, as
mentioned in the above HarfBuzz discussion.
** Better support for displaying Emoji
Emacs is capable of displaying Emoji and some of the Emoji sequences,
provided that its fontsets are configured with a suitable font. To
make this easier out of the box, the following should be done:
*** Populate composition-function-table with Emoji rules
The Unicode Character Database (UCD) includes several data files that
define the valid Emoji sequences. These files should be imported into
the Emacs tree, and should be converted by some script at Emacs build
time to Lisp code that populates composition-function-table with the
corresponding composition rules.
*** Augment the default fontsets with Emoji-capable fonts
The default fontsets set up by fontest.el should include known free
fonts that provide good support for displaying Emoji sequences. In
addition, the rule that the default face's font is used for symbol and
punctuation characters, disregarding the fontsets, should be modified
to exempt Emoji from this rule (since Emoji characters belong to the
'symbol' script in Emacs), so that use-default-font-for-symbols would
not have to be tweaked to have Emoji display by default with a capable
font.
*** Consider changing the default display of Variation Selectors
Emacs by default displays the Variation Selector (VS) codepoints not
composed with base characters as thin 1-pixel space glyphs. The
Unicode FAQ says that if variation sequences cannot be supported, the
VS characters should not be shown, leaving just the base character of
the sequence visible. This could be handled via
glyphless-char-display, by changing the entries for VS codepoints to
'zero-width'.
*** Special face for displaying text presentation of Emoji
Emoji-capable fonts support Emoji sequences with the U+FE0F VARIATION
SELECTOR-16 (VS16) for emoji-style display, but usually don't support
the U+FE0F VARIATION SELECTOR-15 (VS15) for text-style display. There
are other fonts which support the text-style sequences, but not
emoji-style. Since Emacs selects a font based on a single character,
it cannot choose 2 different fonts for displaying both styles of the
same base character. To display both styles in the same buffer, one
could use a special face, placing a 'face' text property on portions
of the text. This special face could specify a specific font known to
support text-style Emoji sequences. Emacs could have such a face
built-in.
See the discussion of bug#39799 for more details about this task.
Another relevant resource is the Unicode Technical Standard #51
"Unicode Emoji" (http://www.unicode.org/reports/tr51/).
** Extend text-properties and overlays
*** Several text-property planes
This would get us rid of font-lock-face property (and I'd be happy to
......
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