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emacs-27 4b9fbdb: ; Update TODO item about ligature support
From: |
Eli Zaretskii |
Subject: |
emacs-27 4b9fbdb: ; Update TODO item about ligature support |
Date: |
Tue, 19 May 2020 11:48:19 -0400 (EDT) |
branch: emacs-27
commit 4b9fbdb5a713745dfdb13042e33ba2345e6860e1
Author: Eli Zaretskii <address@hidden>
Commit: Eli Zaretskii <address@hidden>
; Update TODO item about ligature support
* etc/TODO: Add the todo item for moving cursor "inside" a
ligature.
---
etc/TODO | 31 +++++++++++++++++++++++++++++++
1 file changed, 31 insertions(+)
diff --git a/etc/TODO b/etc/TODO
index 20262a7..f983fa2 100644
--- a/etc/TODO
+++ b/etc/TODO
@@ -264,6 +264,37 @@ of the mode line.
The prettify-symbols-mode should be deprecated once ligature support
is in place.
+A related, but somewhat independent, feature is being able to move the
+cursor "into a ligature", whereby cursor motion commands shows some
+pseudo-cursor on some part of a ligature. For example, if "ffi" is
+displayed as a ligature, then moving by one buffer position should
+show the middle part of the ligature's glyph similar to the cursor
+display: some special background and perhaps also a special
+foreground. There are two possible ways of figuring out the offset at
+which to display the pseudo-cursor:
+
+ . Arbitrarily divide the ligature's glyph width W into N parts,
+ where N is the number of codepoints composed into the ligature, then
+ move that pseudo-cursor by W/N pixels each time a cursor-motion
+ command is invoked;
+ . Use the font information. For example, HarfBuzz has the
+ hb_ot_layout_get_ligature_carets API for that purpose. However,
+ it could be that few fonts actually have that information recorded
+ in them, in which case the previous heuristics will be needed as
+ fallback.
+
+One subtle issue needs to be resolved to have this feature of
+"sub-glyph" cursor movement inside composed characters. The way Emacs
+currently displays the default block cursor is by simply redrawing the
+glyph at point in reverse video. So Emacs currently doesn't have a
+way of displaying a cursor that "covers" only part of a glyph. To
+make this happen, the display code will probably need to be changed to
+draw the cursor as part of drawing the foreground and/or background of
+the corresponding glyph, which is against the current flow of the
+display code: it generally first completely draws the background and
+foreground of the entire text that needs to be redrawn, and only then
+draws the cursor where it should be placed.
+
** Support for Stylistic Sets
This will allow using "alternate glyphs" supported by modern fonts.
For an overview of this feature, see
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