Re: ucs-normalize and diacritics

[Robert Pluim <rpluim@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Robert Pluim <rpluim@gmail.com>
>> Cc: emacs-devel@gnu.org
>> Date: Wed, 25 Jul 2018 09:40:34 +0200
>> 
>> I think Iʼll start by putting pointers to auto-composition-mode in the
>> manual and lispref.
>
> Thanks in advance.

Hereʼs a first stab at it, intended for emacs-26. Probably the unicode
characters below will not survive intact.

diff --git i/doc/emacs/mule.texi w/doc/emacs/mule.texi
index 401c83dd49..925a582a4c 100644
--- i/doc/emacs/mule.texi
+++ w/doc/emacs/mule.texi
@@ -225,6 +225,51 @@ International Chars
   decomposition: (101 770) ('e' '^')
 @end smallexample
 
+@cindex diacritic
+@cindex composition
+  Sometimes Emacs will display a single character even when the buffer
+contains multiple characters, through a process known as @dfn{composition}.
+This is done via @code{auto-composition-mode}, which is enabled by default,
+and can only be done if the characters to be composed all exist within
+the same font.  The exact rules for which characters to compose are
+defined by the Unicode standard, but generally they concern
+diacritical marks such as accents.
+
+  For a successfully composed character, @kbd{C-u C-x =} displays
+details about the base character and the following character(s) it is
+composed with.  For example for @samp{e} composed with @samp{COMBINING
+CIRCUMFLEX ACCENT}, which visually would be very similar to the
+previous example, the output would look like:
+
+@smallexample
+             position: 146 of 147 (99%), column: 0
+            character: e (displayed as e) (codepoint 101, #o145, #x65)
+    preferred charset: ascii (ASCII (ISO646 IRV))
+code point in charset: 0x65
+               script: latin
+               syntax: w 	which means: word
+             category: .:Base, L:Left-to-right (strong), a:ASCII, l:Latin,
+                       r:Roman
+             to input: type "C-x 8 RET 65" or "C-x 8 RET LATIN SMALL LETTER E"
+          buffer code: #x65
+            file code: #x65 (encoded by coding system utf-8-unix)
+              display: composed to form "ê" (see below)
+
+Composed with the following character(s) "Ì‚" using this font:
+  xft:-PfEd-DejaVu Sans Mono-normal-normal-normal-*-27-*-*-*-m-0-iso10646-1
+by these glyphs:
+  [0 1 101 72 16 2 15 21 0 nil]
+  [0 1 770 650 0 4 12 22 -17 [-16 0 0]]
+
+Character code properties: customize what to show
+  name: LATIN SMALL LETTER E
+  general-category: Ll (Letter, Lowercase)
+  decomposition: (101) ('e')
+
+There are text properties here:
+  fontified            t
+@end smallexample
+
 @node Language Environments
 @section Language Environments
 @cindex language environments
diff --git i/doc/lispref/nonascii.texi w/doc/lispref/nonascii.texi
index 4d75d6a1f1..c1943dda1c 100644
--- i/doc/lispref/nonascii.texi
+++ w/doc/lispref/nonascii.texi
@@ -500,10 +500,14 @@ Character Properties
 character's classification.  For unassigned codepoints, the value
 is @code{Cn}.
 
+@cindex diacritic
+@cindex composition
 @item canonical-combining-class
 Corresponds to the @code{Canonical_Combining_Class} Unicode property.
 The value is an integer.  For unassigned codepoints, the value
-is zero.
+is zero.  Emacs can use this to visually compose multiple characters,
+using @code{auto-composition-mode}, if all the characters concerned
+exist in the same font.
 
 @cindex bidirectional class of characters
 @item bidi-class