emacs/README.unicode

                                            -*-mode: text; coding: latin-1;-*-

Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008
  Free Software Foundation, Inc.
See the end of the file for license conditions.

Problems, fixmes and other unicode-related issues
-------------------------------------------------------------

Notes by fx to record various things of variable importance.  handa
needs to check them -- don't take too seriously, especially with
regard to completeness.

 * SINGLE_BYTE_CHAR_P returns true for Latin-1 characters, which has
   undesirable effects.  E.g.:
   (multibyte-string-p (let ((s "x")) (aset s 0 ?£) s)) => nil
   (multibyte-string-p (concat [?£])) => nil
   (text-char-description ?£) => "M-#"

	These examples are all fixed by the change of 2002-10-14, but
	there still exist questionalble SINGLE_BYTE_CHAR_P in the
	code (keymap.c and print.c).

 * Rationalize character syntax and its relationship to the Unicode
   database.  (Applies mainly to symbol an punctuation syntax.)

 * Fontset handling and customization needs work.  We want to relate
   fonts to scripts, probably based on the Unicode blocks.  The
   presence of small-repertoire 10646-encoded fonts in XFree 4 is a
   pain, not currently worked round.

	With the change on 2002-07-26, multiple fonts can be
	specified in a fontset for a specific range of characters.
	Each range can also be specified by script.  Before using
	ISO10646 fonts, Emacs checks their repertories to avoid such
	fonts that don't have a glyph for a specific character.

	fx has worked on fontset customization, but was stymied by
	basic problems with the way the default face is dealt with
	(and something else, I think).  This needs revisiting.

 * Work is also needed on charset and coding system priorities.

 * The relevant bits of latin1-disp.el need porting (and probably
   re-naming/updating).  See also cyril-util.el.

 * Quail files need more work now the encoding is largely irrelevant.

 * What to do with the old coding categories stuff?

 * The preferred-coding-system property of charsets should probably be
   junked unless it can be made more useful now.

 * find-multibyte-characters needs looking at.

 * Implement Korean cp949/UHC, BIG5-HKSCS and any other important missing
   charsets.

 * Lazy-load tables for unify-charset somehow?

	Actually, Emacs clear out all charset maps and unify-map just
	before dumping, and their are loaded again on demand the
	dumped emacs.  But, those maps (char tables) generated while
	temacs is running can't be get rid of from the dumped emacs.

 * Translation tables for {en,de}code currently aren't supported.

	This should be fixed by the changes of 2002-10-14.

 * Defining CCL coding systems currently doesn't work.

	This should be fixed by the changes of 2003-01-30.

 * iso-2022 charsets get unified on i/o.

	With the change on 2003-01-06, decoding routines put `charset'
	property to decoded text, and iso-2022 encoder pay attention
	to it.  Thus, for instance, reading and writing by
	iso-2022-7bit preserve the original designation sequences.
	The property name `preferred-charset' may be better?

	We may have to utilize this property to decide a font.

 * Revisit locale processing: look at treating the language and
   charset parts separately.  (Language should affect things like
   speling and calendar, but that's not a Unicode issue.)

 * Handle Unicode combining characters usefully, e.g. diacritics, and
   handle more scripts specifically (à la Devanagari).  There are
   issues with canonicalization.

 * Bidi is a separate issue with no support currently.

 * We need tabular input methods, e.g. for maths symbols.  (Not
   specific to Unicode.)

 * Need multibyte text in menus, e.g. for the above.  (Not specific to
   Unicode -- see Emacs etc/TODO, but now mostly works with gtk.)

 * There's currently no support for Unicode normalization.

 * Populate char-width-table correctly for Unicode chanaracters and
   worry about what happens when double-width charsets covering
   non-CJK characters are unified.

 * Emacs 20/21 .elc files are currently not loadable.  It may or may
   not be possible to do this properly.

	With the change on 2002-07-24, elc files generated by Emacs
	20.3 and later are correctly loaded (including those
	containing multibyte characters and compressed).  But, elc
	files generated by 20.2 and the primer are still not loadable.
	Is it really worth working on it?

 * Rmail won't work with non-ASCII text.  Encoding issues for Babyl
   files need sorting out, but rms says Babyl will go before this is
   released.

 * Gnus still needs some attention, and we need to get changes
   accepted by Gnus maintainers...

 * There are type errors lurking, e.g. in
   Fcheck_coding_systems_region.  Define ENABLE_CHECKING to find them.

 * You can grep the code for lots of fixmes.

 * Old auto-save files, and similar files, such as Gnus drafts,
   containing non-ASCII characters probably won't be re-read correctly.



New font handling mechanism with font backend method
----------------------------------------------------

Emacs now contains new codes for handling fonts by multiple font
backends.  The old font handling codes still exist completely parallel
to the new codes, and the new codes are used only when you configure
Emacs with the argument "--enable-font-backend".

Which font backends to use can be specified by X resource
"FontBackend".  For instance, if you want to use Xft fonts only,

Emacs.FontBackend: xft

will work.  If this resource is not set, Emacs tries to use all font
backends available on your graphic device.

The configure script, if invoked with "--enable-font-backend", checks
if libraries freetype and fontconfig exist.  If they are both
available, macro "USE_FONT_BACKEND" is defined in src/config.h.  In
that case, the existing of Xft library is checked too.

The new files are:
	font.h -- header providing font-backend related structures
		(most important ones are "struct font" and "struct
		font_driver"), macros, and etc.
	font.c -- main font handling code.
	xfont.c -- font-driver on X for X core fonts.
	ftfont.c -- generic font-driver for FreeType fonts providing
		device-independent methods of struct font_driver.
	xftfont.c -- font-driver on X using Xft for FreeType fonts
		utilizing methods provided by ftfont.c.
	ftxfont.c -- font-driver on X directly using FreeType fonts
		utilizing methods provided by ftfont.c.
	w32font.c -- font driver on w32 using Windows native fonts,
		corresponding to xfont.c

So we already have codes for X.  For the other systems (w32 and mac),
it seems that we need these files:
	atmfont.c -- font-driver on mac using ATM fonts, corresponding
		to xfont.c
As BDF fonts are currently used on w32, we may also implement these:
	bdffont.c -- generic font-driver for BDF fonts, corresponding to
		ftfont.c
	bdfw32font.c -- font-driver on w32 using BDF fonts,
		corresponding to ftxfont.c
But, as FreeType already supports BDF fonts, if FreeType and
Fontconfig are also available on w32, what we need may be:
	ftw32font.c -- font-driver on w32 directly using FreeType fonts
		utilizing methods provided by ftfont.c.

And, for those to work, macterm.c and macfns.c must be changed by the
similar way as xterm.c and xfns.c (the parts "#ifdef USE_FONT_BACKEND"
... "#endif" should be checked).

It may be interesting if Emacs supports a frame buffer directly and
have these font driver.
	ftfbfont.c -- font-driver on FB for FreeType fonts.
	bdffbfont.c -- font-driver on FB for BDF fonts.

Note: The fontset related codes are not yet matuared to work well with
the font backend method.  So, for instance, even if you start Emacs
as something like this:
  % emacs -fn tahoma
Non-ASCII Latin characters will not be displayed by the font "tahoma".
In such a case, please try this:

(set-fontset-font "fontset-default" 'latin '("tahoma" . "unicode-bmp"))


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