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Re: Font back end font selection process

From: Kenichi Handa
Subject: Re: Font back end font selection process
Date: Mon, 08 Jun 2009 11:49:52 +0900

In article <address@hidden>, Adrian Robert <address@hidden> writes:

> I am working on updating the NS font driver to work with script and  
> friends so that correct nonASCII fonts can be chosen using the  
> default fontset skeleton mechanism.  The back end seems to use these  
> methods to request a font from the list() method:

> - registry in the font spec proper
> - :script property in "extra" properties
> - :lang property in "extra"
> - part of the :otf property bundle in "extra"

> I haven't found a way to respond to the first type of query using  
> Cocoa APIs yet.

In that case, you can simply reject any register other than

> The others that get requested, and the order, seems  
> to depend on the language in question.  In particular, for some  
> languages like Thai, only OTF requests ever seem to get made.  It  
> seems like this class might be the scripts requiring compositional  
> rendering, but why, since emacs used to be able to handle  
> compositional rendering without making use of any OTF-specific  
> properties provided by a font driver?

Emacs 23 still can use non-OTF Thai font if the registry is
tis620 or iso8859-11.  The default fontset has this entry
for Thai.

     (thai  ,(font-spec :registry "iso10646-1" :otf '(thai nil nil (mark)))
            (nil . "TIS620*")
            (nil . "ISO8859-11"))

The reason why I added :otf for "iso10646-1" is that now we
have many OTF Thai fonts usable with Xft font-backend (and
perhaps with uniscribe backend).  OTF Thai fonts provide
better Thai rendering than the simple relative stacking
method of Emacs 22.  But, if OTF is not available on Cocoa,
I'll change the entry for Thai to something like this:

     (thai  ,(font-spec :registry "iso10646-1" :otf '(thai nil nil (mark)))
            ,(font-spec :registry "iso10646-1" :scritp 'thai)
            (nil . "TIS620*")
            (nil . "ISO8859-11"))

Does it solve your problem?

> Also, often I have noticed that when given a Chinese text file  
> (encoded in UTF-8), the only request that comes through is :lang=ja.   

?? For han script, the default fontset has this entry:

     (han (nil . "GB2312.1980-0")
          (nil . "JISX0208*")
          (nil . "JISX0212*")
          (nil . "big5*")
          (nil . "KSC5601.1987*")
          (nil . "CNS11643.1992-1")
          (nil . "CNS11643.1992-2")
          (nil . "CNS11643.1992-3")
          (nil . "CNS11643.1992-4")
          (nil . "CNS11643.1992-5")
          (nil . "CNS11643.1992-6")
          (nil . "CNS11643.1992-7")
          (nil . "gbk-0")
          (nil . "gb18030")
          (nil . "JISX0213.2000-1")
          (nil . "JISX0213.2000-2")
          (nil . "JISX0213.2004-1")
          ,(font-spec :registry "iso10646-1" :lang 'ja)
          ,(font-spec :registry "iso10646-1" :lang 'zh))

So, not only `ja', emacs should try `zh' if `ja' is not
available.  Doesn't it happen on Cocoa?

> How should the font driver know to return a kanji font instead of  
> hiragana / katakana?.

A font driver can return any 'ja' iso10646-1 fonts for this
request (even if the font support only kana):

          ,(font-spec :registry "iso10646-1" :lang 'ja)

If the first font in the returned list doesn't support a
specific han character, Emacs tries another font in the
returned list.

> Wouldn't it would be better to  
> request :script=han, adding :lang=ja or :lang=zh only if emacs has  
> some knowledge that the file IS actually in one of these languages?   
> The file encoding might be one piece of information to take into  
> account, but when it is UTF-8 it would need to run some kind of  
> lexical analysis, or query the user.

If the buffer file is in UTF-8, Emacs currently does this.
If the current lang.  env. is "Japanese", try :lang=ja
before :lang=zh.  If the current lang. env. is
"Chinese-XXX", try :lang=zh before :lang=ja.  Otherwise, try
by the order the default fontset is defined (thus :lang-ja
first).  I've thought that should work well in most cases.

"Some kinf of lexical analysis" is surely very good but
currently we don't have that facility.  And, "query the
user" is too annoying.  I think it is better to provide a
good user interface for specifing a font for each script (or
range of characters).

> I also noticed that if no entities are returned from a list() request  
> with a family and a script specified, it next makes a list() request  
> with no family specified.  Instead of this it would be good to  
> request a match() with the family still specified, as this gives the  
> driver the opportunity to find a font that "looks like" the family  
> (e.g. presence of serifs, etc.), instead of just a random font  
> covering the needed characters.  Indeed, I have not noticed match()  
> being called at all when searching for a font for a script -- instead  
> the back end just goes with the ascii font (and rendering boxes)  
> before ever making such a request.

Ah, that sounds a good idea.  Another way is to allow font
drivers to list also fonts of similar families (sorted by
the closeness of family) and modify font_sort_entities to
preserver the order of lists of other properties than family
are the same.

Kenichi Handa

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