On CJK font selection (was Re: [Fwd: Re: Request for review and advice on wqy-bitmap-fonts fontconfig settings])

[Behdad Esfahbod <behdad behdad org>]

Hi Qianqian,

[CC'ing to gtk-i18n-list, so hopefully this is the last time I have to
repeat this.]

On Mon, 2007-12-10 at 18:01 -0500, Qianqian Fang wrote:
> 
> Go back to the digit font change issue as we discussed earlier, I
> spent some time in the past few days, trying to get myself a more
> clear 
> picture on this. I dug out some bug reports from various bugzillas
> (Mozilla, Redbat, Gnome) and gathered a list of similar reports (see
> the bottom of the email). These reports were filed from simplified and
> traditional Chinese users and Japanese users (I believed Korean
> experienced the same problem).  So, one thing that can be said from
> this list is that the "contextual font selection" does seem to be
> bothering CJK users in text formatting. 

Yes, you have identified the problem very accurately.


> I understand that "contextual shaping" is one of the techniques for
> rendering complex scripts. I am not sure how tight is the connection
> between "contextual shaping" and the "contextual format propagation",
> but one thing that I think may put some light to the complains of the
> CJK users is that Chinese (maybe Japanese as well) scripts are not
> contextual sensitive. Chinese characters are relatively independent
> and self-consistent in shapes (while, this statement is not true for
> Chinese calligraphy, where strokes may connect between characters
> depending on layout direction, but the current OSs and font
> technologies are not ready to handle this IMO). The only complexities
> may come from the fact that Hanzi for printing are mostly equal-width,
> and the punctuations among the Hanzi are expected to match the width
> of the surrounding Hanzi. As the full-width punctuations being encoded
> separately by Unicode, together with the contextual punctuation
> support of the input-methods, this seems to be handled very well. So,
> in short, for Chinese text layout, users are generally not expected to
> see contextual-based changes, either encoding/glyph or font faces
> (this may not include some extreme cases). 

And Pango supports those all perfectly fine.  Even vertical writing
using the correct substituted punctuation glyphs.  See:

  http://www.pango.org/ScriptGallery


The main font issue though, is that Chinese (Simplified, Traditional),
Korean, and Japanese share some Unicode code points, but they require
slightly different renderings.  Now if you don't tell Pango which
version is preferred, how can it know which font to choose?  It
explicitly doesn't prefer any one over the others to avoid cultural
problems.

The symptoms of this problem are "multiple fonts used in the same line".
Solution is: Either run under a CJK locale, or give hints to Pango about
your preferred CJK locale using the env var PANGO_LANGUAGE.

Note that theoretically Pango can do text analysis to come up with a
best guess, but doing that would then introduce another bug with
symptoms "changes font when typing a few characters on the same line".


> Now go back to pango, from what I read from the bug reports, pango
> uses PANGO_SCRIPT_COMMON to represent language-independent symbols. I
> have no complain about that. It is a good classification based on the
> semantics of the symbols.

Good.  Let me also note that there's no way to change that.  It's
hardcoded in the Unicode standard.


> What I, and most CJK users, are not satisfied with is the
> contextual-sensitivity of those common scripts when for mating text
> under cjk locales. I know that you have advocated to stick with the
> "face" meaning of SCRIPT_COMMON, which is supposedly to be rendered by
> local languages. But IMO, the face meaning is misleading here. From a
> Chinese user perspective, the difference between the SCRIPT_COMMON to
> Latin is negligible,

Lemme correct you here, "From a Chinese user perspective, the ASCII
digits are considered Latin".  There's sure a lot more than ASCII digits
to SCRIPT_COMMON.  Helps to be precise.


> compared with its difference to CJK characters. Therefore, using CJK
> fonts to render SCRIPT_COMMON is quite odd. Using Latin fonts for
> COMMON is most preferred; even specifying no face ( i.e. using system
> fall-back) is better than assigning Chinese fonts for these scripts
> for that most Chinese fonts have low-quality Latin/common glyphs, even
> the commercial ones.

And this problem has a name: "crappy glyphs and multiple scripts in a
font".  Tell me about it...

I already pointed out a few solutions to it previously:

  - Rip the crap out and everyone will feel better.

  - Use TrueType containers (even for bitmap-only fonts) and put each
script's glyphs into its own face, with all faces having the same name
and put into the same TrueType Collection file.

  - Finish patch for fontconfig to allow configuration to disable
certain Unicode codepoints per font.  The write such configuration for
the crappy glyphs.

Pick whichever you prefer and just do it.


Another symptom, "digits change font after typing character" is in fact
a very cool Pango feature, just badmouthed by the above problem.  Fix
the problem.


> As you see from the bug lists, this problem has existed for many
> years, and I am pretty sure that it will come back again and again, as
> long as the expected rendering is not achieved. If the current pango
> formatting logic is not sufficient to handle the CJK preferences as
> said above, I think to refine the logic to take it into consideration
> is better than stick with a fixed but incomplete logic. 

I consider patches improving Pango's font selection algorithm, but none
that I've seen so far had been an improvement (from my point of view).
If it has words like CJK or "special case", I'm most probably not
interested.  Of the bugs you listed, only the one I opened myself is
valid IMO.  The rest is just left open because no matter how many times
I close them, they will be reopened... Oh well.


> please let me know your thoughts and reasoning on whether this is
> feasible or not, if yes, where to get start.

Does the above make sense?  I understand that it's easier to apply a two
line patch to Pango instead of doing what of the things I listed above,
but that just doesn't fit in the design, and it introduces other
problems you don't see right now.


> thank you for paying attention to this issue.
> 
> Qianqian

Regards,

behdad


> =============================================================== 
> Bug 321113 - Wrong glyph subsituation algorithm for digital characters
> and punctuations
> http://bugzilla.gnome.org/show_bug.cgi?id=321113
> 
> 
> Bug 345072 - changes font when typing different scripts on the same
> line 
> http://bugzilla.gnome.org/show_bug.cgi?id=345072
> 
> 
> Bug 345386 - Language and direction propagation in and between
> PangoLayouts
> http://bugzilla.gnome.org/show_bug.cgi?id=345386  (opened by yourself)
> https://bugzilla.redhat.com/bugzilla/attachment.cgi?id=103679
> 
> 
> Bug 481210 - [All lang] [firefox] - Face of the number is changing
> when enter number + Char, in any Locale
> http://bugzilla.gnome.org/show_bug.cgi?id=481210
> 
> 
> Bug 481188 - ascii text space too narrow for Chinese encodings
> http://bugzilla.gnome.org/show_bug.cgi?id=481188
> 
> 
> Bugzilla Bug 129541: changes font when typing different scripts on the
> same line 
> https://bugzilla.redhat.com/show_bug.cgi?id=129541
> 
> 
> Bugzilla Bug 131218: [RHEL4] Characters get truncated in new pango
> https://bugzilla.redhat.com/show_bug.cgi?id=131218
> 
> 
> Bugzilla Bug 149991: [CJK pango] digits and punctuation in textbox
> give bad eol rendering and cursor placement
> https://bugzilla.redhat.com/show_bug.cgi?id=149991 (filed by Jens
> Petersen)
> 
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=220885 (broken link)
> 
> 
> Bugzilla Bug 228804: [All lang] [firefox] - Face of the number is
> changing when enter number + Char, in any Locale
> https://bugzilla.redhat.com/show_bug.cgi?id=228804
> 
> 
> Bugzilla Bug 221361: [pango] ascii text space and punctuation is
> narrow for CJK
> https://bugzilla.redhat.com/show_bug.cgi?id=221361
> 
> 
> Bug 379125 - chinese punctuations after english letters are wrongly
> displayed
> https://bugzilla.mozilla.org/show_bug.cgi?id=379125
> https://bugzilla.mozilla.org/attachment.cgi?id=263185
> ===============================================================

-- 
behdad
http://behdad.org/

...very few phenomena can pull someone out of Deep Hack Mode, with two
noted exceptions: being struck by lightning, or worse, your *computer*
being struck by lightning.  -- Matt Welsh