[Fontconfig] Marking glyphs as deliberately blank, per font

[Qianqian Fang]

Nicolas Mailhot wrote:
> This is about things like
> http://bugs.freedesktop.org/show_bug.cgi?id=20911
> that should not exist at all if the config syntax was clearer for human
> beings ("with X, ... I don't think it can do any harm at all." famous
> last words of one of the commenters, if you knew how often I read
> something like that from a CJK packager word to see a new bug opened a
> month later)
>   
I see you are teasing me here. I thought on this issue, we
are on the same side.

> Actually, it should have been something like:
>
>  <alias>
>    <font>
>      <family>[genericname]</family>
>      <langrequest>[loc]</langrequest>
>      <langcover>[loc]</langcover>
>    </font>
>    <prefer>
>      <font>
>        <family>[fontname]</family>
>      </font>
>    </prefer>
>  </alias>
>
> ...
>
> Many many CJK fontconfig configuration bugs occur because lang (as a rendering
> context) is confused with lang (as specific glyphs in a font)
look, isn't this the same (sort of) as I proposed in the
Comment#16 in the above bug?

I apologize for causing issues when using the fontconfig
files I wrote for several CJK fonts. But I do want to remind you
that this is largely due to the complex nature of CJK, not by
others. Most non-CJK languages don't have the z(shape)-variant
problems as in CJK; and they also don't need to embed bitmaps
to make their glyph readable. So, the reason you see more
complains about CJK is rather a reflection to the insufficiency
to handle these unique problems in the current font rendering
pipeline (I know most of you hate bitmaps).

Talking about syntax, I am sort of agree on both sides.
Yes, using prepend/prepend_first/postpend..., you can do most
things a specific language requires. Indeed, that's what
I used to do, but being blamed badly in the past [1]. In
Fedora's fontconfig guideline, the only "clean" syntax
for setting font orders is "<alias><prefer></prefer></alias>",
anything else, prepend/prepend_first with/wo strong binding,
will be treated as monsters.
And yes, if <alias><prefer> support language tag inside,
it would make these rules looks a lot nicer, and of course,
help me get away from a lot of blames by using the
"fragile" constructs.

So, in summary, I support to add the proposed syntax to
fontconfig; but if you do not get enough support to add this
to fontconfig, then please don't blame cjk people in the
future by using the low-level operations.

Now back to the original thread, I personally don't
think it is much of a CJK issue. I think what was originally
asked is about the granularity of fontconfig: fontconfig
has font-level fallback and glyph-block (in terms of lang)
level fallback, but it does not seem to have glyph-level
fallback. CJK issue can be pretty much solved by the second
level, but glyph-level is a even finer one. I too think
this is something fontconfig developers need to think
of in the future.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=381311#c0