[HarfBuzz] Question about HarfBuzz-ng CTL Shaper Status

Ed ed.trager at gmail.com
Thu Sep 16 14:43:21 PDT 2010

Thanks for the very useful information, Jonathan!

What I am seeing so far (for Tai Tham) is that I can get GSUB "ccmp"
with "dflt" ligature substitutions to work on both FF-3.6x,
FF-4x-linux, and FF-4x-osx.  But GPOS "mark" with "dflt" mark-to-base
don't work on any of these.

The only way I can get diacritical marks to position somewhat
correctly is by using negative left bearings - which I would prefer to
not do.  But obviously I may have to use negative left bearings if
that is the only way that actually works ...

Overall all I can say is: yuck! :-(

I think what I would like to do, ideally, is create a "pure" version
of the font that uses only proper OpenType features.  Then after that,
I could add certain "corruptions" like negative left bearings.  But as
it currently stands, I don't even know what to use to test the "pure"
version of such a font ...

Any suggestions?

> On Linux, we're not yet using harfbuzz-ng at all (work is in progress to move in that direction, but it's not in current betas). So *all* text there is going through Pango at present.
>> Also, if I am trying to test Tai Tham Unicode fonts which utilize
>> OpenType features such as mark-to-base (mark), mark-to-mark (mkmk),
>> and contextual ligatures (clig) --but doing so using Latin "dflt"
>> since there is no tag for Tai Tham-- is Firefox 4x really using
>> Harfbuzz-ng or not?
>> It seems possible --even likely given the kinds of results I am seeing
>> so far-- that Firefox is perhaps marking the runs of text as being Tai
>> Tham and so would not be using the new Harfbuzz-ng
> That's what I would expect, yes. Firefox divides the text according to scripts (based on the Unicode character codes), and then chooses whether to use harfbuzz-ng or platform text layout libraries for each script run. It currently uses HB *only* for the major "simple" scripts, not for those requiring complex shaping.
>> And is there any possible way to know what renderer FF is really using
>> for any given run of text.  I guess the answer is probably not ...
> Not really, short of reading the source to determine how it will behave. :)
> JK

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