[HarfBuzz] Shaping Thai text

[lars.knoll at nokia.com]

On 2/3/11 8:36 AM, "ext Theppitak Karoonboonyanan" <thep at linux.thai.net>
wrote:

>Oops. Forgot to Cc: list.
>
>-- 
>Theppitak Karoonboonyanan
>http://linux.thai.net/~thep/
>
>On Thu, Feb 3, 2011 at 2:35 PM, Theppitak Karoonboonyanan
><thep at linux.thai.net> wrote:
>> On Thu, Feb 3, 2011 at 1:39 PM, Peter Hunter <phunter at atex.com> wrote:
>>> I'm using Harfbuzz (not Harfbuzz-ng) for shaping Thai text (using
>>>HB_BasicShape/HB_HeuristicPosition)
>>>
>>> Not being familiar with the Thai language, I've been told that the
>>>vowels/marks are positioned "too high", (particularly compared to MS
>>>wordPad/Notepad)
>>> At least with the fonts I have access to, the vowels/marks are already
>>>configured to an appropriate height for most situations and do not
>>>require the extra adjustment provided in "positionCluster"
>>>
>>> Disabling the extra adjustment however causes a problem with the
>>>positioning of the combining above marks when used in conjunction with
>>>0xe1b,0xe1d,0xe1f characters( PO PLA, PO FA and PO FAN). (These
>>>characters all have a trailing vertical stem that overlaps the
>>>combining above marks when left at their default position)
>>> In wordpad/notepad these marks are left at their design height, but
>>>positioned left aligned with the base character.
>>> (The fonts appear to have been designed for this situation, but I
>>>don't know if that is true for Thai fonts in general)

The heuristic positioning code should not get used when the font has a
GPOS table. The heuristic positioning code is there as a fallback to
provide readable results. I don't think we can expect any good
typographical results from it in the general case (even though it does
deliver quite ok results in many cases).

>>
>> I also see this weird rendering on Qt applications, and confirm that
>>this is
>> considered sub-optimal typography. Pango, on the other hand, positions
>> the glyphs more elegantly, either by the aids of PUA glyphs or OpenType
>> GPOS feature.

The GPOS features would get used by Qt and harfbuzz.old as well if they
are available in the font. So either Pango has some hand tuned code for
positioning thai diacritics or it uses some extensions for ligated glyphs
in the PUA. Patches to make use of the PUA glyphs are welcome (the hebrew
shaper can act as an example, it also tries to ligate certain glyphs if
the font supports it).

Cheers,
Lars

>>
>> (The PUA solution is still retained for compatibility with Windows,
>>where
>> the lack of OpenType feature is a norm for Thai fonts, so users can
>>still
>> use fonts designed for Windows on it.)
>>
>>> So, I'd like to know if this issue has been resolved in "-ng" or if
>>>anyone else has experienced and/or resolved this problem
>>
>> I wonder how to test HarfBuzz-ng per se to find it out. To my
>>understanding,
>> HarfBuzz-ng is being used in Pango, but the shaping engine which
>> provides the proper positioning is in Pango layer, not HarfBuzz-ng.
>>
>> Regards,
>> --
>> Theppitak Karoonboonyanan
>> http://linux.thai.net/~thep/
>>
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