On 21/09/10 13:21, Kenichi Handa wrote:
For certain combinations, such as Tai Tham's "Medial R" which needs to
precede the base consonant, using ccmp substitutions will work. This
requires creating a lot of "ligature" glyphs. But it works.
I think it's far from:
a "pure" version of the font that uses only proper OpenType features
It may not be "pure" but it may be pragmatic. If you want a font to work
on Windows and Linux today, then taking advantage of the default GPOS
and GSUB features which are already enabled is a reasonable approach.
The Myanmar 5.1/5.2 Unicode fonts that exist take advantage of features
such as clig, liga, kern and in one case mark/mkmk to give correct
rendering under Uniscribe and in some cases Harfbuzz-NG and Pango
without having a specific shaper.
There are 2 ways to get reordering working without a script specific
shaper. Ligatures is probably faster, but requires a lot of glyphs,
which will probably need to be autogenerated. The other approach is
using multiple context specific substitutions to perform the insertion
and then remove the unwanted glyphs from their original position. The
later requires a lot of lookups and may need guard glyphs to be inserted
temporarily between syllables to prevent reordered glyphs matching
unordered patterns.
I think it is a shame that Firefox 4 is not enabling Harfbuzz-NG for
complex scripts, at least for those which don't have a specific platform
shaper. Uniscribe on Vista and Win 7 should support default features
for these scripts, but many computers still run Win XP and don't have
Complex Script support specifically enabled since it is off by default.
Perhaps harfbuzz could be enabled at least if the web developer actively
uses the CSS3 font-feature-settings or font-language-override. This
might help encourage people to move away from quasi-Unicode fonts for
these scripts, to fonts which properly conform to the standard.
The right approach will be:
(1) Write a specification document something like what
Microsoft does for various scripts:
<http://www.microsoft.com/typography/SpecificationsOverview.mspx>
This should be a base of Tai Tham font& layout-engine
developpers, and for that, it may be good that this document
is officially approved by a proper Tai Tham authority
(perhaps by the one who proposed Tai Tham script to Unicode
or ISO10646).
(2) Implement a Tai Tham layout engine as a pango module or
a harfbuzz-ng's shaper according to (1).
With some scripts it is possible that there is still insufficient
information available to write a reliable shaper that will work with
every language using the script. In the case of Myanmar, a shaper was
written for the original harfbuzz for one interpretation of the
ambiguous Unicode 4 specification for Myanmar. However, that was never
updated for Unicode 5.1 which removed the ambiguities and actively
prevents correct rendering of fonts which would work with the default
shaper. Fortunately Pango doesn't use that shaper, though other apps do.
In the interim, I would suggest that if (1) and (2) are implemented for
a script without a current shaper on all platforms, that it actively
checks whether the font has been written for that shaper e.g. by
checking for the presence of non-default features, which would only be
present in a font specifically targeting that shaper. If those features
are not present, then it should just use the default features. There is
also the danger that even if (1) is written there is no guarantee that
other rendering engines will implement it in the same way, but I admit
it will give a faster rendering solution for the long term.
By the way, for OpenType font testing I use grsvg with its python GTK
user interface. You can build it from
http://scripts.sil.org/svn/graphite/grutils/trunk on Linux. Use CMake
for the build and install the ICU and harfbuzz-ng libraries first. It
will allow you to compare the output of the 2 renderers and if graphite
/ graphiteng is available it will also show the graphite rendering. It
is using SVGs as an intermediate format underneath (not SVG fonts), so
it won't show hinting, but is alright for testing OT/Gr rendering.
Best regards,
Keith Stribley
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