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[Devel] Opentype feature setting for Devanagari
From: |
Vinod Kumar |
Subject: |
[Devel] Opentype feature setting for Devanagari |
Date: |
Fri, 7 Nov 2003 13:17:55 +0530 (IST) |
Consider the Devanagari Unicode sequence
Ka Virama Ssa Nukta
U915 U94d U937 U93c
In our Opentype font JanaHindi.ttf, there are two tables relevant for
this example.
1) Akhnd Table under the AKHN flag
KaGlyph ViramaGlyph SsaGlyph -> KSsaGlyph
2) Nukta table under NUKT flag
KSsaGlyph NuktaGlyph -> KSsaNuktaGlyph
The Nukta table comes after the Akhnd table in the looks up sequence.
a) Under Volt, the Visual opentype editing tool from Microsoft, the Feature
settings and shaping for the input sequence are:
KaGlyph ViramaGlyph SsaGlyph NuktaGlyph
AKHN AKHN AKHN | NUKT NUKT
|_____________|______________| |
| |
V |
KSsaGlyph |
NUKT |
|__________________________________|
|
V
KSsaNuktaGlyph
b) Under FreeType 1.4 the Feature settings are on different glyphs.
The sequence is shaped in FreeType 1.4 showing the likely intermediate
steps
KaGlyph ViramaGlyph SsaGlyph NuktaGlyph
AKHN | NUKT
|_____________|______________| |
| |
V |
KSsaGlyph |
NUKT |
|__________________________________|
|
V
KSsaNuktaGlyph
Notice two differences between Volt and Freetype 1.4. Volt requires
the Feature Flag to be set for all glyphs to be substituted from
a table under that Feature. But Freetype requires the Feature flag
to be set only for the first glyph.
The second difference is that, under Volt, a derived left glyph
(KSsaGlyph) will inherit the Feature flag (NUKT) from its rightmost
source glyph (SsaGlyph). Under Freetype 1.4, the feature is inherited
from the leftmost source glyph (KaGlyph). Probably, the second difference
is a consequence of the first one.
>From our experience with the IndiX project, we find that the
FreeType practice of setting the Feature flag for the first glyph
is simpler and works as well as the Volt method. But the need
to propogate the Feature flag back to the first glyph of the
preceeding glyph cluster complicates the processing. Can Freetype
allow inheritance of feature flag from the rightmost source
glyph to the derived glyph please?
Vinod Kumar
Project IndiX
- [Devel] Opentype feature setting for Devanagari,
Vinod Kumar <=