Pango and Indic scripts

I've been paying some attention recently to the rendering of
various
scripts in Pango. One thing I've noticed is that there are a
number of
cases where Pango seems to fall short in rendering Indic
text. I would
like to help out with this, but I find that the code is
poorly
documented and not easy to follow. As such, I'm presenting
my findings
here in the hope that someone can lend me their expertise or
use what
I've found to improve support. For reference, these are
problems I've
found in 1.17.2. Most are present in 1.16.3 as well, to the
best of my
knowledge.

First, for some reason, the sequence "ka + VIRAMA +
ZWJ" fails to
produce the expected half-form. This should be handled by
the `half'
feature of OpenType, which is apparently supported by the
shaper. It
seems as though the characters are not being given the
proper tag.
However, I'm unable to follow the code well enough to
determine why or
even really what tags things should be given in what
circumstances.

Second, many features seem to be entirely ignored. The
shaper creates a
ruleset with `akhn', `haln', `nukt' and `vatu'. These tags
are never
used, though the Devanagari font that I'm currently looking
at makes use
of all of them. Again, I'm not sure exactly what the
different property
sets are trying to accomplish and I can't follow the code
well enough to
figure out precisely what they're doing. As such, I'm
unclear as to when
these tags should be used.

If anyone could shed some light on the situation, especially
to clarify
the code (particularly indic_ot_reorder in
modules/indic/indic-ot.c), I
would appreciate it very much and I would endeavor to
improve Indic
support in the development branch.


Sean Burke
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On 6/12/07, Sean Burke <leftmostcatgmail.com> wrote:
> I've been paying some attention recently to the
rendering of various
> scripts in Pango. One thing I've noticed is that there
are a number of
> cases where Pango seems to fall short in rendering
Indic text. I would
> like to help out with this, but I find that the code is
poorly
> documented and not easy to follow. As such, I'm
presenting my findings
> here in the hope that someone can lend me their
expertise or use what
> I've found to improve support. For reference, these are
problems I've
> found in 1.17.2. Most are present in 1.16.3 as well, to
the best of my
> knowledge.
>
> First, for some reason, the sequence "ka + VIRAMA
+ ZWJ" fails to
> produce the expected half-form. This should be handled
by the `half'
> feature of OpenType, which is apparently supported by
the shaper. It
> seems as though the characters are not being given the
proper tag.
> However, I'm unable to follow the code well enough to
determine why or
> even really what tags things should be given in what
circumstances.
>

could you tell which font you are using? for quite sometime
devanagari
rendering has been considered working and stable.
I am able to generate same seq. using Lohit Hindi, Gargi,
FreeSans .

> Second, many features seem to be entirely ignored. The
shaper creates a
> ruleset with `akhn', `haln', `nukt' and `vatu'. These
tags are never
> used, though the Devanagari font that I'm currently
looking at makes use
> of all of them. Again, I'm not sure exactly what the
different property
> sets are trying to accomplish and I can't follow the
code well enough to
> figure out precisely what they're doing. As such, I'm
unclear as to when
> these tags should be used.
>

the situation now is there are devanagri fonts which work
well with
pango , but maynot be implementing all opentype rules
strictly (as per
microsoft typography docs).

> If anyone could shed some light on the situation,
especially to clarify
> the code (particularly indic_ot_reorder in
modules/indic/indic-ot.c), I
> would appreciate it very much and I would endeavor to
improve Indic
> support in the development branch.
>

maybe good if you could outline all the problems you are
finding in
rendering so it could be decided whether the fault is (pango
or the
fonts).

Karunakar
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Scríobh G Karunakar:
> could you tell which font you are using? for quite
sometime devanagari
> rendering has been considered working and stable.
> I am able to generate same seq. using Lohit Hindi,
Gargi, FreeSans .
>   
The Kalimati font produced this problem. Using Lohit Hindi,
I was able
to get those features working, but Kalimati seems to have
everything in
place to be working. It just looks like it uses different
mechanisms
than Lohit. (The font is available at
http:
//www.thdl.org/thdl/tools/dls/Kalimati.ttf)
> maybe good if you could outline all the problems you
are finding in
> rendering so it could be decided whether the fault is
(pango or the
> fonts).
>   
With Kalimati, combinations like K.SSA or L.LA fail, as do
all explicit
half-forms. I hadn't tested anything beyond that, but I'm
sure other
features fail. (I don't know Hindi, however, so I have
difficulty coming
up with good test cases.) Now, with Lohit Hindi, there are
some issues
lining up the stem. This occurs in a few places in the
phrase I'm
looking at. I can provide a screencap and the text if so
desired. This
is a relatively minor issue, though, and could be anything
from the font
to FreeType. I'd like to see if Kalimati could be made to
work, however.
Thanks for your swift reply.


Sean Burke
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