Re: [Indic-computing-devel] How are indic fonts rendered?

[Keyur S. <key...@ya...>]

--- Arun Sharma <ar...@sh...> wrote:
> > > 
> > >   - do (A) and (B) have to be done multiple times?
> > 
> > Yes. It has to be done for each syllable.
> 
> Let me pose the question in a different way - can it be
> done with one
> pass over the character codes ?

No. It has to be done for each syllable. While reordering,
we look at the characteristic of characters, e.g., whether
it is base character or matra, etc. Doing it in one pass is
not possible as it mixes all the characters and it becomes
very difficult to decide at which place a character
sequence should move.

> If not, can you give an
> example of a 
> character code string, where multiple passes would be
> necessary to 
> determine the correct sequence of glyphs ?

Take a simple example,

U+0930 U+094D U+0915 U+094D U+0916 U+0917

In the above sequence there are two syllables,
A. U+0930 U+094D U+0915 U+094D U+0916
B. U+0917

In (A), the first two character should be turned into
'Reph' glyph and place on top of glyph for U+0916 since it
is base character. For that the sequence U+0930 and U+094D
should be moved after U+0917 and the first syllable will be
reordered as 

A1. U+0915 U+094D U+0916 U+0930 U+094D

No reordering is necessary for the second syllable since
there is only one character. Please note that this
reordering is somewhat depends on the design of OpenType
font substitution table (GSUB). Microsoft and Adobe has
published guidelines to design OpenType fonts so that this
reordering remains almost same for all fonts.

This can not be done within one pass because it is
difficult to determine whether U+0916 is a base character
or U+0917 and consequently where to move the sequence
U+0930 U+094D.

> 
> Also a related question - are there any algorithms to
> efficiently
> determine how far back do you go in a string of
> characters (for doing
> a re-layout), when somebody hits a backspace ? Stop at
> the word boundary ?

A generally accepted action for Backspace is that it should
remove the last character from previous syllable. However,
Delete key should remove entire next syllable. Similarly,
cursor should move over the syllables not characters. These
are not standards but people generally feel comfortable
with these operations. The same mechanism has been adopted
in MS Windows 2000/XP. Using the same "Syllable Breaking
State Machine" we can determione how far to move back or
forth.

> 
> Is it correct to state that the state machine that you
> talked about is
> specified in the font file and not a special purpose
> library ?

No. Syllable breaking state machine and Reordering of
characters is not a part of font but they are part of
special purpose library. OpenType font contains only logic
for Substitution and Positioning. And Reordering is
absolutely necessary for Substitution.

However, in ISCII/ISFOC standard, even this substitution
logic is part of the library and not the font. But there
font designer must design the font according to ISFOC
standard layout. In this standard whole GlyphSet has been
defined and each glyph must occupy previously defined
position. ISCII/ISFOC doesn't cover information for
positioning of glyphs and does not provide so called "Rich
Typography".

Regards,
Keyur


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