RE: [Indic-computing-devel] Testing OpenType support

[Andy W. <And...@bt...>]

Arun M wrote:

> My knowledge of OpenType is very limited. Here is the issue. 
> http://www.microsoft.com/typography/otfntdev/indicot/features.htm

This document was written at a long time ago, when OpenType for Indic
scripts was still in the early planning stage, it has only been
partially updated. Most of it has been rewritten as separate script
specific documents. Unfortunately, the Malayalam specification has been
left behind.  

> It says for some consonants (eg NNA) halant form will be
> represented by chillaksharam. My understanding is
>  <NNA><virama>  will form chillu NNA.

The form that any given isolated 'Consonant + Virama' will be
subsequently rendered as, is not prescribed in the Unicode standard; so
implementers are free to call any feature they like.  However, all
current implementations call feature 'haln' for the sequence, 'Consonant
+ Virama'.

The sequence Consonant+ Virama+ZWNJ has been prescribed to render as a
consonant with a visible virama (chandrakkala)
All current implementations call feature 'haln' for this sequence also.
Hence it seems that feature 'haln' should render a visible chandrakkala

The Unicode sequence 'Consonant + Virama + ZWJ' is equivalent to the
ISCII sequence 'Consonant + Virama + Nukta'. In ISCII, 'Consonant +
Virama + Nukta' is prescribed to form the chillu forms of consonants. As
Unicode is modelled on ISCII, it follows that Unicode sequence,
'Consonant + Virama + ZWJ' should also be used to render the Chillu
letters.
Current implementations call feature 'half' for this sequence, hence it
seems that feature 'half' should deal with the formation of the chillu.

 
<NNA><virama><ZWNJ> will make virama visible as it is and consonant will
be in base form. 

>How does NNA+virama and NNA+virama+ZWNJ should differ ?
The Unicode standard allows the freedom for 'Consonant + Virama' &
'Consonant + Virama+ ZWNJ' to differ in appearance, if it is deemed
appropriate by an implementation (usually they would not differ).


Andy