Re: Do combinations need to be defined in advance?

[Christopher Fynn <cfynn@xxxxxxx>]

Rich Felker wrote:

> OK, so basically it's a workaround for poor OpenType implementations.
> Got it. Thanks for the explanation.

Well I don't know whether or not the implementations are poor but if you 
use a lot of reverse chaining contextual lookups for Tibetan isn't there 
bound to be a lot more processing overhead? When you can get pages and 
pages of Tibetan text without a paragraph break, entering a single 
character in a block of Tibetan text may mean the whole thing has to be 
re-rendered. A good implementation is going to to things much more 
efficiently than a bad one - but in either case, the more complex the 
lookups, the more processing overhead there is is going to be.

In Microsoft's earlier implementations of Tibetan shaping, positioning 
lookups were *very*, *very* slow - in experiments with large documents 
characters sometimes didn't appear on screen for a second or two after 
you typed them - so I always tried to avoid these lookups. In that 
regard things seem to have improved considerably in latest versions of 
Uniscribe. I also understand that every lookup in a font used to result 
in a separate call to their shaping engine - now I think, for Tibetan, 
everything is applied in one pass.

It seems Pango currently doesn't apply all the OT features required for 
Tibetan script in it's implementation ("ccmp" & "kern" are missing) - 
but a bunch of OT features not used in any Tibetan font ("pref", "pres", 
"blwf", "abvf" &  "pstf") are present.

It would probably also be worthwhile checking if all the types of 
lookups that should be supported under blwm & abvm (GPOS lookups type 4 
& 5) are working properly in Pango. For the above stated reasons I've 
tended to avoid these GPOS lookups in Tibetan fonts so wouldn't know 
whether or not they are working properly in Pango.

> BTW, I heard there was a mailing list specifically for Tibetan
> font/script issues. Is that still active, and if so, how can I
> subscribe?

There are two:

Tibex at Unicode dot org - mostly for Tibetan character encoding issues
not font issues. You can subscribe to that list by sending an email
to ecartis at unicode dot org with subscribe Tibex in the body of the mail.

Tibetscript is a list for Tibetan script issues in general. Again not 
font specific. See:
<http://list.mail.virginia.edu/mailman/listinfo/TibetScript>

For OpenType font issues related to Tibetan script you might have better 
luck on the OpenType list. People involved with writing all the main OT 
layout engines, and many people making fonts for complex scripts are 
subscribed to that list. Microsoft's VOLT list 
MicrosoftVOLTuserscommunity at groups dot msn dot com can also be useful.

 - Chris


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