From: Peter Constable (petercon@microsoft.com)
Date: Wed Mar 22 2006 - 09:55:20 CST
> From: Philippe Verdy [mailto:verdy_p@wanadoo.fr]
> I don't think that the Saysettha OT font looks like typewriter style. It
> has multiple glyphs for several internal variant sizes and positions of
> diacritics or base letters, and they are chosen using only
> substitution/positioning basic TrueType tables, and reordering issues (at
> code point level) can also be achieved using glyph IDs within substitution
> tables (however I wonder what is the limitation of the subst tables in
> TrueType engines, in terms of recursive substitutions).
Lao does not require any re-ordering.
Substitution and positioning require OpenType Layout tables. The statement was that a Lao font can be implemented without any OpenType Layout, and my clarification was that this entails not having any substitution or positioning, which gives a typewriter look.
> A OpenType engine would perform glyph substitution faster using much less
> complex feature rules in fonts and internal rules specific for a script,
> but this does not mean that these rules can't be coded in classic
> subst/posn TrueType tables.
There are no "classic subst/posn" TrueType tables. Apple implemented an extension to TrueType known as TrueType GX, later updated to become AAT, and that added additional tables for substitution and reordering. Microsoft implemented an extension to TrueType known as TrueType Open, later updated to become OpenType, and that added additional tables for substitution and positioning. "Classic TrueType" does not have any mechanisms to support substitution, reordering or positioning.
> Indic scripts are not so complex to encode given that they normally work
> with clusters that are encoded in a normalized order:
...
> Once you realize that this is the normal way to write those scripts for
> normal language, the substitution tables to create half-forms, special
> forms, subjoined forms and ligatures are easy to generate automatically
> from a set of linguistic rules for that script...
IMO, you are once again speaking as though you have a good understanding of the topic when you do not.
Peter Constable
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