Re: logical order (and Tamil)

Date: Mon Jul 29 2002 - 12:19:12 EDT

On 07/27/2002 01:17:38 PM "Michael \(michka\) Kaplan" wrote:

>The incompatibility of Linear Tamil in its current form is not with
>technically; it is with the existing mechanisms of shaping engines used by
>Apple, Microsoft, and others.

I think you'll find that there is no incompatibility with existing Apple
shaping engines. There may be incompatibility with some existing AAT fonts,
but not the engine itself. Similarly, both re-ordrant and "linear" Tamil
rendering can be implemented in Graphite without requiring any changes in
the rendering engine. Of the three smart-font implementations I'm aware of,
it's only OpenType -- more precisely, the OT-related rendering engines
(Uniscribe, etc.) -- that this applies to.

Graphite, AAT and OT can all support implementations that allow rendering
of Tamil *both* ways, with the choice controlled by a font feature. This
can be done now in AAT and Graphite without requiring any changes in the
shaping engine, but it would require changes in OT-related shaping engines
to support this.

The key distinction is that AAT and Graphite have all script behaviour
encapsulated in state machines in the font, whereas in the OT model,
handling of script behaviour is divided between the font and the layout
engine, specifically with any re-ordering issues handled by the layout
engine. There are pros and cons of each approach. Situations like this --
wanting to implement (even if only a prototype for demonstration purposes)
"linear" Tamil rendering -- run into the cons of the OT model. (Fortunately
for OT implementers, these situations aren't likely to be common.

- Peter

Peter Constable

Non-Roman Script Initiative, SIL International
7500 W. Camp Wisdom Rd., Dallas, TX 75236, USA
Tel: +1 972 708 7485
E-mail: <>

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