From: John Hudson <>
Date: Tue, 23 Aug 2011 15:57:30 -0700

Philippe Verdy wrote:

> Rereading closely the OpenType spec...

I suggest you read also the script-specific OT layout specifications.

You'll note, for example, that the Arabic font spec doesn't even mention
BiDi, because it is assumed that this has been resolved before glyph
runs for OTL processing are even identified. This makes sense to me
because BiDi is a character-centric operation.

The Microsoft font specs describe what Uniscribe (and DWrite) do with
text and fonts for particular scripts, and there may be some differences
in other implementations. For example, Uniscribe performs s invalid mark
sequence checks that others, preferring to see this as a task for
spellcheckers, do not. But the glyph selection and positioning results
should be the same across implementations. Font makers need to know how
text is processed and OTL features applied in order to make fonts that
work with resulting glyph runs and input strings. Changing the point in
the glyph string resolution when BiDi is applied breaks everything. It's
a complete non-starter.


Tiro Typeworks
Gulf Islands, BC
The criminologist's definition of 'public order
crimes' comes perilously close to the historian's
description of 'working-class leisure-time activity.'
  - Sidney Harring, _Policing a Class Society_
Received on Tue Aug 23 2011 - 18:02:53 CDT

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