From: Michael Everson <>
Date: Mon, 22 Aug 2011 01:03:14 +0100

On 22 Aug 2011, at 00:37, Asmus Freytag wrote:

> If your implementation supported the directional overrides, it would be possible to use these to lay out any RTL text in a portable manner. Just enclose any RTL run with RLO and PDF (pop directional formatting).
> No impact on any existing implementation, no impact on the standard.

Useful for RTL'ing the Phaistos Disc text or even Latin for the Jabberwocky text. Not so desirable for nonce or novel Arabic (or other RTL script) characters intended to be used within RTL text strings.

> Those who produce rendering engines that do not support these overrides today could be leaned on to upgrade their implementations - that change would benefit users of non-PUA RTL languages as well (because sometimes, the bidi-algorithm can fail, such as for part numbers, and being able to use RLO is a simple way to stabilize such problematic text).

The problem is that existing PUA characters are all strong L.

> Treating PUA characters as ON is very problematic - their display would become context sensitive in unintended ways. No users of CJK characters would think of using LRM characters, but if text is inserted or viewed in RTL context, it could behave randomly.

Easy to fix: Add RTL PUA characters.

> In contrast, always supplying a RLO override for RTL text (containing PUA characters) would be a simple thing to remember and to get right.

Not, I think, practical and certainly not putting RTL and LTR users on the same level in terms of PUA usage.

Michael Everson *
Received on Sun Aug 21 2011 - 19:06:02 CDT

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