From: Peter Constable <>
Date: Thu, 25 Aug 2011 02:39:44 +0000

From: [] On Behalf Of Philippe Verdy

> But I suspect that the strong opposition given by Peter Constable...

Yet again, I think you're putting words in my mouth. The only thing I think I've explicitly spoken against in this thread is changing the default bidi category of PUA characters to ON.

> In fact when Peter says that the Bidi processing and the OpenType layout
> engine are in separate layers (so that the OpenType layout works in a lower
> layer and all BiDi processing is done before any font details are inspected),
> I think that this is a perfect lie:

The Unicode Bidi Algorithm uses _character_ properties and operates on _characters_. OpenType Layout tables deal only with glyphs.

> At least the Uniscribe layout already has to inspect the content of any OpenType
> font, at least to process its "cmap" and implement the font fallback mechanism,
> just to see which font will match the characters in the input string to render.

> If it can do that, it can also inspect later a table in the selected font to see which
> PUAs are RTL or LTR. And it can do that as a source of information for BiDi ...

In theory, that could be done. A huge problem with your suggestion, though, is that the bidi algorithm deals only with characters and makes no references whatsoever to font data, and for that reason -- I would hazard to guess -- most implementations of the Unicode bidi algorithm do not rely in any way on font data and would need significant re-engineering to do so.

Received on Wed Aug 24 2011 - 21:42:21 CDT

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