RE: RTL PUA?

Several RTL scripts do not require shaping nor ligatures.

Jony

> -----Original Message-----
> From: unicode-bounce_at_unicode.org [mailto:unicode-bounce_at_unicode.org] On
> Behalf Of Philippe Verdy
> Sent: Sunday, August 21, 2011 10:29 AM
> To: Michael Everson
> Cc: unicore UnicoRe Discussion; Unicode Discussion List
> Subject: Re: RTL PUA?
>
> 2011/8/19 Michael Everson <everson_at_evertype.com>:
> > There is plenty of space. There would be no difficulty in assigning some
> rows to a RTL PUA. Mucking about with the directionality of the existing
> PUA would be extremely unwise.
> >
> >> Conceivably certain closed user-groups could be using closed-
> distribution rendering engines which would support bidi and glyph
> reordering or such for PUA codepoints.
> >
> > Not everyone is a programmer and can devise a rendering engine. But lots
> of people can make fonts that could support a RTL conscript or some
private
> Arabic characters.
>
> Hmmm.... Given the current standard in OpenType, and the fact that
> OpenType fonts cannot reorder glyphs to support the BiDi algorithm and
> correctly handle featues like ligatures, I have serious doubt about
> the feasibility of an OpenType font capable of supporting an RTL
> conscript or some private Arabic characters, that will work with
> existing OpenType engines, simply because there's absolutely nothing
> to describe such properties.
>
> This would be possible only if the engine can not only use the
> existing OpenType fonts, but also include some supplementary character
> properties tables for PUA assignments used in that font, or these
> custom properties can be integrated in extension tables added in the
> OpenType fonts, notably: directionality and mirroring, but also as
> well the combining classes, some decomposition mappings, and probably
> also fallback mapping. There would also be the need to represent a
> finite state machine needed to recognize grapheme cluster boundaries,
> at least, and list the feature names in which the substitution &
> positioning rules for recognized sequences of PUA characters (or their
> mapped glyphs).
>
> What this means is that, in practice, PUA are only usable in fonts for
> characters with strong LTR directionality, excluding all reordering
> and mirroring. Those conscripts will then have to be represented in
> PUAs as if they were completely with strong LTR characters, like the
> sinograms. It's not impossible to do that, but you have to completely
> forget the logical encoding order and only use a strict visual order
> for these PUA-encoded conscripts, and even for unencoded rare Arabic
> letters/clusters for which you'd want to just use a PUA.
>
> The alternative is to not use OpenType features, but use one of the
> alternatives: Apple's AAT or SIL's Graphite, which are less restricted
> than OpenType, or some newer font formats (in this case, you won't
> need any newer PUA ranges with strong RTL properties, you can just use
> the existing assignments).
>
> -- Philippe.
Received on Sun Aug 21 2011 - 03:24:52 CDT