Several RTL scripts do not require shaping nor ligatures. Jony
> -----Original Message----- > From: [email protected] [mailto:[email protected]] 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 <[email protected]>: > > 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.
