From: [email protected] [mailto:[email protected]] On Behalf Of Philippe Verdy
>> I agree that OpenType font tables cannot to glyph re-ordering. But totally >> incorrect in saying that it cannot handle ligatures. > I meant "recognizing and generating ligatures in the context where > re-ordering has been performed externally by the renderer". That statement isn't adequate: the results of re-ordering may result in contexts in which ligatures will occur. That can happen, for instance, in displaying Indic scripts. > Ligatures can only be recognized in OpenType, provided that the layout > engine has performed the reordering itself, because OpenType fonts > won't recognize ligatures with glyphs in arbitrary order or intersperced > with other unrelated characters coming from an unreordered glyph sequence. I'm not sure what it means to create a ligature of glyphs in arbitrary order. If you mean a rule to substitute [g1 g2] with [g3] won't apply if the sequence processed by the OpenType Layout lookup processor is [g2 g1], then that's true: if the behaviour of the script is such that glyph re-ordering is appropriate, then a rendering engine for OpenType should do that reordering, and substitution lookups in OpenType fonts should be written to assume that that reordering has taken place. >>> What this means is that, in practice, PUA are only usable in fonts >>> for characters with strong LTR directionality, excluding all >>> reordering and mirroring. >> >> In the OpenType specification, the only data related to glyph mirroring >> that a rendering engine is assumed to have is the bidi mirroring data from >> TUS 5.1. (See >> http://www.microsoft.com/typography/otspec/TTOCHAP1.htm#ltrrtl.) >> All other glyph mirroring is to be handled using glyph substitution data in >> OpenType Layout tables in fonts. > > Exactly, but mirroring data for remapping glyphs will not be be part of that > font. Um... Why not? If the mirroring isn't in reflected in http://www.unicode.org/Public/5.1.0/ucd/BidiMirroring.txt, then it must be handled by glyph substitution in the font as a normal GSUB operation. Peter
