On Mon, 27 Jul 2015 18:18:09 +0300 Eli Zaretskii <[email protected]> wrote:
> I no longer see where this is going. If there's still some goal, > something you think we should agree or discuss, perhaps you could > spell that out. Otherwise, I think it' time to quit. It's basically to establish that for UBA-compliant bidirectional support of some characters, a font must have both a left-to-right and a right-to-left glyph for the character. > Some random comments: > > > Date: Mon, 27 Jul 2015 15:32:01 +0100 > > From: Richard Wordingham <[email protected]> > > Cc: [email protected] > > > > > U+2140 DOUBLE-STRUCK N-ARY SUMMATION gets mirrored, but its > > > > glyph is not replaced by any other character's glyph. Or are > > > > you claiming that left-to-right U+2140 and right-to-left U+2140 > > > > are two different characters? > > > > > > I'm saying that "providing a mirrored glyph" entails coming up > > > with a character whose glyph can play that role, AFAIU. > > > > I'll take that as 'No' - the left-to-right and right-to-left forms > > are the same character. (Unicode has no consistency in this > > matter.) > > I don't know what is meant by "left-to-right and right-to-left forms" > here. To me, a character has only one form. I trust you've just forgotten that that's not true. Soft-dotted characters like 'i' and 'j' lose their dot when a mark above (ccc=230) is attached, e.g. <U+0069 LATIN SMALL LETTER I, U+1DC4 COMBINING MACRON-ACUTE>. Indic scripts have some more spectacular variations. In a font that supports both left-to-right and Arabic right-to-left maths, U+222B INTEGRAL will have at least two forms, one for left-to-right and one for right-to-left. > > > If you are saying that the "rendering system" here is the shaping > > > engine using the rtlm OTF feature, then you are in fact saying > > > that the mirroring of these characters cannot be implemented with > > > most fonts in wide use today, and with most shaping engines. > > > That would be a very strange claim, IMO, tantamount to saying > > > that those characters cannot, or don't need to, be mirrored at > > > all in most use cases. Is this an expression of disbelief, or a lament that the UBA demands too much? If it's a lament, I believe I've made my point. > > OpenType can handle it - feature rtlm effectively provides a > > supplementary an RTL cmap, and ltrm an LTR cmap. It's conceivable > > that DirectWrite and Uniscribe don't support it, but that's > > unlikely. > > Most popular fonts don't, so this support is basically useless, if it > turns out to be a must. No, it's a 'shall'. One won't be arrested for not doing it. > > The decision to mirror is entirely up to the font. > > Not at all. A display engine can make those decisions on its own, > even if it consults the fonts while making those decisions. If application of the rtlm and rtla features do not change the glyph used for U+222B INTEGRAL, then the font has refused to mirror the character. Now it is possible, in this circumstance, that the rendering enging might synthesise a reflected glyph. The font could then deceive the rendering engine by substituting an identical glyph. > > If you still don't believe me, please explain why U+222B INTEGRAL > > has Bidi_Mirrored=Yes but Bidi_Mirroring_Glyph=<none>. > > The explanation is in the file: there's no glyph for that. You mean, I hope, that there's no other character with the glyph for that rôle. > > I didn't say boustrophedon text was subject to the UBA. I said a > > boustrophedon renderer may modify the text to be rendered so that > > the UBA will layout the text properly. > > Given the directional overrides, this is a trivium, I think. Yes. I couldn't see why you were making such a fuss about it. > > The UBA specifies the appearance of an opening parenthesis. > > That's bidirectional, not unidirectional There may not be any more point in arguing about what is unidirectional and what is bidirectional. Richard.
