On 08/14/2002 02:36:37 PM "William Overington" wrote: >> U+0360 COMBINING DOUBLE TILDE > >> U+035D COMBINING DOUBLE BREVE >> U+035E COMBINING DOUBLE MACRON >> U+035F COMBINING DOUBLE LOW LINE > >I also note U+0361 COMBINING DOUBLE INVERTED BREVE and U+0362 COMBINING >DOUBLE RIGHTWARDS ARROW BELOW in the code chart. > >I wonder if someone could please clarify how an advanced format font would >be expected to use such codes.
In a "dumb" font, support for these character can be implemented by having a glyph that has zero advance width, with the outline extending beyond both side-bearings. In a "smart" font, one could position the glyph for one of these combining marks using attachment points (i.e. the outline of the glyph for the base character includes a target point, and the outline for the combing mark includes a specific point that the layout engine aligns over the target point), or one could look for certain base + combining mark combinations and substitute the sequence of glyphs for a single composite glyph. The latter approach has limitations in that you have to choose ahead of time exactly which combinations you will support, and there can only be a limited number of such combinations. Attachment points, in general, have the advantage that they can be designed to work with arbitrary combinations -- any possible combination. With the double-width combining marks, though, things are rather trickier. First, you may need to substitute a variant glyph for the combining mark that has a width to match the particular pair of base characters -- potentially quite messy; and then you have to deal with positioning in relation to two base characters at once, which has additional complexity. For instance, when positioning a double macron over (say) "la", you need to adjust the height to the taller of the two glyphs; but you need to make the same adjustment for "al". One of my co-workers implemented such behaviour in a font using Graphite a couple of years ago; my recollection is that there isn't an easy way to accomplish this with OT, but I haven't worked with OT enough to know for sure. >I understand from an earlier posting in this thread that the format to use >in a Unicode plain text file would be as follows. > >first letter then combining double accent then second letter Yes. >As first letter and second letter could be theoretically almost any other >Unicode characters, would the approach be to just place all three glyphs >superimposed onto the screen and hope that the visual effect is reasonable That's one possibility, what I would refer to as the "dumb" rendering implementation. >or would a font have a special glyph within it for each of the permutations >of three characters which the font designer thought might reasonably occur >yet default to a superimposing of three glyphs for any unexpected >permutation which arises? This is a possible implementation in a "smart-font" rendering context. >As a matter of interest, how many characters are there where such double >accents are likely to be used please? Is it just a few or lots? This really isn't easy to answer. Someone could tell you, "these 29 combinations..." but they might not -- probably do not -- know about what every user in the world might have ever needed or will ever need. >While in this general area, could someone possibly say something about how >and why U+034F COMBINING GRAPHEME JOINER is used please? Please read the relevant portions of the standard (see on section 13.2 in clause IV of TR#28), and then come back with questions for clarification, if needed. - Peter --------------------------------------------------------------------------- Peter Constable Non-Roman Script Initiative, SIL International 7500 W. Camp Wisdom Rd., Dallas, TX 75236, USA Tel: +1 972 708 7485 E-mail: <[EMAIL PROTECTED]>
