From: John Hudson (john@tiro.ca)
Date: Tue Feb 12 2008 - 14:13:36 CST
Peter Constable wrote:
>> However, there is at least one orthography which places a "simple"
>> diacritic centered on a digraph (*without* an additional double-width
>> diacritic)...
> Sometime last year, UTC considered a similar case involving an orthography for a language in (IIRC) Panama, and the decision was that this should be handled as a contextual kerning adjustment -- no additional characters needed to be encoded. Of course, that implies specially-designed fonts.
> If that is not adequate for the case you've mentioned, you might submit a proposal for some other solution.
A 'kerning' adjustment, contextual or otherwise, is clearly not adequate, since kerning is
a cumulative positioning operation that affects the horizontal relationship of all glyphs
in the sequence and, hence, dragging adjacent glyphs with them. But I presume the UTC
recommendation can be interpreted as meaning a contextual mark positioning adjustment,
which affects only the mark relative to the digraph and not adjacent glyphs.
As Peter indicates, this requires specialised fonts: either fonts designed specifically
for the language in question, with the assumption that any instance of the two letter
sequence is a digraph, or font with language-specific layout behaviours in which the
treatment of the two letter sequence is a digraph for some languages and not for others
(the latter relying, of course, on applications that support such distinctions).
From a practical perspective, the easiest way to implement mark-to-digraph positioning is
to first ligate the two letters into a single digraph glyph, but to class the resulting
glyph as simple rather than as a ligature per se, meaning that like an individual letter
glyph it will take only one anchor for mark positioning in each position (above, below,
etc.) This is easier than trying to correctly position the mark on the second letter glyph
in context of the first letter glyph.
John Hudson
-- Tiro Typeworks www.tiro.com Gulf Islands, BC tiro@tiro.com The Lord entered her to become a servant. The Word entered her to keep silence in her womb. The thunder entered her to be quiet. -- St Ephrem the Syrian
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