Re: Combining marks with two letters

From: John Hudson (
Date: Tue Feb 12 2008 - 14:13:36 CST

  • Next message: John Hudson: "Re: Combining marks with two letters"

    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
    Gulf Islands, BC
    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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