Re: Combining marks with two letters

From: John Hudson (
Date: Mon Feb 11 2008 - 18:56:39 CST

  • Next message: James Kass: "Re: Combining marks with two letters"

    James Kass wrote:

    > What about OpenType GPOS look-ups? Would the CGJ break
    > any mark-to-mark positioning?

    That depends...

    Since CGJ is a kind of control character, it may or may not be 'painted' during line
    layout, i.e. a glyph may or may not be displayed, and this depends on the individual
    layout engine. If a glyph is displayed -- even an invisible, zero-width glyph -- then it
    will break mark-to-mark positioning unless that glyph is accounted for in the GPOS
    lookups. It can be accounted for, of course, because CGJ is technically a combining
    character itself, and can certainly be classed as a mark in the font GDEF table. So what
    you could do is position the indisible CGJ on the preceding mark as if it were another
    mark and then position marks relative to the CGJ with a 0,0 offset. The trouble with this
    approach is the ambiguity of the CGJ as mark: is it an above mark, a below mark, an
    overlay mark? In order to function within GPOS lookups it needs to be able to play
    multiple roles, mimicking the position of above marks in some situations and below marks
    in other situations. That's tricky, although not impossible with a little creative
    contextual GSUB accessing different CGJ glyphs despending on preceding context.

    A preferable situation is one in which CGJ is not painted, and so does not interrupt the
    glyph sequences for OTL lookups.

    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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