RE: Phetsarat font, Lao unicode

From: Peter Constable (
Date: Mon Jul 09 2007 - 21:56:01 CDT

  • Next message: James Kass: "RE: Phetsarat font, Lao unicode"

    From: [] On Behalf Of Philippe Verdy

    > One problem is that fonts (at least with TrueType/OpenType) are not designed
    > to support reordering and positioning with an unbound number of base
    > characters. For example the GSUB/GPOS tables in TrueType require listing
    > somewhere the complete list of codepoints where such reordering and
    > positioning may be applied, something that can't be performed in fonts with
    > the current format, because they don't allow defining character classes in
    > them, and assigning them pseudo-glyph IDs that can be used in GSUB tables.

    This description, unfortunately, does not at all convey actual facts. The GSUB and GPOS tables in an OpenType font do *not* have to list codepoints where re-ordering is applied. In fact, the OpenType Layout tables do not have any support for glyph re-ordering actions; rather, it is up to client software (such as Uniscribe) that controls glyph re-ordering.

    GSUB and GPOS tables *can* use glyph classes to describe certain aspects of glyph actions; and development tools for creating OpenType Layout tables such as VOLT do allow any aspect of a glyph action to be expressed in terms of glyph classes. But, of course, since re-ordering actions aren't supported in OpenType Layout, re-ordering in terms of classes can't be done.

    In the case of a different font format such as Graphite that does support glyph re-ordering actions, then it would be necessary to specify what glyphs (or classes of glyphs) should interact with re-ordering actions in what way.


    This archive was generated by hypermail 2.1.5 : Mon Jul 09 2007 - 21:59:11 CDT