Understanding PUA - (was) Re: Re: Ligatures For Indic languages

From: Mahesh T. Pai (paivakil@gmail.com)
Date: Fri May 30 2008 - 03:20:10 CDT

  • Next message: Mahesh T. Pai: "Re: [unicode] Re: Ligatures For Indic languages"

    mpsuzuki@hiroshima-u.ac.jp said on Fri, May 30, 2008 at 04:02:53PM +0900,:

    > I guess you're looking for some standard ligature collections
    > for Indic scripts, and you want to assign PUA codepoints to
    > them. I think PRC's precomposed Tibetan glyph collection
    > (GB/T 20542:2006) is similar approarch. However, such effort
    > is out of scope of ISO/IEC 10646 and Unicode, and the explicit
    > utilization of PUA codepoint (to point an Indic ligature) may
    > not be welcomed in Unicode oriented softwares.

    This brings me to a question I asked quite a few people some years
    ago, and did not receive an answer.

    How can one ensure that text created on one rendering system
    (rendering/layout engine + font) which uses one method of using glyphs
    from the PUA renders same on another system which may or may not use a
    different method of using the PUA.

    Somebody told me something on IRC, and I have extrapolated that
    information to understand that (1) the truetype fonts have a method of
    naming glyphs, which can be used uniformly, irrespective of the
    position of the glyph in the PUA (2) the layout engine has to have a
    mapping from a sequence to a named glyph (3) once the layout engine
    encounters a code sequence which has a predefined mapping to a named
    glyph, the glyph is substituted, irrespective of position of the glyph
    in the PUA. (4) The OpenType specs take the sequence <> glyph mapping
    out of the rendering engine's realm and places the onus on the font
    file itself.

    Can somebody please elaborate on this? Or at lease provide some
    pointers to documentation?

     Mahesh T. Pai <<>> http://paivakil.blogspot.com/
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