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

From: John H. Jenkins (
Date: Fri May 30 2008 - 10:24:04 CDT

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    On May 30, 2008, at 2:20 AM, Mahesh T. Pai wrote:

    > 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

    TrueType fonts *may* give glyphs names but aren't required to. Nor is
    there any formal requirement that the glyph names are consistent
    across fonts. Adobe does provide guidelines for naming glyphs but
    many fonts do not follow them, and many fonts don't use named glyphs
    at all.

    > (2) the layout engine has to have a
    > mapping from a sequence to a named glyph

    No, it doesn't. There is no requirement that the layout engine do this.

    > (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.


    Glyph names are used, at this point, largely as a convenience for the
    font designer and as a mechanism for reverse mapping glyphs to Unicode
    characters. There is no expectation that you can usefully map text to
    glyphs using the glyph names and, in actual practice, this would be
    impossible to do anyway.

    John H. Jenkins

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