From: James Kass (firstname.lastname@example.org)
Date: Tue Jan 08 2008 - 04:00:49 CST
Using GSUB (glyph substitution), the font's tables can provide
the outline data from a pre-composed glyph for a Unicode string
consisting of a base letter plus a mark.
Using GPOS (glyph positioning), the font's tables can position that
mark relative to the base letter and provide the system with the
exact same outline data as it would have done using the pre-composed
glyph mentioned above.
GSUB vs GPOS is a matter of individual preference. There should
be no rendering difference when the outline data the font provides
is the same.
Anyone using certain modern systems should be able to display base
letters with marks properly. In a perfect world, it would not be
necessary to put such anticipated combinations in the P.U.A. as
pre-composed glyphs. In the real world, there is no need to use
the P.U.A. to *encode* such sequences as unique "characters". There
is, however, still a desire to be able to *display* such sequences
properly. MUFI provides a method by which this can be accompished,
even on some older systems.
Reading through some of the MUFI documentation, it is clear that
proper caveats about P.U.A. use are in place.
P.S. - Related to stacking marks on base letters, this page from
Andrew West concerning tests made with Doulos SIL may be of
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