At 19:55 7/2/2002, Marco Cimarosti wrote:
>The OpenType specs published on the Adobe site states that table GSUB has a
>subtable to handle ligatures ("LookupType 4: Ligature Substitution
>Subtable": http://partners.adobe.com/asn/developer/opentype/gsub.html#LSF1).
>
>It says that "A Ligature Substitution (LigatureSubst) subtable identifies
>ligature substitutions where a single glyph replaces multiple glyphs"
>(multiple *glyphs*, not multiple characters).
>
>OK: literally speaking, it is true that OT maps single characters to single
>glyphs, but then it maps multiple glyphs to ligature glyphs, so what's the
>difference?
>
>I mean: isn't this two-step mapping:
>
> code point -> glyph ID
> component glyph ID's -> ligature glyph ID
>
>functionally equivalent to an hypothetical one-step mapping?
>
> component code points -> ligature glyph ID
>
>Am I missing something?
Well, what you might be missing is layout feature support, in which case
the sequences described might be 'dysfunctionally equivalent'. It is useful
to maintain a distinction between direct (cmap) and indirect (GSUB lookup)
glyph-to-character mapping, because of the system and application support
issues involved in getting the latter to work.
John Hudson
Tiro Typeworks www.tiro.com
Vancouver, BC tiro@tiro.com
Language must belong to the Other -- to my linguistic community
as a whole -- before it can belong to me, so that the self comes to its
unique articulation in a medium which is always at some level
indifferent to it. - Terry Eagleton
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