Le 02/06/2011 20:26, Arthur Reutenauer a écrit :
So that such an analysis must be done depends only on the tags themselves?
Yes, there is a semantic value to the feature tags, as opposed to the
lookups that implement them (otherwise there would be little point in having
two different levels).
The two levels made sense to me as a handle or button (the tag) and a
machine (the lookup). Actually, I was wondering why there were two
levels since they did the same job (i.e. tell you what should be done to
implement a feature).
This is why there can be "typographic" and "linguistic" features (smcp would
be of the former type, init of the latter).
I.e. you have to know that even though init and e.g. smcp point to similar
lookups (simple substitution), they shouldn't be treated similarly: in the case
of smcp, the lookup suffices, while in the case of init it doesn't. It is
surprising that nothing in the font signals such a difference...
The features an OpenType font contains are only half the story. The
complete implementation of an OpenType-compliant system needs to take into
account the specification of the layout engine (that commands to apply some
features in a specific order, to do contextual analysis, etc.). There is
indeed an implementation choice here, but it has been made at the very
beginning of OpenType. In my opinion, it provides for a more balanced workload
between the font designer and the implementer of the typesetting system.
Thank you very much for the explanation.
What confused me, I think, was that I thought tags could be made up at
will by font designers, unlike lookup types, and so applications
couldn't be expected to ``understand'' them, so to speak. Now I've
checked that they must be registered beforehand, so they aren't so
unruly after all, and it makes more sense to assign a meaning to them.
Best,
Paul
