The FeatureParam subtable needs to be implemented.
AdobeJenson is the only font I'm aware of that uses this field
currently, for the "size" feature. See the Adobe feature registry.
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Here's another one: Adobe Minion Pro.
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There are fundamental problems with this feature:
1) Adobe got the offset to the FeatureParams block of data wrong
2) It doesn't contain a length field, so the size if the data depends on the
context, ie. in *which* feature it is used. Luckily, so far it's only defined
for the 'size' feature, but what if other features start using it?
On the OpenType list, Adobe have admitted that there is a problem, and
that they're working on an entirely different solution.
I have no interest in working around all this nonsense, so I'm tempted to
close this bug as "won't fix". At least I'm going to stop feeling bad about
not supporting FeatureParams for while, again...
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Quite a number of Adobe fonts use this feature now.
MinionPro does too.
Also FontForge will generate them when converting a TeX font
into an otf format (TeX fonts also contain design size info).
The 'size' feature has the added joy that it does not
conform to the OpenType spec. The Feature Param offset is
relative to the wrong thing. I'm including a email that
appeared on the opentype mailing list about this issue:
Since the 'size' layout feature has come up, I should write
about its current status. A while back, we learned that the way
'size' has been implemented in Adobe's fonts is buggy. (We
absolute and relative offsets - oops!) This problem went
for so long because we don't know of any product which
'size' yet. The longer I'm here, the more I appreciate the
it's not tested, it's broken." <sigh>
Although it would be trivial to fix the code which generated
incorrect 'size' values, we've delayed, in order to consider a
proposal that would obviate 'size' with a new table
information. I consider this approach much more useful, but
yet managed to get enough of the decision makers into the
I'm sorry to say this is likely to continue moving slowly for a
while, because the people who would implement the feature
fully occupied with more urgent projects for several more
For what it's worth.
- David Lemon
I don't know FontForge, but I would strongly suggest that any new
software should NOT output the feature, especially since Adobe
considers abandoning it altogether. I would even go so far as to say
that the feature is evil. It must die.
All FeatureParams are implemented in my tree now: https://github.com/behdad/fonttools
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