[MPEG-OTSPEC] "application interface" in feature descriptions

[Peter Constable]

I've drafted proposed revisions to the "Application interface" portion of features 'unic' to 'zero', removing content that was simply repeating general info on GSUB/GPOS processing, and replacing it with info that seems more relevant. I've given details for the draft in the discussion area of  Issue 601<https://github.com/MicrosoftDocs/typography-issues/issues/601>. Feedback is invited.


Peter

From: Peter Constable
Sent: Monday, September 7, 2020 11:16 AM
To: MPEG OT Spec list (mpeg-otspec at lists.aau.at) <mpeg-otspec at lists.aau.at>
Subject: "application interface" in feature descriptions

In feature descriptions in the OT and OFF feature registry, one of the fields of information is "Application interface". The wording of most (all?) of these is strange because they're worded as though a feature is a function. E.g.,

"Application interface: For GIDs found in the 'valt' coverage table, the application passes the GIDs to the table and gets back positional adjustments (YPlacement)."

While it might be clear to most what is meant, strictly this is nonsense since neither Feature tables nor the associated lookup (sub)tables are functions: they're just data.

This was called out by Simon Cozens in discussion of an issue for one of the feature descriptions, but it's a more general issue. I've opened an issue<https://github.com/MicrosoftDocs/typography-issues/issues/601> against the OT spec for this and have drafted revised wording for several descriptions (for u to z) to provide a sense of what better wording might look like.

Having said that, it seems to me that the Application interface information (at least, for most descriptions) is completely redundant as it simply repeats what is defined in the GPOS and GSUB chapters (how are lookup subtables used in processing glyph sequences). So, it makes me wonder...

Q: Would it be better if the Application interface portion of feature descriptions were removed?

Q: Is there any feature for which there is some useful application information that isn't a repeat of GPOS/GSUB that should be kept?



Peter
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