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

[Peter Constable]

> The “fina” feature and its buddies as well as the “frac” feature have additional information that needs to be maintained.

Yes, since writing this mail earlier today, I've been working on other revisions in various feature descriptions and noticed there are some that are not simply repeating GSUB/GPOS details.

But there are a lot that are only repeating.

-----Original Message-----
From: Norbert Lindenberg <mpeg-otspec at lindenbergsoftware.com> 
Sent: Monday, September 7, 2020 10:43 PM
To: Peter Constable <pgcon6 at msn.com>
Cc: MPEG OT Spec list (mpeg-otspec at lists.aau.at) <mpeg-otspec at lists.aau.at>
Subject: Re: [MPEG-OTSPEC] "application interface" in feature descriptions

As I pointed out in
https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FMicrosoftDocs%2Ftypography-issues%2Fissues%2F290&data=02%7C01%7C%7Ca398e2ae98f64a7d762c08d853ba00f5%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637351405623767434&sdata=r78PLOMWQUXCPdfBlqbtv%2FXhfckkQpBIAARPHIB7Hic%3D&reserved=0
talking about applications in this context is misleading. In reality, you’re talking about the interpretation of features in rendering systems.

To the extent that an “application interface” section just describes how the lookups of the feature are applied, I’d agree that it can be replaced with a reference to a generic description of that process in the GSUB or GPOS documentation, as applicable.

The “fina” feature and its buddies as well as the “frac” feature have additional information that needs to be maintained. There may be others. You’ll need to read each one to decide whether there’s useful information or whether it’s just redundant.

Best regards,
Norbert
Lindenberg Software LLC



> On Sep 7, 2020, at 11:16, Peter Constable <pgcon6 at msn.com> wrote:
> 
> 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 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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