[mpeg-OTspec] Interaction between kern table and GPOS table

[Thomas Phinney]

I'm strongly in favor of Sairus' proposal, on both the general level (we
need some definition of expected behavior of consumers), and on the
specifics (I think that his proposal is pretty much the ideal approach,
given where we are today).

Font developers have asked for this sort of thing, because without it they
don't know how to build kerning that works well in legacy environments, yet
also takes advantage of the newer OpenType / OFF capabilities such as class
kerning for modern environments.

Regards,

T

On Tue, Jan 19, 2010 at 6:47 PM, Levantovsky, Vladimir <
vladimir.levantovsky at monotypeimaging.com> wrote:

>
>
>  Dear all,
>
>
>
> I haven’t heard any objections to the proposal from Sairus. Please review
> his proposed wording once again and let me know by the end of the day
> Wednesday, Jan. 20 if anyone has any objection to include the proposed text
> to the description of the kern table.
>
> The primary concern is to not invalidate any existing implementations – if
> there are no objection voiced, I will add the proposed wording to the draft
> text of the corrigendum. Please note that this decision to add the language
> is not final – we will have a chance to review this and change or remove the
> text via ISO ballot comments.
>
>
>
> Thank you and regards,
>
> Vladimir
>
>
>
>
>
> *From:* mpeg-OTspec at yahoogroups.com [mailto:mpeg-OTspec at yahoogroups.com] *On
> Behalf Of *Sairus Patel
>
> *Sent:* Tuesday, January 12, 2010 9:46 PM
> *To:* Behdad Esfahbod
> *Cc:* mpeg-OTspec at yahoogroups.com; multiple recipients of OpenType - sent
> by
> *Subject:* RE: [mpeg-OTspec] Interaction between kern table and GPOS table
>
>
>
>
>
> Hello Behdad,
>
> > Doesn't this belong to the OpenType list though?
>
> I was hesitant to send it to both lists: I thought/hoped there would be
> sufficient overlap by now, and didn't want to clog folks' Inboxes with
> duplicates. But I'm happy to cc the OT list on this thread now, and perhaps
> in the future err on the side of clogging Inboxes rather than leaving some
> folks out of the discussion.
>
> > I'm leaning towards not applying 'kern' if the font has any 'GPOS' table
> though.
>
> Reasons not to do as you suggest:
> 1. We've seen fonts with "stub" or empty GPOS tables, I believe produced by
> VOLT. If we do not apply the kern table in this case, no kerning would occur
> in OT layout.
> 2. MS had revised Arial and Times New Roman in Vista to add a GPOS table
> with only non-kern features in it; the kern table remained. If we do not
> apply the kern table in this case, no kerning would occur in OT layout.
>
> Best,
> Sairus
>
>
> -----Original Message-----
> From: Behdad Esfahbod [mailto:behdad.esfahbod at gmail.com<behdad.esfahbod%40gmail.com>]
> On Behalf Of Behdad Esfahbod
> Sent: Tuesday, January 12, 2010 3:49 PM
> To: Sairus Patel
> Cc: mpeg-OTspec at yahoogroups.com <mpeg-OTspec%40yahoogroups.com>
> Subject: Re: [mpeg-OTspec] Interaction between kern table and GPOS table
>
> Hi Sairus,
>
> Thanks for bringing this up. Doesn't this belong to the OpenType list
> though?
>
> I'm leaning towards not applying 'kern' if the font has any 'GPOS' table
> though.
>
> That said, from a technical point of view, it's also worth noting that GPOS
> is
> applied on the logical glyph string while 'kern' is applied on the visual
> string (ie. after reversing for RTL runs).
>
> behdad
>
> On 01/12/2010 06:31 PM, Sairus Patel wrote:
> >
> >
> > Reason:
> >
> > Sometimes a font can have both a kern table and a GPOS table, and the
> > GPOS may or may not have a kern feature for some scripts and language
> > systems. How is an OFF layout engine to decide which source of kern
> > data, if any, to apply? This proposal specifies this. Question: It is
> > proposed to be in the Recommendations section; is there a better place
> > in the specification for something like this?
> >
> > Proposal:
> >
> > ADD the following paragraphs to the end of the 'kern' Table sub-section
> > in the Recommendations section:
> >
> > When a kern table and GPOS table are both present in a font, and an OFF
> > layout engine is requested to apply kerning to a run of text of a
> > particular script and language system: (a) If the number of kern feature
> > lookups in the resolved language system in the GPOS table is zero, then
> > the kern table must be applied, followed by any remaining GPOS features
> > requested. (b) If the number of kern feature lookups in the resolved
> > language system in the GPOS table is non-zero, then all GPOS lookups,
> > including the kern lookups, must be applied in the usual way and the
> > kern table data ignored.
> >
> > If a kern table but no GPOS table is present in the font, then an OFF
> > layout engine must apply the kern table to the text, regardless of the
> > resolved language system of the text.
> >
> > If compatibility with legacy environments is not a concern, vendors are
> > encouraged to use the GPOS table, and not the kern table, for kerning.
> >
> > Sairus
> >
> >
>
>    
>



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