[mpeg-OTspec] RE: [OpenType] Kerning

[Sairus Patel]

John wrote:
> I think what you have raised in the past few days is a different issue: what to do if a font contains a kern table and non-kerning GPOS.

This situation is in the original proposal; it hasn't just been brought up in the last few days. The relevant snippet from the proposal is:

> (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.

"The number of kern feature lookups" can be zero if either (i) no kern feature is present, or (ii) a kern feature is present but has no lookups. The language of the proposal is intended to cover both these cases.

Would it be clearer if instead of " If the number of kern feature lookups in the resolved language system is zero..." we said something like "If there is no kern feature or if the number of kern feature lookups in the resolved language system is zero..."?

I'll also take this opportunity to remind folks once again that:

a. Important fonts such as Times New Roman and Arial in Vista have a kern table and a GPOS table with no kern feature. We found this out when people reported that kerning didn't work for these fonts in InDesign -- not something we can say "Too bad!" to very easily.
b. If fonts with both kern and GPOS tables are rare cases, I could see the spec/recommendations being silent on how to handle them. But as I'd pointed out earlier, 4 fonts in XP SP2, 65 fonts in Vista, and 81 fonts in Windows 7 Enterprise have both kern and GPOS tables.
c. The proposal is in the recommendations section and not in the spec proper.

Here is the Adobe current proposal in its entirety (with 3 "must"s changed to "should"s):

<<<
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 should 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, should 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 should 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, font vendors are encouraged to record kerning in the GPOS table's kern feature and not in the kern table.
>>>

Concrete suggestions for how to modify the proposal so as to be more satisfactory are welcome.

Sairus

-----Original Message-----
From: listmaster at indx.co.uk [mailto:listmaster at indx.co.uk] On Behalf Of John Hudson 
Sent: Thursday, March 25, 2010 5:17 PM
To: multiple recipients of OpenType - sent by
Subject: Re: [mpeg-OTspec] RE: [OpenType] Kerning

Message from OpenType list:


Behdad Esfahbod wrote:

> I think you are missing the point of the discussion.  This whole thread is
> about recommending what layout engines should do when both kern and some
> GSUB/GPOS is available.  Aren't you getting rid of the question instead of
> answering it?

I'm wondering whether it is a question that needs to be answered in 
terms of the OpenType specification which is -- notoriously so -- a font 
format specification not a layout implementation specification.

 > If the recommendation is "no recommendation", sure, that's an
> option too.  But that's not what's currently on the table from my understanding.

I understand what is currently on the table, in terms of the spec 
ammendment proposed by Sairus, as regarding what happens when a font 
contains both a kern table and GPOS *kerning*. I think what you have 
raised in the past few days is a different issue: what to do if a font 
contains a kern table and non-kerning GPOS.

The presence or absence of GSUB seems to me tangential or, at least, it 
should be tangential, because to make decisions about positioning based 
on anything other than the presence or absence of GPOS seems a bad idea.

J.


-- 

Tiro Typeworks        www.tiro.com
Gulf Islands, BC      tiro at tiro.com

Car le chant bien plus que l'association d'un texte
et d'une mélodie, est d'abord un acte dans lequel
le son devient l'expression d'une mémoire, mémoire
d'un corps immergé dans le mouvement d'un geste
ancestral.  - Marcel Pérès


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