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

Sairus Patel sppatel at adobe.com
Wed Jan 20 20:43:39 CET 2010

Actually, that's the very statement I was thinking of *removing* from the proposal. I left it in because it seems to reflect the general consensus.

What motivates our wanting to encourage vendors to move from the kern table to the GPOS table for kerning?

If we envision some "pure" OT layout engine in the future that doesn't support the kern table, that might be a reason. However, given that practically every font in a Windows system that has kerning expresses it in a kern table (sometimes in addition to a GPOS feature), I don't see such an engine happening anytime soon. (The alternative may be to convert a kern table into a GPOS feature on-the-fly as input to such an engine, but that still involves knowing about the kern table and parsing it!)

One advantage of the kern table (before the advent of the 'DFLT' script in the spec, that is) was that vendors didn't have to think about what scripts/languages to organize the data under, as they would have had to do with the GPOS. Yes, the GPOS may offer superior compression in some cases but a vendor might not care. Yes, having kern data in the GPOS would allow a vendor to more precisely control its interaction with other GPOS features, but a vendor might not have any other GPOS features.

It seems to me that the kern table is pretty much here to stay in the OT spec.

All that said, it may be OK for that last sentence to be strengthened or to otherwise indicate the kern table as "deprecated",  but it would be good to be clear on why we're doing that. Suggestions for revised wording are welcome.

(John, BTW, I don't know if your statement "The interaction of kerning and mark positioning is the biggest weak spot in OT Layout: there is no easy way to address it." on the " GPOS kerning; pure anchor-based accents in Latin?" thread on the OT list 12/8/09 is related to this.)


-----Original Message-----
From: John Hudson [mailto:john at tiro.ca] 
Sent: Tuesday, January 19, 2010 7:53 PM
To: Levantovsky, Vladimir
Cc: Sairus Patel; Behdad Esfahbod; mpeg-OTspec at yahoogroups.com; opentype-migration-list at indx.co.uk
Subject: Re: [mpeg-OTspec] Interaction between kern table and GPOS table

I wouldn't mind seeing the concluding statement of Sairus' proposal 

	If compatibility with legacy environments is not
	a concern, vendors are encouraged to use the GPOS
	table, and not the kern table, for kerning.

Overall, I think the proposal is fine.



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

More information about the mpeg-otspec mailing list