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

Levantovsky, Vladimir vladimir.levantovsky at monotypeimaging.com
Thu Jan 21 04:59:54 CET 2010

Since I did not see any objections to Sairus' proposal I will go ahead and add this text to the corrigendum.
I also suggest that we should discuss in details the points that John brought up in his email. I think his arguments are valid and emphasizing the benefits of GPOS-based kerning would be a good thing. We can discuss it and, if agreed, make changes in the corrigendum via ISO ballot process. (which will be opened for three months once the draft corrigendum is approved).

Thank you and best regards,

> -----Original Message-----
> From: John Hudson [mailto:john at tiro.ca]
> Sent: Wednesday, January 20, 2010 4:11 PM
> To: Sairus Patel
> Cc: Levantovsky, Vladimir; 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
> Sairus wrote:
> Perhaps the problem is that I am misunderstanding what you mean by
> 'vendor' in the statement, so that should be clarified. I thought you
> were talking about application makers, but perhaps you meant font
> makers.
> > What motivates our wanting to encourage vendors to move from the kern
> table to the GPOS table for kerning?
> For me, the grounds for encouragement are that one can do a lot more
> with GPOS kerning than one can with kern table kerning. For some
> scripts, I would argue that GPOS kerning is essential because
> interglyph
> spacing needs to be contextual: pair kerning as implemented in the kern
> table simply isn't up to the task of correctly spacing these scripts.
> But even for 'simple' scripts like Latin, the limitation on encoded
> glyphs in the kern table is a major handicap. So long as application
> developers imagine that they can get by with supporting the kern table
> instead of GPOS kerning, fonts and typography will be handicapped by
> the
> inertia of this obsolete technology.
> > 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),
> On whose Windows system? Most of the fonts on my system have GPOS
> kerning only, and this is also true of most of the fonts that I have
> personally made for MS. The MS ClearType fonts had kern tables
> retroactively fitted to them when Office decided they wanted to make
> Calibri etc. standard fonts, but I take this as a good example of what
> I'm talking about: the inertia of app developers imagining that a kern
> table is an acceptable substitute, or even an equivalent, for GPOS
> kerning.
> Because of the size limitation of the kern table*, no effort was made
> to
> extend the kern table kerning for the Windows 7 extensions to ClearType
> fonts: its a frozen legacy table supporting a small subset of the
> interglyph spacing adjustments in these fonts. Even ignoring the fact
> that it doesn't apply any kerning to unencoded glyph variants, it
> doesn't even apply kerning to all the encoded glyphs.
> * As documented on the OT list by Joshua Hadley and Karsten Luecke in
> July 2007.
> > It seems to me that the kern table is pretty much here to stay in the
> OT spec.
> Even though its size limitations mean that an increasing number of
> fonts
> cannot be properly spaced using this table?
> > 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.
> The only situation I can see in which I might opt for a kern table as a
> reasonable solution to interglyph spacing adjustments is if I were
> making a font that a) had a relatively small glyph set, b) required
> only
> adjustments to the spacing of encoded glyphs, and c) required backwards
> compatibility. Because of the backwards compatibility issue, I doubt if
> we're yet in a place in which we could formally deprecate the kern
> table, but I think a strong statement pointing out its limitations and
> discouraging its use would help move us towards an eventual
> deprecation.
> > (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.)
> No, not directly related. The difficulties of spacing and mark
> positioning interaction are specific to GPOS, i.e. they're the problems
> that arise even when one is using the best tools that OpenType has to
> offer. If one were using the kern table instead of GPOS, one wouldn't
> even be able to approach that particular set of challenges because the
> presence of the combining marks would break any kerning.
> We're well over a decade into the 'OpenType era' now. Is disturbing to
> me that major applications are still relying of something as limited as
> the kern table while some of us are getting increasingly concerned with
> the limitations of OpenType itself.
> JH
> --
> 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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