[mpeg-OTspec] Interaction between kern table and GPOS table [1 Attachment]

Michelle Perham mihill at microsoft.com
Tue Feb 9 18:40:06 CET 2010

Thanks Vlad. At Microsoft we have some concerns over the kern table recommendations. Here is a summary of our concerns that was compiled by Sergey:

- It is hard to define exact behavior that will work for everybody. Does it make sense to make it per-script or we just cover legacy font with no kerning lookup at all? Should we skip mark glyphs while looking for kerning pairs? There is no strict answer for these questions, so it opens door for incompatible implementations and, therefore, hacky fonts.

- It is dangerous to combine kern and GPOS lookups. It may seem to help with some fonts, but will mess up less obvious cases. I remember how easily we broke font lookups by zeroing widths of mark glyphs outside of GPOS. Applying kerning may have the same effect.

- It is really easy to generate GPOS lookups out of ‘kern’ table, especially with my kern2volt toolJ. We did this backwards for ClearType fonts without much effect on overall font size. FontLab also should be able to generate both from one source.

- I do not see this as implementation spec, it has appropriate level of detail for desired behavior. But it does not mean I like this proposed behavior. It is trying to cover some class of legacy cases, will open door for future hack,  have danger of breaking old fonts and may not work as Adobe expects it to work. And I do not even know how common this case is.

- Too many ‘must’s.


From: mpeg-OTspec at yahoogroups.com [mailto:mpeg-OTspec at yahoogroups.com] On Behalf Of Levantovsky, Vladimir
Sent: Wednesday, January 20, 2010 8:14 PM
To: Sairus Patel; John Hudson
Cc: mpeg-OTspec at yahoogroups.com
Subject: RE: [mpeg-OTspec] Interaction between kern table and GPOS table [1 Attachment]

[Attachment(s) from Levantovsky, Vladimir included below]

Here is the latest version of the draft corrigendum (attached) - please review and let me know if any corrections are needed.

Thank you,

> -----Original Message-----
> From: Sairus Patel [mailto:sppatel at adobe.com<mailto:sppatel%40adobe.com>]
> Sent: Wednesday, January 20, 2010 11:06 PM
> To: Levantovsky, Vladimir; John Hudson
> Cc: mpeg-OTspec at yahoogroups.com<mailto:mpeg-OTspec%40yahoogroups.com>; opentype-migration-list at indx.co.uk<mailto:opentype-migration-list%40indx.co.uk>
> Subject: RE: [mpeg-OTspec] Interaction between kern table and GPOS
> table
> John Hudson 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.
> Yes, I meant font makers. Here is a revision of the last sentence of my
> proposal:
> "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."
> Vladimir, could you please replace the last sentence of my proposal
> with the above.
> Thanks,
> Sairus
> -----Original Message-----
> From: Levantovsky, Vladimir
> [mailto:Vladimir.Levantovsky at MonotypeImaging.com<mailto:Vladimir.Levantovsky%40MonotypeImaging.com>]
> Sent: Wednesday, January 20, 2010 8:00 PM
> To: John Hudson; Sairus Patel
> Cc: Behdad Esfahbod; mpeg-OTspec at yahoogroups.com<mailto:mpeg-OTspec%40yahoogroups.com>; opentype-migration-
> list at indx.co.uk<mailto:list%40indx.co.uk>
> Subject: RE: [mpeg-OTspec] Interaction between kern table and GPOS
> table
> 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,
> Vladimir
> > -----Original Message-----
> > From: John Hudson [mailto:john at tiro.ca<mailto:john%40tiro.ca>]
> > Sent: Wednesday, January 20, 2010 4:11 PM
> > To: Sairus Patel
> > Cc: Levantovsky, Vladimir; Behdad Esfahbod; mpeg-
> > OTspec at yahoogroups.com<mailto:OTspec%40yahoogroups.com>; opentype-migration-list at indx.co.uk<mailto:opentype-migration-list%40indx.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<http://www.tiro.com>
> > Gulf Islands, BC tiro at tiro.com<mailto:tiro%40tiro.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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