[mpeg-OTspec] Vertical ligatures

[David Lemon]

At 9:27 AM -0700 4/24/12, John Hudson wrote:
>Over on the CSS3 discussion list, John Daggett wrote something that I
>think should be discussed re. use of vertical ligatures in OpenType fonts:
>
>	The OpenType spec defines a basic set of defaults
>	for some scripts in the horizontal case. But for
>	the vertical case, there really isn't a clear model
>	defined in the spec or in any public documentation
>	that I've seen.  For example, is the "common
>	ligatures" feature ('liga') enabled for upright
>	text? The Kazuraki font from Adobe depends on
>	'liga' and 'vert' being enabled for vertical runs
>	to get proper vertical ligatures. However, I'm not
>	really sure this is the right model for the general
>	case, since enabling common ligatures enables lots
>	ligatures intended for horizontal runs. For example,
>	in stacked Latin you don't want fi-ligatures to be
>	used. I think for vertical runs there really should
>	be a separate and distinct feature ('vlig'?) for
>	vertical ligatures rather than overloading the common
>	ligatures ('liga') feature.

Back in 2004 when we developed the first version of Kazuraki we had 
an extensive conversation with Microsoft about whether to register a 
'vlig' layout feature, and (clearly) agreed not to in the end. A key 
philosophical aspect is that we believe layout features should be 
defined and used as generally as practical - don't define two when 
one will do. (There are already too many layout features we wouldn't 
have defined separately if we had it to do over again.)

I'd be interested in a realistic case where 'liga' is problematic in 
vertical setting. John D's example is unrealistic, because 
proportional Latin is rotated (not stacked) and monospaced Latin 
wouldn't be ligated. Kazuraki has five f ligatures in its Latin 
(which is proportional), and they're fine there.

>The notion of separate features for vertical layout suggests, at least,
>not only 'vlig' but also 'dvlg' (discretionary vertical ligatures), so
>that ligatures may be classed according to default behaviour, as with
>horizontal ligatures. [Some other horizontal ligature features seem to
>me redundant, so I don't think we need 'cvlg' for contextual ligatures
>or 'hvlg' for 'historical' ligatures, but perhaps others may disagree.]

Indeed; we concluded that there was no real need to duplicate a set 
of horizontal layout features for vertical use. (See "philosophy" 
above.) By putting the 'vert' feature first for vertical text (as Ken 
noted), the font determines which glyphs get ligated.
- thanks,
-- 
David Lemon
Sr Manager, Type Development
Adobe Systems, Inc.

408 536 4152
lemon at adobe.com
http://blogs.adobe.com/typblography