Vertical ligatures

[John Hudson]

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.

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.]


JH


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