[XeTeX] [off-topic] proper OpenType tag for hanging diacritics

[Pablo Rodríguez]

Hi there,

this is mainly a question for OpenType experts, but I guess anyone can
enjoy the beauty of the texts composed with XeTeX.

At http://diosas-nubes.blogspot.com/2009/09/mas-ligaduras.html, you can
find an ancient Greek text typeset with XeTeX and ledmac.

There are two images, with and without Greek ligatures enabled. The font
used is GFS Baskerville. But there is a problem that you can see at the
end of line 14 on the first image: the space before the uppercase letter
with diacritics is wrong.

The problem comes from the font itself, since it has a negative value
for the left bearing of capital letters with diacritics.

The explanation for the issue was to properly align Greek capital
letters with hanging diacritics at the beginning of the line in a left
aligned paragraph. This was a convention in Greek poetry (as Yannis
Haralambous described in section 3.5 from
http://omega.enstb.org/yannis/pdf/thessaloniki.pdf).

I'm afraid that this was the worst way of solving the issue, because the
effects when composing non-verse texts are obvious, since they can
contain words beginning with capital letters with diacritics in the
middle of a sentence..

A way to avoid this would be to add a kerning pair with the space
character and add the value from the left bearing to the kerning. But
this is only a workaround and not the proper fix.

I thought there might be a proper feature for this in OpenType (from the
GPOS table). After having a quick look at the list of registered
features (http://www.microsoft.com/typography/otspec/featurelist.htm), I
think that the optical bounds is the proper feature to do this
(http://www.microsoft.com/typography/otspec/features_ko.htm#opbd). The
example doesn't refer to Greek capital letters with diacritics, but I
think it describes the same case.

I wonder whether optical bounds are the right choice for hanging
diacritics. And I would like to know whether XeTeX would support this
feature with poetry.

Thanks for your help,


Pablo