Re: Basic question: types of diacritics marks

From: John Hudson (
Date: Thu Jul 18 2002 - 20:08:07 EDT

At 12:34 AM 19-07-02, Michael Everson wrote:

>At 00:56 +0200 2002-07-19, Adam Twardoch wrote:
>>I have a very basic question. What would be the implementation differences
>>of diacritics marks in a font? For example, we'd consider:
>>U+00B4 acute accent
>This is a spacing acute accent.
>>U+02CA modifier letter acute accent
>This is also spacing, and is used to mark tone in some Asian languages.
>>U+0301 combining acute accent
>This is the one that you can use productively as a nonspacing combining
>diacritic over a, p, x, or letters in other scripts.

In an OpenType font, it is possible for the combining diacritics to
actually be spacing glyphs, since the width can be set to zero using a GPOS
metric adjustment as a first stage of a <mark> positioning feature, but
this should not encourage anyone to double-encode a single glyph to both
the spacing and combining diacritic codepoints. It is necessary to provide
separate glyphs for the spacing and combining forms, since the kind of
glyph processing lookups you will want to involve them in will differ.

Of course, the combining diacritic can be a composite of the spacing form,
placed on a zero width.

John Hudson

Tiro Typeworks
Vancouver, BC

Language must belong to the Other -- to my linguistic community
as a whole -- before it can belong to me, so that the self comes to its
unique articulation in a medium which is always at some level
indifferent to it. - Terry Eagleton

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