Re: Merging combining classes, was: New contribution N2676

From: John Cowan (
Date: Wed Oct 29 2003 - 13:53:58 CST

Jim Allan scripsit:

> << For example, it is crucial that the combining class of the cedilla be
> lower than the combining class of the dot below, although their exact
> values of 202 and 220 are not important for implementation. >>
> This is not explained, but obviously the reason why it is "crucial" that
> the combining class of cedilla be lower is that the cedilla indeed might
> interact with characters in other combining classes and so must belong
> to a combining class which is applied first so that other diacritics
> that might be applied beneath are positioned below the cedilla which
> cannot have any other diacritic between it and the base character.

By no means: you are over-interpreting. The reason it is critical that cedilla
comes before dot below is that Unicode has declared so, by fiat. A
rendering engine is *not* entitled to misbehave if it receives <a, dot-below,
cedilla> and try to place the dot between the "a" glyph and the cedilla;
this is a direct consequence of the conformance requirement that processes
not distinguish (unless they have special purposes in doing so) between
canonical equivalents.

The intent of these remarks is that the particular numbers 202 and 220 are
not significant, as long as the correct order of combining classes is maintained;
it would be conformant to multiply all combining classes by 10 or subtract
5 from each, or (more practically) to map the classes actually in use onto
consecutive integers.

> >The implication is that a font designer
> >cannot assume that glyph positioning adjustments are required only
> >between adjacent characters in canonical order, despite what the
> >"interact typographically" rule might suggest.

Quite so. In particular contexts, the usual inside-outward rule
does not apply (e.g. Vietnamese diacritics above).

John Cowan
If a soldier is asked why he kills people who have done him no harm, or a
terrorist why he kills innocent people with his bombs, they can always
reply that war has been declared, and there are no innocent people in an
enemy country in wartime.  The answer is psychotic, but it is the answer
that humanity has given to every act of aggression in history.  --Northrop Frye

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