Canonical equivalence in rendering: mandatory or recommended?

From: Peter Kirk (
Date: Wed Oct 15 2003 - 05:19:06 CST

I note the following text from section 5.13, p.127, of the Unicode
standard v.4:

> Canonical equivalence must be taken into account in rendering multiple
> accents, so that any two canonically equivalent sequences display as
> the same. This is particularly important when the canonical order is
> not the customary keyboarding order, which happens in Arabic with
> vowel signs, or in Hebrew with points. In those cases, a rendering
> system may be presented with either the typical typing order or the
> canonical order resulting from normalization, ...

> Rendering systems should handle any of the canonically equivalent
> orders of combining
> marks. This is not a performance issue: The amount of time necessary
> to reorder combining
> marks is insignificant compared to the time necessary to carry out
> other work required
> for rendering.

The word "must" is used here. But this is part of the "Implementation
Guidelines" chapter which is generally not normative. Should this
sentence with "must" be considered mandatory, or just a recommendation
although in certain cases a "particularly important" one?

The conformance chapter does state the following, p.82, which can be
understood as implying the same thing, and refers to section 5.13 in a
way which suggests that the "information" there is relevant to conformance:

> If combining characters have different combining classes... then no
> distinction of graphic form or semantic will result. This principle
> can be crucial for the correct appearance of combining characters. For
> more information, see “Canonical Equivalence” in /Section 5.13,
> Rendering Nonspacing Marks./

Does everyone agree that "This is not a performance issue"?

Peter Kirk (personal) (work)

This archive was generated by hypermail 2.1.5 : Thu Jan 18 2007 - 15:54:24 CST