L2/01-478

A Use for Variation Selector VS2
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2001-12-10
Asmus Freytag


Occasionally we have the following paradoxial situation in the standard.
The set of normally acceptable glyphic variations for character X
{X1, X2,....} may contain one or more Xi that also are acceptable
glyphs for a different character Y, such that Xi = Yj.

However, at the same time there exist documents in which X and Y must
be distinguished at all times.

The classical, but by no means only example is the use of variant
glyphs for letters of the Greek alphabet as independent symbols for
technical and mathematical usage. (IPA is another example).

For example, when writing a Greek text, the character 03C6 will be
used for indicating the lower case phi, but whether the glyph is
the 'straight' or 'loopy' one is entirely a matter of the font that
is being used. In some cases, printers will even substitute a font
using the opposite glyph from the one used in on-screen composition
of the text. For the purposes of Greek text usage, there is no
possible confusion.

For the purpose of technical and mathematical usage, it is however
very important that the two characters (in this example 03C6 and
03D5) are always distinguished, and always associated with the
same glyph.

My proposal is to reserve VS2 for use with such characters, not
to select and alternate glyph, but to indicate the intent of
restricting the choice of glyph, so that a distinction with the
other character can be guaranteed. In practical terms, VS2 would
have the effect of indicating that a glyph identical or very close
to the reference glyph be chosen.

As with VS1, this action would be restricted to the known set of
characters that are affected by this particular issue

Character  Contrasting
0061		0251
0067		0261
03C6		03D5
03B5		03F5
03B2		03D0
03B8		03D1
(probably some other Greek characters as well)


There are some characters where some fonts are not acceptable for
some common and specific usage, but fully acceptable for more
general use. Examples include the use of 2113 for the liter and
2133 for the historical German Mark symbol, which both require
very specific glyph shapes.

Using any of the non-traditional fonts used for 'script' characters
in some mathematical publications would provide an incorrect
rendering for the cited unifications, while at the same time
being perfectly acceptable for use as mathematical variables.

Characters affected
2113
2133

Use of VS2 would allow the same restriction of glyph variation
to ensure the intended semantics are not inadvertently lost.

The alternative to defining such use of VS2 would be to always
code up to three characters, whenever a symbol with limited
glyph repertoire is unified with a symbol of unrestricted glyph
repertoire:

o a character with variable appearance
o a character with restricted appearance
o (where needed) a character with contrasting appearance

Supporting a mechanism such as glyph-range restriction via VS2
allows font designers to be free in designing the generic glyph
for the character, based on wider use, while still being
able to accommodate users that need guaranteed access to a more
specific glyph.

In the interim, those that do not support this mechanism, would
treat the extraneous VS2 character like any other unsupported
Variation Selector, by ignoring it.