Re: Multiple private agreements (was: RE: Code pages and Unicode)

2011/8/23 Doug Ewell <doug_at_ewellic.org>:
> srivas sinnathurai <sisrivas at blueyonder dot co dot uk> wrote:
>
>> If same codes within PUA becomes standard for different purposes,
>
> They aren't "standard."  Two different private agreements could assign
> different characters to the same PUA code points.
>
>> how to get both working using same font?
>
> You can't.

I do agree.

>> How to instruct text docs, what font if different fonts are used?
>
> There's no standard way to specify even one font or private agreement in
> plain text, let alone how to switch between them within the same
> document.  This is not an intended use of the PUA.

There exists such standard in the context of plain-text rendering,
because of font fallback mechanisms (in Windows with Uniscribe, such
fallback mechanism is not tunable per user preferences, as the list of
alternative fonts that are tried is fixed by the implementation of
Uniscribe; but anyway it still exists), which implies that multiple
fonts will be scanned with an order of preference; font fallback is
involved each time a character is not mapped on the selected font but
may be mapped in another font.

Such mechanism is exactly similar to the explicit fallback mechanism
in CSS (where one provides an ordered comma-separated list of
font-family names), but that also extends this list of fonts
automatically using the default font fallback mechanisms used for
plain-text rendering.

In other words, even if you can't instruct a plain-text to use glyphs
from one font or from another for the same code point (PUA here), such
possibility still exists in rich-text rendering, because all glyphs
can become selectable as variants (including the variants listed in
the same font for the same glyph, in standardized OpenType features,
provided that the rich-text application implements such
glyph-selection mechanism).

PUAs are effectively not meant to supply the PUA agreement. This has
to be provided elsewhere, but a font can perfectly transport this
agreement (for the font as a whole which is separately selectable,
just like its designed glyph variants are individually selectable by
some typographic feature tables, as well as by index, for example
several swash variants of the same letter with more or less
decorations).

If you can use font fallbacks, then you can render the same text
containing distinct PUAs designed for distinct PUA agreements (and
this demonstrates the utility of the conscript registry, which allows
cooperation between authors of separate agreements, that have accepted
to encode their PUA characters with non-conflicting PUA code point
assignments).

-- Philippe.
Received on Tue Aug 23 2011 - 16:44:54 CDT