Re: RTL PUA?

On 08/19/2011 08:18 PM, Michael Everson wrote:
>> I entirely agree! There then should be an effort to officially
>> change the BC of these characters to ON, would you say? I mean,
>> what kind of implementations could such a change affect adversely?
>
> There is plenty of space. There would be no difficulty in assigning
> some rows to a RTL PUA.

It is not a question of availability of space. It is a question of
principles.

> Mucking about with the directionality of the
> existing PUA would be extremely unwise.

... which would mean that it is extremely unwise to change the BC of the
existing PUA characters and defeat your purpose.

That said, you don't mean to say that existing rendering engines
actually take notice of the BC=L of these characters, do you? Again, if
you wish for some PUA ranges to have their BC changed to R, it is
necessary that such engines *don't* take notice of the BC of PUA characters.

In effect, changing the existing BC=L to ON is no worse than changing it
to R.

>> Conceivably certain closed user-groups could be using
>> closed-distribution rendering engines which would support bidi and
>> glyph reordering or such for PUA codepoints.
>
> Not everyone is a programmer and can devise a rendering engine. But
> lots of people can make fonts that could support a RTL conscript or
> some private Arabic characters.

Heh -- you are a font expert and presumably can whip a font up at short
notice. The same can't be said for others!

Others are programming experts and easily extend their rendering engine
to a script in the PUA.

Abilities are different. But if you have the need to support a
particular RTL script in the PUA, just like you have to create an
appropriate font, you should be ready to write programming for rendering
support. Remember that for handling Indic-type scripts (involving
reordering etc) in the PUA, support is needed from the rendering engine
side anyway.

It may be difficult to do this for closed-source rendering engines,
especially OpenType ones that place half the rendering logic in compiled
code, but with OSS rendering engines, or using technologies like
Graphite, you can achieve proper rendering for Indic scripts. I presume
the same is extensible to RTL too.

>> In which case, the only change that needs to be done to affirm that
>> the PUA can be used for both LTR and RTL scripts is to change the
>> BC of all those characters to ON.
>
> I wouldn't support that.

OK -- but I entreat you to look at the bigger picture by which
allocating RTL ranges in the PUA would not solve the situation for
unencoded Indic-style scripts.

-- 
Shriramana Sharma