Proposal for BiDi in terminal emulators

[Egmont Koblinger via Unicode]

Hi Frédéric,

> I guess Arabic shaping is doable through presentation form characters,
> because the latter are character inherited from legacy standards using
> them in such solutions. But if you want to support other “arabic like”
> scripts (like Syriac, N’ko), or even some LTR complex scripts, like
> Myanmar or Khmer, this “solution” cannot work, because no equivalent of
> “presentation form characters” exists for these scripts

Unfortunately my knowledge ends here, I'm not familiar with shaping
for Syriac and other similar scripts. I'd really appreciate input from
experts here.

I outline in the document problems that arise from the terminal
emulator performing shaping on its contents in "explicit" mode, which
is to be used by Emacs and others. The terminal emulator is not aware
of the characters that are chopped off at the edge of the screen,
required for shaping. The terminal emulator is not aware of which
characters happen to be placed next to each other, but belong to
semantically different UI elements, that is, shouldn't be shaped.

(And as a side note, FriBidi doesn't provide a method for doing
shaping on _visual_ order. I'm unsure about other libraries, and
unsure if there's an algorithm for it at all.)

Honestly, I have no idea how to best address all these problems at
once. This is where we can think of extensions "expliti mode level 2",
use control characters that explicitly specify how to shape certain
glyphs. This is subject to further research.

cheers,
egmont