Proposal for BiDi in terminal emulators

[Eli Zaretskii via Unicode]

> Date: Wed, 30 Jan 2019 15:49:34 +0100
> Cc: unicode at unicode.org
> From: Egmont Koblinger via Unicode <unicode at unicode.org>
> 
> 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.

Personally, I think we should simply assume that complex script
shaping is left to the terminal, and if the terminal cannot do that,
then that's a restriction of working on a text terminal.  There's
nothing you can do here, because correct shaping requires too many
features that applications running on text terminals cannot use, and
OTOH a terminal emulator who wants to perform shaping needs
information from the application (like the directionality of the text
and its language) that there's no way for the application to provide.