Proposal for BiDi in terminal emulators

[Eli Zaretskii via Unicode]

> From: Egmont Koblinger <egmont at gmail.com>
> Date: Wed, 30 Jan 2019 14:36:42 +0100
> Cc: unicode at unicode.org
> 
> - GNU Emacs reshuffles the characters according to the BiDi algorithm,
> expecting that the terminal emulator doesn't do any BiDi.

Yes, users are told to disable bidi reordering of the terminal, if the
terminal supports that.

> - It doesn't do Arabic shaping.

It doesn't do _any_ shaping.  Complex script shaping is left to the
terminal, because it's impossible to do shaping in any reasonable way
without controlling the fonts being used and accessing the font
information, and this is not possible when you run on a terminal --
the user configures the terminal emulator, and the emulator chooses
the fonts it likes/needs according to that configuration.

(Emacs does support complex script shaping on GUI displays.)

> - When it comes to visually wrapping a line because it doesn't fit in
> the current width, Emacs goes its own way which doesn't match what the
> Unicode BiDi algorithm says.

Yes, this deviation is documented in the Emacs manuals.  The reason
for that is that the Emacs implementation of the UBA reorders
characters on the fly (i.e., it implements a function that can be
called character by character, and returns the next character in the
visual order).  This was done due to a special structure of the Emacs
display engine and efficiency considerations.

In practice, this problem happens very rarely.