Proposal for BiDi in terminal emulators

Egmont Koblinger via Unicode unicode at
Fri Feb 1 06:40:48 CST 2019

Hi Eli,

> Arabic presentation forms are more like an exception than a rule, I
> hope you understand this by now.  Most languages/scripts don't have
> such forms, and even for Arabic they cover only a part of what needs
> to be done to present correctly shaped text.  Complex script shaping
> is much more than just substituting some glyphs with others, it
> requires an intimate knowledge of the font being used and its
> capabilities, and the ability to control how various glyphs of a
> grapheme cluster are placed relative to one another, something that an
> application running on a text terminal cannot do.
> So I suggest that you don't consider Arabic presentation forms a
> representative of the direction in which terminal emulators supporting
> such scripts should evolve.

Thanks a lot for this information!

I now understand that presentation forms isn't an ideal possible
approach, and the recommendation should be improved here.

Until it happens, I'm uncertain whether using presentation form
characters is a decent low hanging fruit that significantly improves
the readability in some situations (e.g. "good enough" in some sense
for Arabic), or is a dead end we shouldn't propagate.

I still do not agree however that the entire responsibility can be
shifted to the emulator. There are certain important bits of
information that are only available to the application, and not the
emulator – as with many other aspects, such as reordering,
copy-pasting, searching in the data in BiDi-aware text editors using
the terminal's explicit mode, which are all pushed to the application
because the emulator cannot do them correctly.

I believe we should further study the situation, e.g. see whether
ECMA-48's SAPV (8.3.18) parameters 5..8 (to explicitly specify whether
to use isolated/initial/medial/final form for each character) are
flexible enough to convey all this information, or perhaps a new, more
powerful means should be crafted. At this point I lack sufficient
knowledge to fix the design, I'd need to spend a lot of time studying
the situation and/or working together with you guys, if you're up for

Thanks a lot,

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