Re: Verticality and future of display engine and lines (bis) [Was: Re: RTL lines]

[Eli Zaretskii <eliz@gnu.org>]

> From: Alexandre Garreau <galex-713@galex-713.eu>
> Date: Wed, 27 Oct 2021 21:54:11 +0200
> 
> > So you are not only talking about a revolution in the display engine,
> > you are also talking about a revolution in scrolling commands.
> 
> That’s less than half a dozen of commands and the change is trivial, it’s 
> just level of abstraction.

I guess you are unaware, or perhaps forgot, that the display engine
itself scrolls the window when it finds that necessary.

> > I don't see any excess height of the lines, I think if you see that in
> > Emacs, you have a faulty font or something.
> 
> Did you really look at the screenshots? don’t you see all the blank 
> between the lines?

I'm talking about what I see in my Emacs session where I read your
email.  If any Emacs session displays that as you describe, that's
either a font configuration problem or some rendering bug that isn't
present in my build of Emacs.

> > Showing just one diacritic is TRT in this case.  Anything else is a
> > display bug.
> 
> So how do you display languages which *need* several diacritics?

Not every arbitrary combination of the diacritics on the same base
character is valid.  Lumping the same diacritic several times on the
same base character certainly isn't.  For valid combination of the
diacritics, we will display them the way the font and the shaping
engine (HarfBuzz) tell us to do.

IOW, find a real-life case with several diacritics on the same
character, and then let's talk.  The example you provided makes no
sense in real life, and the correct reaction of the Emacs display
engine to that nonsensical example is to show just one diacritic.

> > > Btw it would be nice if emacs supported such tweaking of
> > > directionality
> > > (although my friend wouldn’t benefit from it since he’s a user of vim,
> > > but I’m pretty sure that would be a point in advertising emacs to
> > > him).
> > You can have this with special bidirectional formatting control
> > characters, like LRO and RLO.  Emacs supports them.
> 
> Yes I know but I mean to have that systematically without having to 
> manually enter them at each script change (and avoiding to put them in a 
> possibly shared file with people with different opinions regarding 
> directionality)

Using those formatting controls _is_ the systematic way of changing
the text directionality.  Emacs is not a playground for arbitrarily
messing with the text layout, it follows relevant standards of
layout.  And in those standards, the _only_ way of controlling the
text directionality is by using directionality controls and
overrides.

You could, of course, go overboard and modify the basic properties of
the characters, which Emacs holds in char-tables.  But that is not
recommended, and I'm not even sure I understand why would you like to
do something like that.