Re: Display of undisplayable characters: \U01F3A8 instead of diamond

[Alan Mackenzie]

Hello, Eli.

On Sun, Sep 04, 2022 at 16:01:53 +0300, Eli Zaretskii wrote:
> > Date: Sun, 4 Sep 2022 11:23:43 +0000
> > Cc: rms@gnu.org, gregory@heytings.org, emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > > > A few days ago you said it could use char-displayable-p
> > > > to determine this.  If that works, it solves the problem of finding out
> > > > which characters to display as diamond.

> > > It works, but looping over all the Unicode characters is very slow, so
> > > doing that at each startup would be unacceptable.  If someone needs, I
> > > can show a simple code to be run by the user, once, on the Linux
> > > console, that would allow then to determine the codepoint ranges for
> > > which the U+FFFD (or any other) replacements are needed, so that users
> > > could run that code, and then put such customizations in their init
> > > files.  We could even have a special command to do that, if people
> > > think it's important and helpful.

> > > But all I hear till now is flat rejection of that solution, which is a
> > > very weak incentive (to say the least) to work on such a command.

> > I will start working on this soon (if nobody else does in the meantime).

> No need, it's already done.  See the new command
> standard-display-by-replacement-char.

Thanks, that works well.

> > I envisage having a minor mode specific to the Linux console, which when
> > enabled for the first time will do the loop over all Unicode characters
> > testing them for glyps, and create the display table.

> This loop takes a long time (25 sec in my unoptimized build, likely 5
> sec in optimized ones), and needs to be run only once on a given
> terminal.  So instead the command I added produces a buffer with Lisp
> code that you could eval and also add to your init file, to be run
> every startup.

The second time I tried it (having rebooted the machine between times)
standard-display-by-replacement-char ran in around 0.077 seconds, which
scarcely counts as slow.  I don't know why it is fast on my machine, but
slower on others'.  Still, the code it produced is now firmly in my
..emacs.

> > This can then be
> > stored in .emacs in the customisation section.  When the mode is
> > enabled, this display table will be used to output the \ufffd's.  When
> > disabled, the glyphless characters will get printed as hex strings, as
> > currently.  Possibly this will involve having two display tables which
> > will get swapped when the mode is en/disabled.  Or something like that.

> I don't really see a reason to reset the display table to its default
> state, especially since it isn't clear what is that default.  E.g.,
> the startup code could add a few entries in the display-table (to show
> the Help buffers), as does info.el.  You don't want to lose those
> setting, and there's no easy way of discerning them from the ones
> produced by the command we are discussing.

OK.  I suppose that people only rarely change the fonts they are using
(more precisely, the character ranges of those fonts).

-- 
Alan Mackenzie (Nuremberg, Germany).