From: Eli Zaretskii <eliz@gnu.org>
To: Ihor Radchenko <yantar92@gmail.com>
Cc: 57693@debbugs.gnu.org
Subject: bug#57693: 29.0.50; Is there a more reliable version of `char-displayable-p'?
Date: Tue, 13 Sep 2022 14:19:01 +0300 [thread overview]
Message-ID: <834jxb5xp6.fsf@gnu.org> (raw)
In-Reply-To: <878rmoujyx.fsf@localhost> (message from Ihor Radchenko on Tue, 13 Sep 2022 09:44:06 +0800)
> From: Ihor Radchenko <yantar92@gmail.com>
> Cc: 57693@debbugs.gnu.org
> Date: Tue, 13 Sep 2022 09:44:06 +0800
>
> Eli Zaretskii <eliz@gnu.org> writes:
>
> > If the terminal's encoding is UTF-8, and it doesn't support the method
> > we use to query about individual glyphs, char-displayable-p may return
> > non-nil when the character cannot be displayed, i.e. will show as an
> > empty box or something. Otherwise, char-displayable-p does provide
> > the correct answer for TTY frames.
>
> Do I understand correctly that:
> 1. `font-at' is not sufficient for terminals, and we also need to use
> `char-displayable-p'
> 2. `char-displayable-p' works on many terminals but may be inaccurate in some
More or less. More accurately:
. font-at is not useful on text-mode frames
. char-displayable-p, when invoked on text-mode frames, in most cases
reports only whether the character can be safely encoded by
terminal-coding-system
. if terminal-coding-system is UTF-8, char-displayable-p will
generally trivially return non-nil, unless the terminal supports
queries about glyphs that it has for specific characters
. the only terminal I know of that supports the above queries is the
Linux console, which I think is not very popular among Emacs users
. for terminals that do support such queries, you can use this:
(internal-char-font nil CHAR)
which will return a positive number if CHAR can be displayed,
negative number if it cannot, or nil if the query is not supported.
So an efficient method of testing this for terminal would be:
. if terminal-coding-system is UTF-8, call internal-char-font to see
if the character is supported, and if it returns a number,
consider the character supported if the number is positive, not
supported otherwise
. otherwise, if terminal-coding-system is UTF-8, consider character
supported (and pray)
. otherwise call char-displayable-p and judge by its value
This is more efficient than calling char-displayable-p because I
expect most terminals these days to use UTF-8 encoding, and for them
the above is optimized.
next prev parent reply other threads:[~2022-09-13 11:19 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-09 13:25 bug#57693: 29.0.50; Is there a more reliable version of `char-displayable-p'? Ihor Radchenko
2022-09-09 13:38 ` Eli Zaretskii
2022-09-09 22:25 ` Stefan Kangas
2022-09-10 6:01 ` Eli Zaretskii
2022-09-10 6:06 ` Ihor Radchenko
2022-09-10 6:35 ` Eli Zaretskii
2022-09-10 6:44 ` Ihor Radchenko
2022-09-10 4:48 ` Ihor Radchenko
2022-09-10 6:24 ` Eli Zaretskii
2022-09-10 6:37 ` Ihor Radchenko
2022-09-10 7:56 ` Eli Zaretskii
2022-09-10 8:17 ` Ihor Radchenko
2022-09-10 8:42 ` Eli Zaretskii
2022-09-11 9:31 ` Ihor Radchenko
2022-09-11 9:43 ` Eli Zaretskii
2022-09-11 10:07 ` Eli Zaretskii
2022-09-12 5:46 ` Ihor Radchenko
2022-09-12 11:15 ` Eli Zaretskii
2022-09-13 1:44 ` Ihor Radchenko
2022-09-13 11:19 ` Eli Zaretskii [this message]
2022-09-14 1:52 ` Ihor Radchenko
2022-09-14 2:43 ` Eli Zaretskii
2023-02-17 19:18 ` Stefan Kangas
2023-02-17 19:29 ` Eli Zaretskii
2023-02-17 22:31 ` Stefan Kangas
2023-02-18 6:56 ` Eli Zaretskii
2023-02-18 9:00 ` Stefan Kangas
2023-02-18 9:18 ` Eli Zaretskii
2023-02-18 11:32 ` Ihor Radchenko
2023-02-18 11:56 ` Eli Zaretskii
2023-02-19 11:31 ` Ihor Radchenko
2023-02-19 11:55 ` Eli Zaretskii
2023-02-19 12:08 ` Ihor Radchenko
2023-02-19 12:23 ` Eli Zaretskii
2023-02-19 14:19 ` Ihor Radchenko
2023-02-19 15:02 ` Eli Zaretskii
2023-02-18 11:35 ` Ihor Radchenko
2023-02-18 12:00 ` Eli Zaretskii
2022-09-12 5:24 ` Ihor Radchenko
2022-09-09 13:46 ` Robert Pluim
2022-09-09 22:25 ` Stefan Kangas
2022-09-10 4:56 ` Ihor Radchenko
2022-09-10 6:02 ` Eli Zaretskii
2022-09-10 4:53 ` Ihor Radchenko
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