From: Paul Eggert <eggert@cs.ucla.edu>
To: Eli Zaretskii <eliz@gnu.org>
Cc: user42_kevin@yahoo.com.au, 26396@debbugs.gnu.org
Subject: bug#26396: 25.1; char-displayable-p on a latin1 tty
Date: Fri, 14 Apr 2017 11:56:32 -0700 [thread overview]
Message-ID: <24bd0681-08d5-137f-29df-715eefddf2ae@cs.ucla.edu> (raw)
In-Reply-To: <83inm7go6o.fsf@gnu.org>
On 04/14/2017 05:37 AM, Eli Zaretskii wrote:
>> This should not be a problem, as the Linux console has only
>> single-width characters.
> Are you sure? AFAIU, the Linux console supports the BMP, and some of
> the characters in the BMP are double-width (a.k.a. "full-width"), for
> example U+1100, U+231A, U+2B1B, and others. What does the Linux
> console do when these characters are sent to the screen driver?
I haven't experimented with it, so I'm not 100% sure. However, as I
understand the implementation, the console driver can support at most
512 simultaneously-displayable characters, as this is a property of the
classic IBM VGA design that is the greatest common denominator of
current or recent (post-1990) PC graphics hardware. The user can specify
what each character looks like down to the pixel level, but cannot alter
character sizes on a character-by-character basis. In theory one could
display double-wide characters by splitting them into halves and
displaying each half separately, but I don't know of anyone who does
that (it would not be practical due to that 512 limit).
>
>>> And what does "display as-is" means in practice? Should we send to
>>> the console the glyph codes corresponding to Unicode points, or should
>>> we send UTF-8 encoded characters?
>> It depends on whether the console is in UTF-8 mode. If so, send UTF-8;
>> if not, send a byte that is transformed according to the current mapping
>> table into a Unicode value. I hope we don't need to bother with the
>> latter possibility.
> What software puts the console in UTF-8 mode? Is that the locale
> setting?
It's done at boot time. The escape sequences ESC % G (or ESC % 8) and
ESC % @ get you into and out of UTF-8 mode; see
<http://man7.org/linux/man-pages/man4/console_codes.4.html>. Common
practice is to stay in UTF-8 mode as the alternative is worse (it has
only 256 simultaneously-displayable characters).
> http://www.tldp.org/LDP/LG/issue91/loozzr.html
> http://man7.org/linux/man-pages/man4/console_codes.4.html
> that seems to be just the tip of an iceberg. Or maybe the
> issue is easier than I envisioned.
Both, I hope. :-)
> Suppose we only wanted to use this feature for UTF-8 locales.
> Assuming that the OS takes care of putting the console in UTF-8 mode,
> we don't need any changes in Emacs, since Emacs already sends UTF-8
> sequences to the screen driver. As the Linux console only supports
> the BMP, we could then simply amend the code of char-displayable-p to
> check whether a character is within the BMP, when the terminal is the
> Linux console. Do you agree with this conclusion?
No, because a character is displayable only if it's in that set of
at-most-512 characters.
> OTOH, now I'm not sure I understand the need for terminal_glyph_code.
> What does it do that a simple check for a Linux console and UTF-8
> terminal encoding, plus a character being inside a BMP, don't?
terminal_glyph_code gets the current set of at-most-512 displayable
characters from from the kernel.
next prev parent reply other threads:[~2017-04-14 18:56 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-08 2:20 bug#26396: 25.1; char-displayable-p on a latin1 tty Kevin Ryde
2017-04-08 7:42 ` Eli Zaretskii
2017-04-09 5:16 ` Kevin Ryde
2017-04-10 6:47 ` Eli Zaretskii
2017-04-10 7:05 ` Eli Zaretskii
2017-04-10 7:45 ` Eli Zaretskii
2017-04-13 6:19 ` Paul Eggert
2017-04-13 7:16 ` Eli Zaretskii
2017-04-13 20:58 ` Paul Eggert
2017-04-14 3:01 ` Kevin Ryde
2017-04-14 18:59 ` Paul Eggert
2017-04-14 12:37 ` Eli Zaretskii
2017-04-14 18:56 ` Paul Eggert [this message]
2017-04-15 8:48 ` Eli Zaretskii
2017-04-15 21:12 ` Paul Eggert
2017-04-16 5:59 ` Eli Zaretskii
2017-04-16 20:25 ` Paul Eggert
2017-04-17 6:19 ` Eli Zaretskii
2017-04-17 3:00 ` Kevin Ryde
2017-04-17 3:26 ` Paul Eggert
2017-04-17 5:56 ` Paul Eggert
2017-04-17 7:33 ` Eli Zaretskii
2017-04-17 17:22 ` Paul Eggert
2017-04-17 6:24 ` Eli Zaretskii
2017-04-17 6:41 ` Paul Eggert
2017-04-17 7:27 ` Kevin Ryde
2017-04-17 8:08 ` Eli Zaretskii
2017-04-17 18:08 ` Paul Eggert
2017-04-17 18:32 ` Eli Zaretskii
2017-04-18 17:49 ` Paul Eggert
2017-04-18 18:19 ` Eli Zaretskii
2017-04-13 22:07 ` Richard Stallman
2017-04-13 22:18 ` Paul Eggert
2017-04-14 19:48 ` Richard Stallman
2017-04-11 7:22 ` Kevin Ryde
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