Re: Can watermarking Unicode text using invisible differences sneak through Emacs, or can Emacs detect it?

[Tomas Hlavaty]

On Sat 05 Feb 2022 at 16:12, Eli Zaretskii <eliz@gnu.org> wrote:
>> If it's a console, does it support the ioctl issued by
>> calculate_glyph_code_table?
>
> I guess the answer is NO, because if it did, you'd see t as the return
> value, not 'unicode'.

ok, but is it rare or surprising, that my linux console does not support
the discussed ioctl?  I did not do anything with the linux console, I am
simply using it the way it is, except choosing the font.

I found it surprising that Richard did not see the boxes I saw and that
he actually could read it.

I also found surprising you saying something about runtime test to
detect supported characters and choosing replacements accordingly.
That would be great if feasible.

What is the recommended setting for the linux console,
in order to see less boxes and more readable characters?

> So then it isn't surprising that you get false positives when your
> terminal-coding-system is UTF-8:

There is no terminal-coding-system variable in my Emacs 27.2.
How can it be UTF-8?

M-x occur on ~/.emacs shows me these utf-8 settings:
current-language-environment "UTF-8"
prefer-coding-system 'utf-8

> that coding-system can encode any
> character, and Emacs has no way of knowing which of them actually have
> glyphs in the console font if the console doesn't support the
> GIO_UNIMAP ioctl we use to find out which glyphs are actually
> available.

Maybe the ioctl is not the sufficient source of information; emacs would
need to query the current console font and then read the avalable glyphs
from the font?