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

[Eli Zaretskii]

> Date: Sat, 05 Feb 2022 16:06:42 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: psainty@orcon.net.nz, luangruo@yahoo.com, kevin.legouguec@gmail.com,
>  rms@gnu.org, emacs-devel@gnu.org
> 
> > char-displayable-p returns non-nil even though I see boxes.
> 
> Is this the Linux console or is this a terminal emulator?
> 
> 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'.

So then it isn't surprising that you get false positives when your
terminal-coding-system is 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.