Re: XTerm char-width handling

[GOTO Masanori <gotom@debian.or.jp>]

From: Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
Subject: Re: XTerm char-width handling 
> When characters can now be either 1 or 2 cells wide, what would be the
> preferred semantics for the cursor control sequences? Should they
> position the cursor on the n-th cell or on the n-th character in a line?

I think, they should be the n-th character.

> Should backspace move back the cursor by one or by two columns, if the
> character left of the cursor is a CJK double-cell character?

two columns. kterm do such a behavier.

> Is there already some well-established existing practice on how to
> interpret VT100 cursor control commands in the context of double-cell
> characters, or is it up to us to set the new standard here?

I don't know. But, please check kterm.

> For reasons of simplicity, I think I'd prefer a strictly cell-based
> cursor control concept. If half a double-cell character gets overwritten
> by text that follows a cursor repositioning, then the other half can be
> substituted with some default character such as space. This seems to me
> to be both most easy to understand and most easy to implement.

In such a situation, other half character vanishes.
If the other half is substituted with other characters,
almost users are maybe embarassed.

> The whole cursor control question will come up again, when we start
> thinking about really advanced stuff such as combining characters and
> ligature substitutions, but these issues are much more tricky and
> probably much less urgent than CJK wide-chars at the moment.

Decomposition characters like Korean Hangle (U+1100 -) is
more more tricky. It seems that Unicode require us changing
the character handling routine, if we want to support
full Unicode specification.

If you want more imformation about wide char, 
see http://www.unicode.org/unicode/reports/tr11/ , etc.

--
GOTO Masanori
Department of Computational Intelligence and Systems Science,
Tokyo Institue of Technology.
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