Re: \not\in doesn't work properly

[Khaled Hosny <khaledhosny@eglug.org>]

On Mon, Dec 03, 2012 at 09:07:43PM +0100, Hans Hagen wrote:
> On 11/30/2012 10:26 AM, Jeong Dal wrote:
> >Hi,
> >
> >After updating ConTeXt, math command '\not\in' is not shown properly.
> >
> >For example,
> >
> >$a \not\eq b$ is good.
> >
> >but $a \not\in A$ shows two symbols separately.
> >
> >
> >I am using Mac OS X 10.8 and the versions of LuaTex and ConTeXt are
> >
> >This is LuaTeX, Version beta-0.70.2-2012052309 (TeX Live 2012)
> >ConTeXt ver: 2012.11.23 17:35 MKIV fmt: 2012.11.26 int: english/english
> 
> I assume that you use lm ... before I start looking into it I'd like
> to know Khaleds point of view on this.

The main issue here is that \not in was largely a hack in CM fonts, the
glyph was specially crafted so that it overlays the next symbol by
having zero width and -ve right side bearing and it was centered over
equal and similar symbols, anything with different width would not look
good.

This can't be relied on with OpenType fonts mainly because we don't
control all the fonts and since U+0338 is a combining mark it tends to
have -ve left side bearing (the opposite of CM).

Previously I thought it should be handled as special math accent that
don't get shifted above, but this didn't work because accent noads do
not inherit the spacing of their nuclei, and changing this is not simple
AFAICT. When I checked Word it had no notion of negations either, you
can only use pre-composed negated symbols (but accents respect the
spacing of its accentee, BTW).

The solution currently used by unicode-math package is to look ahead and
see if the next symbol after \not has a precomposed negated form and use
it (such a list can be obtained by checking Unicode characters that
decompose to something + U+0338), else just output U+0338. This probably
can be even enhanced by inserting a kern between \not and the next
symbol that would center their bounding boxes together, to handle cases
why no precomposed symbol exists. My be we need a new primitive
\Unegation that would do this in the engine?

Regards,
Khaled
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