# Re: Stretchy equal sign for commutative diagrams

From: Frédéric WANG <fred.wang@free.fr>
Date: Mon, 25 Mar 2013 21:50:22 +0100
Message-ID: <5150B88E.6070808@free.fr>

Hi Ross,

Thanks for you feedback.

I should say that I'm considering the AMScd package, so very basic
commutative diagrams. For more complex diagrams, I don't think it's a
good idea (or even possible) to use pure MathML but it's better instead
to rely on SVG for the graphical elements. IIUC, that's what does XyJax
(Xy-pic package for MathJax) or another package being developed for LaTeXML.

AMScd only implements diagrams with vertical/horizontal arrows. So here
we can just use a table with stretchy operators inside. All arrows and
bars usable in AMScd are already defined stretchy in the MathML operator
dictionary. Only the case of the equal sign is not clear, thus my message.

On 25/03/2013 21:34, Ross Moore wrote:
> Hi David, and others.
>
> When designing and developing Xy-pic, many years ago, we decided that the most convenient representation was putting an '=' as a label above a single line, with all the stretchiness being in the line. This fits with labeling using other characters for different special meanings.
> Authors can choose to use a double line, if they like, but although the spacing between the lines can be controlled, there is no easy way to match it to the actual separation of a specific font character. The syntax to use a double line is quite different to that of placing a single character.
>
> Then in CDs you also need to consider what to do with diagonal lines, going at arbitrary angles.
> Placing the relational symbol as a label is surely the better option, at least for high quality printable mathematics.
>
> Is it really right to try to make a font character stretchy, when it has not been designed that way in the font itself? If you do it for one character, how many other relational symbols should it also be done for? Surely the 'stretch' attribute is for vertical delimiters, and horizontal enclosures (over/under braces, wide-tilde, etc.). Applying it to horizontal stretching of arbitrary symbols leads to a lack of proper control of the resulting aesthetics, which surely is not a good thing.
>
> Just my thoughts.
>
>       Ross
>
>

--
Frédéric Wang
maths-informatique-jeux.com/blog/frederic
Received on Monday, 25 March 2013 20:54:28 UTC

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