# Re: Stretchy equal sign for commutative diagrams

From: Ross Moore <ross.moore@mq.edu.au>
Date: Tue, 26 Mar 2013 07:34:51 +1100
Cc: "www-math@w3.org" <www-math@w3.org>
To: David Carlisle <davidc@nag.co.uk>
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

On 25/03/2013, at 20:42, David Carlisle <davidc@nag.co.uk> wrote:

> On 25/03/2013 09:25, Frédéric WANG wrote:
>> Thank you David and Daniel. My question was not really about how to
>> implement the stretching but about the markup. The choice <mo
>> stretch="true">=</mo> seems the most natural (and preserves the
>> semantics of the diagram). It is used by LaTeXML and the MathJax
>> extension. Since I don't see constructions for stretchy equal sign in
>> Gecko or the STIX Open Type Math table, I wanted to be sure that
>> everybody agree about that choice, and font designers and MathML
>> implementers do the necessary to make the equal sign extendible.
>
>
> Yes I was just musing really. I think using = is going to be the most
> natural markup however it's implemented. Using a different character
> would possibly have the advantage that you could make it stretchy by
> default, but that would be offset by the fact that = is so much easier
> to enter, and, as you say, probably fits with the semantic of the
> diagram as well.
>
> David
>

Received on Monday, 25 March 2013 20:35:27 UTC

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