Remove/Deprecate mfrac@bevelled

[fred-wang]

Just opening for discussion. I wonder if this is really used in practice.

[davidcarlisle]

The equivalent layout in tex is provided by several popular packages, nicefrac, xfrac for example. Not used so much in scientific documents but quite a lot for fractions in other fields. Arguably of course in a web context one should use characters such as ¾ rather than constructed fractions for many of these uses, but still mapping between TeX and MathML is easier with this in, I think.

[fred-wang]

Thanks for the info.

[dani31415]

I understand that it is difficult to implement. But since we have the icon in MathType, I assume some people are using it.

[fred-wang]

I understand that it is difficult to implement.

It's not really about the difficulty to implement but about
(1) implementing in a way that is compatible with web engines's layout approach.
(2) not adding extra code/complexity or duplicate features to web engines if it has limited usage. Again we need #55

---

cc @rwlbuis, @emilio :

Currently it's only implemented in Gecko. Regarding (1), the issue is that the width of the slash might strongly depend on the vertical metrics of numerator/denomiator preventing to get accurate intrinsic widths. See https://dxr.mozilla.org/mozilla-central/source/layout/mathml/nsMathMLmfracFrame.cpp where intrinsic width and actual width calculation differ:

 // Set the width of the slash
 if (aWidthOnly) {
   mLineRect.width = mLineThickness + slashMaxWidthConstant;
 } else {
   mLineRect.width =
       mLineThickness +
       std::min(slashMaxWidthConstant,
                (mBoundingMetrics.ascent + mBoundingMetrics.descent) /
                    slashRatio);
 }

We have a similar issues for vertical stretchy operators, but at least the width generally does not depend too strongly on the height so it's acceptable to take the max widths for preferred width calculation... except of course for U+2044 FRACTION SLASH !!

The equivalent layout in tex is provided by several popular packages, nicefrac, xfrac for example. Not used so much in scientific documents but quite a lot for fractions in other fields. Arguably of course in a web context one should use characters such as ¾ rather than constructed fractions for many of these uses, but still mapping between TeX and MathML is easier with this in, I think.

In general one can probably implement the slash fraction layout <mpadded>NUMERATOR</mpadded><mo>&#x2215;</mo><mpadded>DENOMINATOR</mpadded></mrow> with some mpadded attributes to adjust the position of numerator and denominator. This suggests that (1) duplicates existing features (2) could be implemented via polyfills ; hence should not be in core.

[NSoiffer]

According to the stats we have so far, the DLMF and arXiv converters don't support it and NAG doesn't use it. However, I've seen it many times in textbooks. I think we should defer a decision until we get stats from Pearson. That represents a group where I think it is used, but the stats may say something different.

[davidcarlisle]

I think bevelled is only really used for small fractions in mostly non technical works and is essentially the same usage as U+2044 (fraction slash)

123⁄456

Seems to be supported by most browsers (well chrome and firefox at least) and I'd be happy if core only supported this for the case of mfrac with two token elements and

<mfrac type="bevelled"><mn>123</mn><mn>456</mN></mfrac>

was treated as

<mn>123⁄456</mn>

In which case it might be cleaner if core didn't support mfrac bevelled at all, and that mapping was left to convertors from full.