Plain text custom fraction input

Marcel Schneider charupdate at
Thu Jul 23 09:47:58 CDT 2015

On 23 Jul 2015, at 14:57,  Khaled Hosny  wrote:

> On Thu, Jul 23, 2015 at 10:25:22AM +0200, Marcel Schneider wrote:
> > The remaining question would then be: What was the idea when at font
> > design, the fraction slash was given left and right kerning, so that a
> > preceding superscript digit will take exactly the place it has as a
> > part of a precomposed fraction, and a following subscript takes place
> > like if it were a denominator in one of the precomposed fractions?
> What says that this kerning is there for super/subscript glyphs, it can
> be equally (and more likely) be there for the numerator and denominator
> glyphs.

You are right, the fraction slash's kerning helps the rendering engine when it's flagged to use the numerators and denominators. I should be able to look inside a font with Western Arabic super- and subscripts and with glyphs for numerator and for denominator, to see whether the numerator glyphs are mapped to the superscript glyphs, and the denominator glyphs to the subscript glyphs. As Denis Jacquerye wrote, this is, if ever, not the case in all fonts, some of them having different glyphs for the two classes. 

The fraction formatting works also when the slash is not a fraction slash but a common slash. Here too it would be interesting to know whether the slash is then mapped to U+2044, or the rendering engine performs the whole.

if the synergy between the fraction slash and the super- and subscripts is purely fortuitous, plain text fraction input would be categorized as a hack, a shortcut which works around the legal process. I would be glad if that weren't true, because I think that the shortest way, if correct, is the best. Again, this short way is practicable only under certain circumstances.

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