[smufl-discuss] Ligatures and the control characters in SMuFL

[Grzegorz Rolek]

Dear All,

The recent thread on fractional time signatures has reminded me of a problem that keeps coming back to me for quite a while, namely, that for all those non-linear text capabilities within SMuFL, there’s probably a slight overuse of ligatures as a means of dealing with combining control characters. While ligatures in themselves are totally fine as one of different ways of rendering those special character sequences as required, it’s probably not a good thing when the control characters are defined as if ligatures were the only means available.

If we take the fractional time signatures as an example, there’s absolutely no need to have two separate control characters for ‘numerator’ and ‘denominator’ digits. It’s actually a bit inefficient, if not impractical: not only one has to deal with two different control chars for a simple concept (the digits above vs. the subsequent digits below), but also, in cases of multi-digit fractional times, each digit requires it’s own copy of the appropriate control code point. (Please correct me if I'm missing something here.)

Please compare this to how a similar case, the fraction slash, is already modelled out in Unicode. Here's what The Unicode Standard, Core Specification, Version 7.0, p. 273, says about it:

> Fraction Slash. U+2044 fraction slash is used between digits to form numeric fractions, such as 2/3 and 3/9. The standard form of a fraction built using the fraction slash is defined as follows: any sequence of one or more decimal digits (General Category = Nd), followed by the fraction slash, followed by any sequence of one or more decimal digits. Such a fraction should be displayed as a unit, such as [diagonally aligned 3⁄4] or [vertically stacked 3/4]. The precise choice of display can depend on additional formatting information.
>
> If the displaying software is incapable of mapping the fraction to a unit, then it can also be displayed as a simple linear sequence as a fallback (for example, 3/4). If the fraction is to be separated from a previous number, then a space can be used, choosing the appropriate width (normal, thin, zero width, and so on). For example, 1 + THIN SPACE + 3 + FRACTION SLASH + 4 is displayed as [one followed by diagonally aligned 3/4].

Thus fractional time signature could be defined as follows: any sequence of zero or more time signature digits, followed by the (one and only) fractional time control character, followed by any sequence of zero or more time signature digits. Such a time signature might then be displayed as vertically stacked one, either by auxiliary glyph positioning, font-defined glyph positioning, ligatures, or whatever one likes best. Note also the ‘zero or more’ phrasing — that’s for those cases where there’s a need for only either the nominator or denominator in such a vertical setting.

Revisions like this might, of course, be too late of a change at this point, but I wanted to raise this issue nevertheless, if only for the record, or for consideration in any future developments.

Regards,
Grzegorz Rolek


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