Set Numbers in Small Caps by Default

Question

I am using the fontspec package with LuaLaTeX. I'm curious if there is a way to automatically set numbers in small caps by default similar to how you can set numbers in OldStyle/Lowercase by default via the fontspec package options Numbers=OldStyle or Numbers=Lowercase

Note: Not all fonts support numbers in small caps. I use a free font in my MWE that supports small caps numbers.

My motivation for asking this question is that I am currently testing if typesetting numbers in small caps is easier or nicer to read than the usual uppercased, monospaced numbers. Having an easier way to output numbers in small caps would be wonderful.

\documentclass{article}
\newcommand{\alphabet}{abcdefghijklmnopqrstuvwxyz}
\newcommand{\digits}{0123456789}
\usepackage{fontspec}
\setmainfont{Source Serif Pro}%Available for free on Google Fonts.
    [
    %Numbers=OldStyle%
    %Numbers=Lowercase%
    ]
\begin{document}
\textbf{Regular letters and digits:}
\par
\alphabet\quad\digits
\par
\textbf{Small caps letters and digits:}
\par
\textsc{\alphabet\quad\digits}
\par
\textbf{Regular letters and Oldstyle digits:}
\par
\alphabet\quad\oldstylenums{\digits}
\end{document}

EDIT 1: I added a photo displaying numbers with small caps.

EDIT 2: I am looking for a solution, if possible, that makes numbers display as small caps by default without having to use \textsc or any other in-text macro. Thus, \textsc{1234} and 1234 should look identical. If this isn't possible, of course, I can always fall back on making an in-text macro.

Answer 1

So, what you're after are digits that match small caps in size (which have not really a name to my knowledge, since the term "small caps" is usually only used for letters), but you want them printed in this style regardless of the sourrounding style of the text.

You can select specific OpenType features using fontspec via the option RawFeature=+xxxx where xxxx is the abbreviation of the relevant OpenType feature. So, you could select RawFeature=+smcp to enable small caps in general. Sadly, Source Sans Pro does not contain a feature that lets you select only these small numbers without affecting other letters, and on the other hand it seems that it is not directly possible to only select such features only for numbers using fontspec.

However, you can use fonts.handlers.otf.addfeature and enhance the font with your own custom OpenType feature which you can then use with fontspec. In the following example, I created the feature xnum which replaces the digit glyphs with the small-caps variants (you'd need to look up the relevant glyph names in the font, though):

\documentclass{article}
\usepackage{fontspec}

\directlua{
    fonts.handlers.otf.addfeature {
        name = "xnum",
        type = "substitution",
        data = {
            ["zero"] = "zero.sc",
            ["one"] = "one.sc",
            ["two"] = "two.sc",
            ["three"] = "three.sc",
            ["four"] = "four.sc",
            ["five"] = "five.sc",
            ["six"] = "six.sc",
            ["seven"] = "seven.sc",
            ["eight"] = "eight.sc",
            ["nine"] = "nine.sc",
        },
    }
}

\setmainfont[RawFeature=+xnum]{Source Serif Pro}

\newcommand{\alphabet}{abcdefghijklmnopqrstuvwxyz}
\newcommand{\digits}{0123456789}

\begin{document}

\textbf{Regular letters and digits:}
\par
\alphabet\quad\digits

\end{document}

---

Another way would be to create a combofont using the package with the same name (which is based on the mechanisms provided by the luaotfload package). It is important to note that adding features such as smcp to the second font only works with setting mode=base.

\documentclass{article}
\usepackage{combofont}
\setupcombofont{sourceserifpro-regular}{
    {name:Source Serif Pro-Regular:mode=base;language=DFLT;+tlig} at #1pt,
    {name:Source Serif Pro-Regular:mode=base;language=DFLT;+tlig;+smcp} at #1pt
}{
    {} ,
    0x30-0x39
}
\setupcombofont{sourceserifpro-bold}{
    {name:Source Serif Pro-Bold:mode=base;language=DFLT;+tlig} at #1pt,
    {name:Source Serif Pro-Bold:mode=base;language=DFLT;+tlig;+smcp} at #1pt
}{
    {} ,
    0x30-0x39
}

\DeclareFontFamily{TU}{sourceserifpro-combo}{}
\DeclareFontShape {TU}{sourceserifpro-combo}{m}{n} {<->combo*sourceserifpro-regular}{}
\DeclareFontShape {TU}{sourceserifpro-combo}{bx}{n}{<->combo*sourceserifpro-bold}{}

\newcommand{\alphabet}{abcdefghijklmnopqrstuvwxyz}
\newcommand{\digits}{0123456789}

\begin{document}

\fontfamily{sourceserifpro-combo}\selectfont

\textbf{Regular letters and digits:}
\par
\alphabet\quad\digits

\end{document}

The output is the same as above.