How to specify relative font sizes?

[davidchisnall]

Looking at the code for the verbatim environment, it sets the font size to the current size - 3 (pt?). I was not able to work out how to write that in the declarative syntax. The manual mentions that you can write things in terms of the page height or width.

[Omikhleia]

it sets the font size to the current size - 3 (pt?)

It still does? This is likely wrong, there's no reason for using a "hard-coded" 3pt smaller font, not depending on what was the original size. ^^
What you are looking for might be a font size in "em" (e.g. 0.8em = slightly smaller than the current font size)

[Omikhleia]

N.B. I see from some traces of yours in #1837 that you've resilient.sile installed, so be aware that when using the "resilient.book" class (instead of the default "book" class), the \verbatim:font hook (and actually the whole default verbatim package) is not used in principle.
Anyhow, this -3pt correction is a bad default, and the right way to pick a smaller or bigger font than the current font is to use a relative unit, typically in "em". If your initial font, say, is at 10pt, then 1em for it = 10pt, and an environment switching to 0.9em would thus be in 9pt (and so on, the same environment nested would be in 8.1pt)

[Omikhleia]

P.S. Just in case old-school typographers pass by, using "em" ratios is a technical answer, and not a "typographical" one in the stricter sense (in the old era of metal typography, a compositor would have frown upon the above 8.1pt of course).

[Omikhleia]

Yes, this does look wrong.

Opened as #2235 = fixed in SILE 0.15.10 see next answer.

[Omikhleia]

So now that SILE 0.15.10 is available, we should be able to answer the question fully.

Relative font size can be expressed in a font-relative unit.

• 1em = the current font size
• (1en = 0.5em)
• 1ex = the height of an "x" in the current font.

The em is the most used e.g. for bigger title headings, smaller block quotes, etc.
So if you had, say, a current font at 10pt, and then use \font[size=0.9em]{smaller}, the "smaller text" will now 90% smaller, hence at 9pt.

The ex unit expresses a size relative to the x-height and is also sometimes used, mostly for scaling and raising characters in the same font. A poor's man superscript 1 could for instance be \raise[height=0.7ex]{\font[size=1.5ex]{1}} -- raised at 70% of a lowercase x, and made smaller theoretically in regular fonts, as the x-height is usually some ratio < 50% of the font size). This is where it gets weird, so let's illustrate with "Libertinus Serif " at 10pt. We can measure: 1em = 10pt (by definition), 1ex = 4.31pt (measured), and hence our "naive superscript here would be raised by approx. 3pt and use a 6.465pt font size.

(For the sake of completeness, other relative units exist, and may or may not be font-relative. A paragraph indent or a baseline leading will usually be font-relative, although this is by no mean mandatory.)

But as one can start pondering, the em/ex ratio depends on the font design and doesn't really tell how characters fit in that size. Different fonts at the same size (= having the same em) can vary a lot in the size of their letters within that "boundary". In other terms, fonts have different "eyes". Let's illustrate again with the "Hack" font at 10pt, measuring: 1em = 10pt (by definition), 1ex = 5.46875pt (measured)... So when one switches between different fonts, they don't match well side-by-side at the same font size. Here, a lowercase letter in "Hack" is much bigger that a lowercase letter in "Libertinus Serif", and such effects are (most of the time) quite ugly.

Yet, verbatim / code are usually about switching between a regular text font and a (monospace) code font. Unless the text font also comes with a well-made monospace code-looking equivalent (e.g. the Libertinus family has a "Libertinus Mono" font), some font adjustment must be made for a better-looking result.

This is now available SILE 0.15.10: \font[family=Hack, adjust=ex-height] will compute the target font size so that the "ex" matches that of the current font. (The other possible adjustment is cap-height). Let's use this to switch from Libertinus Serif at 10pt, and measure our Hack text: 1ex = 4.31pt (by construction ± rounding), 1em=7.88pt (measured) -- So we are now using a smaller absolute font size for Hack, but the lowercase letters match. It usually looks nicer... It is always a compromise here!

SILE 0.15.10 now uses this technique in \code and \verbatim:font. For the record, resilient.sile 2.8.0 also supports it, though one may have to tweak their existing style files as explained in the release note.

@davidchisnall Does this answer satisfies your initial question?

[Omikhleia]

Just to slightly update and correct my earlier response:

1ex= the height of an "x" in the current font.

To be really precise, this is wrong or somewhat approximate. As discussed elsewhere, the "x" character in a font might have a different height than the intended font-designer's actual "ex" size.
See #2317 (and related PR) for details and better explanations. But other than that, the above text remains valid (almost, i.e. with slightly different numbers in the examples).

[Omikhleia]

@davidchisnall Is there anything else needed for marking this topic as answered?

[alerque]

The easiest ways is to use relative units. For example the em unit is typically the size of the current font, so \font[size=0.95em]{this is 5% smaller} would make a smaller size relative to the surrounding context.

Relative sizes are typically not specified as with an absolute value offset from a relative size. You could do that, but you'd probably need to make a small wrapper function of your own to accomplish it:

SILE.registerCommand("three-smaller", function (options, content)
   options.size = SILE.types.measurement("1em"):absolute() - SILE.types.measurement("3pt")
   SILE.call("font", options, content)
end)

Then in your content, you could use \three-smaller{this is 3 hard points smaller than the surrounding context}.