What is the designed behaviour of font `covers` in math equations?

Y.D.X · July 10, 2025, 3:31pm

What is the designed behaviour of the following? Does the font A only covers 0 in texts in math, or all 0 in math, or anything?

#show math.equation: set text(font: (
  (name: "A", covers: regex("0")),
  "Some math font",
))

Full code

#set page(width: auto, height: auto, margin: 2em)

#set text(fallback: false)
#set raw(lang: "typc")
#show raw: set text(font: ("DejaVu Sans Mono", "Noto Serif CJK SC"))

#align(center, `$ a 0 9 "a09" $`)

= Example A

+ New Computer Modern Math
$ a 0 9 "a09" $

= Example B

+ Fira Math
#show math.equation: set text(font: "Fira Math")
$ a 0 9 "a09" $

= Example C1

+ Fira _Math_, covers `regex("0")`
+ New Computer Modern Math
#show math.equation: set text(font: (
  (name: "Fira Math", covers: regex("0")),
  "New Computer Modern Math",
))
$ a 0 9 "a09" $

= Example C2

+ Fira _Sans_, covers `regex("0")`
+ New Computer Modern Math
#show math.equation: set text(font: (
  (name: "Fira Sans", covers: regex("0")),
  "New Computer Modern Math",
))
$ a 0 9 "a09" $

My guess

For texts in math.equation, render the character using the first font that covers and contains it.
For real math in math.equation, render the chracter using the first font that support OpenType math features, ignoring covers configurations.

Background

I am trying to debug Setting fonts repeatedly with different `covers` is not effective · Issue #6566 · typst/typst · GitHub.

Y.D.X · July 11, 2025, 10:20am

It looks like there was no math-specific code in Support for defining which charset should be covered by a font by peng1999 · Pull Request #5305 · typst/typst · GitHub?

And the only two references of family.covers() are both in crates/typst-layout/src/inline/shaping.rs: one in insert_hyphen, another in shape_segment.

DeepWiki:

What is the designed behaviour of font `covers` in math equations? In the following example, does the font A only covers 0 in texts in math, or all 0 in math, or anything? #show math.equation: set te…Show full text - Search | DeepWiki
数学字体是怎么选择的？ - Search | DeepWiki

Conclusions

So my guess is right: find_math_font only concerns about family.as_str() and ignores covers.

github.com/typst/typst

crates/typst-layout/src/math/mod.rs

026453492


      
          fn find_math_font(
              engine: &mut Engine<'_>,
              styles: StyleChain,
              span: Span,
          ) -> SourceResult<Font> {
              let variant = variant(styles);
              let world = engine.world;
              let Some(font) = families(styles).find_map(|family| {
                  let id = world.book().select(family.as_str(), variant)?;
                  let font = world.font(id)?;
                  let _ = font.ttf().tables().math?.constants?;
                  Some(font)
              }) else {
                  bail!(span, "current font does not support math");
              };
              Ok(font)
          }

github.com/typst/typst

crates/typst-library/src/text/mod.rs

026453492


      
          /// A lowercased font family like "arial".
          #[derive(Debug, Clone, PartialEq, Hash)]
          pub struct FontFamily {
              // The name of the font family
              name: EcoString,
              // A regex that defines the Unicode codepoints supported by the font.
              covers: Option<Covers>,
          }

github.com/typst/typst

crates/typst-library/src/text/mod.rs

026453492


      
          /// The lowercased family name.
          pub fn as_str(&self) -> &str {
              &self.name
          }

mkorje · July 11, 2025, 10:53am

The font handling in math currently is very rudimentary, and doesn’t do any fallback nor supports covers. If/when Support multiple fonts in math by mkorje · Pull Request #6365 · typst/typst · GitHub gets merged, the situation in math will match normal text.

Y.D.X · July 11, 2025, 10:54am

Thanks! I am trying to document the behaviour in TextElem, and GitHub also reminds me the existence of #6365.

Update: Wow, You are the author of #6365!