Add support for variable font weight detection/conversion

[condemil]

OS: macOS 26.1 / Debian 13.2

Font: https://fonts.google.com/specimen/Nunito+Sans

Font is loaded:

$ resvg --list-fonts | grep Nunito                      
/Users/foo/Library/Fonts/NunitoSans-Italic[YTLC,opsz,wdth,wght].ttf: 'Nunito Sans (English, United States)', 0, Italic, 200, Normal
/Users/foo/Library/Fonts/NunitoSans[YTLC,opsz,wdth,wght].ttf: 'Nunito Sans (English, United States)', 0, Normal, 200, Normal

SVG to reproduce:

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1000 1000" fill="currentColor" stroke="none" >
    <text x="0" y="100" font-family="Nunito Sans" font-weight="bold" font-size="100">test</text>
</svg>

[Its-Just-Nans]

Note:

I think it's a SVG2 setting

• resvg/docs/svg2-changelog.md

  Line 152 in 17cde9e

  - [ ] Since CSS Fonts Module Level 4, the [`font-weight`](https://www.w3.org/TR/css-fonts-4/#font-weight-prop) property allows any value in a 1..1000 range.

[condemil]

I tried to use bold and bolder, but it doesn't work either. I see the following 16 styles available in macOS Font Book app:

[Its-Just-Nans]

It looks like it's because Nunito is a variable font

python -m pip install fonttools
fonttools varLib.instancer "NunitoSans-VariableFont_YTLC,opsz,wdth,wght.ttf" wght=800
# this command creates the "NunitoSans-VariableFont_YTLC,opsz,wdth,wght-partial.ttf"
# install the partial non variable font

# then
resvg file.svg file.png

Seems lilke resvg is not detecting that

cargo run -- --list-fonts | grep Nuni
fonts/NunitoSans-Italic-VariableFont_YTLC,opsz,wdth,wght.ttf: 'Nunito Sans (English, United States)', 0, Italic, Weight(200), Normal
fonts/NunitoSans-VariableFont_YTLC,opsz,wdth,wght-partial.ttf: 'Nunito Sans (English, United States)', 0, Normal, Weight(800), Normal
fonts/NunitoSans-VariableFont_YTLC,opsz,wdth,wght.ttf: 'Nunito Sans (English, United States)', 0, Normal, Weight(200), Normal

We can see

• the variable - italic mode
• the static 800 exported
• the variable - normal mode

[condemil]

Just tested in Debian 13 docker container, same issue:

$ sudo apt install resvg wget
$ mkdir -p ~/.fonts
$ wget -O ~/.fonts/NunitoSans.ttf https://raw.githubusercontent.com/google/fonts/refs/heads/main/ofl/nunitosans/NunitoSans%5BYTLC%2Copsz%2Cwdth%2Cwght%5D.ttf
$ resvg --list-fonts | grep Nunito
/root/.fonts/NunitoSans.ttf: 'Nunito Sans (English, United States)', 0, Normal, 200, Normal

[Its-Just-Nans]

Yep from what I can see the font datbase is made of FontInfo which contains pub weight: Weight, (not an array and no such pub variable: bool)

[condemil]

I assume it should be possible to implement normal | bold | bolder | lighter detection from a variable font in a similar manner as it's done in Font Book app

[Its-Just-Nans]

From fontdb docs

fontdb stores and matches font faces, not fonts. Therefore, after loading a font collection with 5 faces (for example), the database will be populated with 5 FaceInfo objects, all of which will be pointing to the same file or binary data.

[condemil]

From fontdb readme:

For font properties querying you can use [ttf-parser](https://github.com/RazrFalcon/ttf-parser)

I see that ttf-parser is already used in resvg.

[Its-Just-Nans]

Looks like the ttf_parser is correclty showing the variable=true

let variable = fontdb
    .with_face_data(face.id, |font_data, face_index| {
        let font = ttf_parser::Face::parse(font_data, face_index).unwrap();
        font.is_variable()
    })
    .unwrap();

File("fonts/NunitoSans-Italic-VariableFont_YTLC,opsz,wdth,wght.ttf") is variable: true
File("fonts/NunitoSans-VariableFont_YTLC,opsz,wdth,wght-partial.ttf") is variable: true
File("fonts/NunitoSans-VariableFont_YTLC,opsz,wdth,wght.ttf") is variable: true

I will check fontdb later if I have time

[condemil]

Maybe this example will help to understand how to extract styles/weights from variable font:

https://github.com/harfbuzz/ttf-parser/blob/main/examples/font-info.rs

[Its-Just-Nans]

I think the problem is that fontdb is not supporting variable fonts right now

I think

• the function parse_o2() https://github.com/RazrFalcon/fontdb/blob/62cfd96671eba4debe73eeecf541b1a3a051d223/src/lib.rs#L1145-L1165

could be updated to also have

const FVAR_TAG: ttf_parser::Tag = ttf_parser::Tag::from_bytes(b"fvar");
let table = match raw_face
    .table(FVAR_TAG)
    .and_then(ttf_parser::fvar::Table::parse)
{
    Some(table) => {
        // add the font with fvar to the database 
    },
    None => // do default,
};

but then i think the https://docs.rs/fontdb/latest/fontdb/struct.Weight.html is not suitable for variable font

I think @RazrFalcon (hope you're going well) know more about that

[RazrFalcon]

I'm not sure how it should be implemented either.

[Its-Just-Nans]

I'm not sure how it should be implemented either.

I thought of something like that RazrFalcon/fontdb#81
I appreciate if you can give a feedback - (but fontdb is on an unmaintained state, right ?)