Missing instance records in fvar::Table

[zimond]

related #120.

I think based on the spec, there are instance records in fvar. But currently ttf-parser only parses axis record?

[RazrFalcon]

Yes, it is not implemented.

[zimond]

@RazrFalcon do you mind if I submit a PR for this?

[RazrFalcon]

Sure. Note that the PostScript name field is optional, making parsing more convoluted, since you cannot use read_array16

You can check it using:

let has_post_script_name = instance_length == (axes_count * 4) + 6;

I guess the only way to implement it is by creating a custom wrapper for InstanceRecords data blob with a get method that will do the parsing.

[inferiorhumanorgans]

If an iterator pattern works then something like this is possible:

let names = face.tables().name.unwrap().names;

let fvar = face.tables().fvar.unwrap();

for instance in fvar.instances() {
    let subfamily = names
        .into_iter()
        .find(|n| n.name_id == instance.subfamily_name_id && n.is_unicode())
        .unwrap()
        .to_string()
        .unwrap();

    let ps_name = instance
        .ps_name_id
        .and_then(|name_id| {
            names
                .into_iter()
                .find(|n| n.name_id == name_id && n.is_unicode())
        })
        .and_then(|n| n.to_string());

    println!("subfamily: {subfamily}, ps_name: {ps_name:?}");
    for (axis, value) in fvar.axes.into_iter().zip(instance.user_tuples) {
        let axis_name = axis.tag.to_string();
        println!("\t{axis_name} {value:?}")
    }
    println!();
}

e.g.

./target/debug/examples/fvar ~/Library/Fonts/RobotoCondensed-VariableFont_wght.ttf
subfamily: Thin, ps_name: Some("RobotoCondensed-Thin")
        wght Fixed(100.0)

subfamily: ExtraLight, ps_name: Some("RobotoCondensed-ExtraLight")
        wght Fixed(200.0)

subfamily: Light, ps_name: Some("RobotoCondensed-Light")
        wght Fixed(300.0)

subfamily: Regular, ps_name: Some("RobotoCondensed-Regular")
        wght Fixed(400.0)

subfamily: Medium, ps_name: Some("RobotoCondensed-Medium")
        wght Fixed(500.0)

subfamily: SemiBold, ps_name: Some("RobotoCondensed-SemiBold")
        wght Fixed(600.0)

subfamily: Bold, ps_name: Some("RobotoCondensed-Bold")
        wght Fixed(700.0)

subfamily: ExtraBold, ps_name: Some("RobotoCondensed-ExtraBold")
        wght Fixed(800.0)

subfamily: Black, ps_name: Some("RobotoCondensed-Black")
        wght Fixed(900.0)

https://github.com/inferiorhumanorgans/ttf-parser/tree/fvar-instance

I'll hold off on a PR until someone who actually wants this feature can chime in with a use case and whether or not this is sufficient.