build MVAR table from variable GlobalMetrics

Fixes #531

Author: anthrotype

fontbe/src/error.rs

@@ -50,6 +50,8 @@ pub enum Error {
     OutOfBounds { what: String, value: String },
     #[error("Unable to compute deltas for {0}: {1}")]
     GlyphDeltaError(GlyphName, DeltaError),
+    #[error("Unable to compute deltas for MVAR {0}: {1}")]
+    MvarDeltaError(Tag, DeltaError),
     #[error("Unable to assemble gvar")]
     GvarError(#[from] GvarInputError),
     #[error("Unable to read")]

fontbe/src/font.rs

@@ -8,7 +8,7 @@ use write_fonts::{
     tables::{
         avar::Avar, cmap::Cmap, fvar::Fvar, gdef::Gdef, glyf::Glyf, gpos::Gpos, gsub::Gsub,
         gvar::Gvar, head::Head, hhea::Hhea, hmtx::Hmtx, hvar::Hvar, loca::Loca, maxp::Maxp,
-        name::Name, os2::Os2, post::Post, stat::Stat,
+        mvar::Mvar, name::Name, os2::Os2, post::Post, stat::Stat,
     },
     types::Tag,
     FontBuilder,
@@ -50,6 +50,7 @@ const TABLES_TO_MERGE: &[(WorkId, Tag, TableType)] = &[
     (WorkId::Post, Post::TAG, TableType::Static),
     (WorkId::Stat, Stat::TAG, TableType::Variable),
     (WorkId::Hvar, Hvar::TAG, TableType::Variable),
+    (WorkId::Mvar, Mvar::TAG, TableType::Variable),
 ];
 
 fn has(context: &Context, id: WorkId) -> bool {
@@ -72,6 +73,7 @@ fn has(context: &Context, id: WorkId) -> bool {
         WorkId::Post => context.post.try_get().is_some(),
         WorkId::Stat => context.stat.try_get().is_some(),
         WorkId::Hvar => context.hvar.try_get().is_some(),
+        WorkId::Mvar => context.mvar.try_get().is_some(),
         _ => false,
     }
 }
@@ -97,6 +99,7 @@ fn bytes_for(context: &Context, id: WorkId) -> Result<Option<Vec<u8>>, Error> {
         WorkId::Post => to_bytes(context.post.get().as_ref()),
         WorkId::Stat => to_bytes(context.stat.get().as_ref()),
         WorkId::Hvar => to_bytes(context.hvar.get().as_ref()),
+        WorkId::Mvar => to_bytes(context.mvar.get().as_ref()),
         _ => panic!("Missing a match for {id:?}"),
     };
     Ok(bytes)
@@ -127,6 +130,7 @@ impl Work<Context, AnyWorkId, Error> for FontWork {
             .variant(WorkId::Post)
             .variant(WorkId::Stat)
             .variant(WorkId::Hvar)
+            .variant(WorkId::Mvar)
             .variant(WorkId::LocaFormat)
             .variant(FeWorkId::StaticMetadata)
             .build()

fontbe/src/lib.rs

@@ -11,6 +11,7 @@ pub mod hvar;
 pub mod kern;
 pub mod marks;
 pub mod metrics_and_limits;
+pub mod mvar;
 pub mod name;
 pub mod orchestration;
 pub mod os2;

fontbe/src/mvar.rs

@@ -0,0 +1,329 @@
+//! Generates an [MVAR](https://learn.microsoft.com/en-us/typography/opentype/spec/MVAR) table.
+
+use std::collections::{BTreeMap, BTreeSet, HashMap};
+
+use fontdrasil::orchestration::AccessBuilder;
+
+use fontdrasil::{
+    coords::NormalizedLocation,
+    orchestration::{Access, Work},
+    types::Axis,
+};
+use fontir::{
+    ir::GlobalMetricValues, orchestration::WorkId as FeWorkId, variations::VariationModel,
+};
+use write_fonts::types::MajorMinor;
+use write_fonts::{
+    tables::{
+        mvar::{Mvar, ValueRecord},
+        variations::{ivs_builder::VariationStoreBuilder, VariationRegion},
+    },
+    types::Tag,
+    OtRound,
+};
+
+use crate::{
+    error::Error,
+    orchestration::{AnyWorkId, BeWork, Context, WorkId},
+};
+
+#[derive(Debug)]
+struct MvarWork {}
+
+pub fn create_mvar_work() -> Box<BeWork> {
+    Box::new(MvarWork {})
+}
+
+/// Helper to build MVAR table from global metrics sources.
+struct MvarBuilder {
+    /// Variation axes
+    axes: Vec<Axis>,
+    /// Sparse variation models, keyed by the set of locations they define
+    models: HashMap<BTreeSet<NormalizedLocation>, VariationModel>,
+    /// Metrics deltas keyed by MVAR tag
+    deltas: BTreeMap<Tag, Vec<(VariationRegion, i16)>>,
+}
+
+impl MvarBuilder {
+    fn new(global_model: VariationModel) -> Self {
+        let axes = global_model.axes().cloned().collect::<Vec<_>>();
+        let global_locations = global_model.locations().cloned().collect::<BTreeSet<_>>();
+        let mut models = HashMap::new();
+        models.insert(global_locations, global_model);
+        MvarBuilder {
+            axes,
+            models,
+            deltas: BTreeMap::new(),
+        }
+    }
+
+    fn add_sources(&mut self, mvar_tag: Tag, sources: &GlobalMetricValues) -> Result<(), Error> {
+        let sources: HashMap<_, _> = sources
+            .iter()
+            .map(|(loc, src)| (loc.clone(), vec![src.into_inner() as f64]))
+            .collect();
+        if sources.len() == 1 {
+            assert!(sources.keys().next().unwrap().is_default());
+            // spare the model the work of computing no-op deltas
+            return Ok(());
+        }
+        let locations = sources.keys().cloned().collect::<BTreeSet<_>>();
+        let model = self.models.entry(locations).or_insert_with(|| {
+            // this glyph defines its own set of locations, a new sparse model is needed
+            VariationModel::new(sources.keys().cloned().collect(), self.axes.clone()).unwrap()
+        });
+        let deltas: Vec<_> = model
+            .deltas(&sources)
+            .map_err(|e| Error::MvarDeltaError(mvar_tag, e))?
+            .into_iter()
+            .filter_map(|(region, values)| {
+                if region.is_default() {
+                    return None;
+                }
+                // Only 1 value per region for our input
+                assert!(values.len() == 1, "{} values?!", values.len());
+                Some((
+                    region.to_write_fonts_variation_region(&self.axes),
+                    values[0].ot_round(),
+                ))
+            })
+            .collect();
+        // don't encode no-op deltas
+        if deltas.iter().all(|(_, delta)| *delta == 0) {
+            return Ok(());
+        }
+        self.deltas.insert(mvar_tag, deltas);
+        Ok(())
+    }
+
+    fn build(self) -> Option<Mvar> {
+        let mut builder = VariationStoreBuilder::new(self.axes.len() as u16);
+        let delta_ids = self
+            .deltas
+            .into_iter()
+            .map(|(tag, deltas)| (tag, builder.add_deltas(deltas)))
+            .collect::<Vec<_>>();
+
+        let (varstore, index_map) = builder.build();
+
+        let records = delta_ids
+            .into_iter()
+            .map(|(tag, temp_id)| {
+                let varidx = index_map.get(temp_id).unwrap();
+                ValueRecord::new(
+                    tag,
+                    varidx.delta_set_outer_index,
+                    varidx.delta_set_inner_index,
+                )
+            })
+            .collect::<Vec<_>>();
+
+        (!records.is_empty()).then(|| Mvar::new(MajorMinor::VERSION_1_0, Some(varstore), records))
+    }
+}
+
+impl Work<Context, AnyWorkId, Error> for MvarWork {
+    fn id(&self) -> AnyWorkId {
+        WorkId::Mvar.into()
+    }
+
+    fn read_access(&self) -> Access<AnyWorkId> {
+        AccessBuilder::new()
+            .variant(FeWorkId::StaticMetadata)
+            .variant(FeWorkId::GlobalMetrics)
+            .build()
+    }
+
+    /// Generate [MVAR](https://learn.microsoft.com/en-us/typography/opentype/spec/MVAR)
+    fn exec(&self, context: &Context) -> Result<(), Error> {
+        let static_metadata = context.ir.static_metadata.get();
+        let metrics = context.ir.global_metrics.get();
+        let var_model = &static_metadata.variation_model;
+
+        let mut mvar_builder = MvarBuilder::new(var_model.clone());
+        for (metric, values) in metrics.iter() {
+            // maybe we should get rid of GlobalMetric variants that aren't MVAR-relevant?
+            if let Some(mvar_tag) = metric.mvar_tag() {
+                mvar_builder.add_sources(mvar_tag, values)?;
+            }
+        }
+        let mvar = mvar_builder.build();
+
+        context.mvar.set_unconditionally(mvar.into());
+
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::str::FromStr;
+
+    use fontdrasil::{
+        coords::{CoordConverter, UserCoord},
+        types::Axis,
+    };
+    use write_fonts::{
+        dump_table,
+        read::{
+            tables::{mvar as read_mvar, variations::ItemVariationData},
+            FontData, FontRead,
+        },
+        types::F2Dot14,
+    };
+
+    use super::*;
+
+    fn axis(tag: &str, min: f32, default: f32, max: f32) -> Axis {
+        let min = UserCoord::new(min);
+        let default = UserCoord::new(default);
+        let max = UserCoord::new(max);
+        Axis {
+            name: tag.to_string(),
+            tag: Tag::from_str(tag).unwrap(),
+            min,
+            default,
+            max,
+            hidden: false,
+            converter: CoordConverter::unmapped(min, default, max),
+        }
+    }
+
+    fn new_mvar_builder(locations: Vec<&NormalizedLocation>, axes: Vec<Axis>) -> MvarBuilder {
+        let locations = locations.into_iter().cloned().collect();
+        let model = VariationModel::new(locations, axes).unwrap();
+        MvarBuilder::new(model)
+    }
+
+    fn add_sources(
+        builder: &mut MvarBuilder,
+        mvar_tag: &str,
+        sources: &[(&NormalizedLocation, f32)],
+    ) {
+        let sources = sources
+            .iter()
+            .map(|(loc, value)| ((*loc).clone(), (*value).into()))
+            .collect::<HashMap<_, _>>();
+        builder
+            .add_sources(Tag::from_str(mvar_tag).unwrap(), &sources)
+            .unwrap();
+    }
+
+    fn delta_sets(var_data: &ItemVariationData) -> Vec<Vec<i32>> {
+        (0..var_data.item_count())
+            .map(|i| var_data.delta_set(i).collect::<Vec<_>>())
+            .collect()
+    }
+
+    #[test]
+    fn smoke_test() {
+        let regular = NormalizedLocation::for_pos(&[("wght", 0.0)]);
+        let bold = NormalizedLocation::for_pos(&[("wght", 1.0)]);
+        let mut builder = new_mvar_builder(
+            vec![&regular, &bold],
+            vec![axis("wght", 400.0, 400.0, 700.0)],
+        );
+
+        add_sources(&mut builder, "xhgt", &[(&regular, 500.0), (&bold, 550.0)]);
+
+        let Some(mvar) = builder.build() else {
+            panic!("no MVAR?!");
+        };
+
+        let bytes = dump_table(&mvar).unwrap();
+        let mvar = read_mvar::Mvar::read(FontData::new(&bytes)).unwrap();
+
+        assert_eq!(mvar.version(), MajorMinor::VERSION_1_0);
+        assert_eq!(mvar.value_records().len(), 1);
+
+        let rec = &mvar.value_records()[0];
+        assert_eq!(rec.value_tag(), Tag::new(b"xhgt"));
+        assert_eq!(rec.delta_set_outer_index(), 0);
+        assert_eq!(rec.delta_set_inner_index(), 0);
+
+        let Some(Ok(varstore)) = mvar.item_variation_store() else {
+            panic!("MVAR has no ItemVariationStore?!");
+        };
+
+        assert_eq!(varstore.variation_region_list().unwrap().region_count(), 1);
+        assert_eq!(varstore.item_variation_data_count(), 1);
+
+        let vardata = varstore.item_variation_data().get(0).unwrap().unwrap();
+        assert_eq!(vardata.region_indexes(), &[0]);
+        assert_eq!(delta_sets(&vardata), vec![vec![50]]);
+    }
+
+    #[test]
+    fn no_variations_no_mvar() {
+        let regular = NormalizedLocation::for_pos(&[("wght", 0.0)]);
+        let bold = NormalizedLocation::for_pos(&[("wght", 1.0)]);
+        let mut builder = new_mvar_builder(
+            vec![&regular, &bold],
+            vec![axis("wght", 400.0, 400.0, 700.0)],
+        );
+
+        add_sources(&mut builder, "xhgt", &[(&regular, 500.0), (&bold, 500.0)]);
+        add_sources(&mut builder, "cpht", &[(&regular, 800.0), (&bold, 800.0)]);
+
+        // hence no MVAR needed
+        assert!(builder.build().is_none());
+    }
+
+    struct MvarReader<'a> {
+        mvar: read_mvar::Mvar<'a>,
+    }
+
+    impl<'a> MvarReader<'a> {
+        fn new(mvar: read_mvar::Mvar<'a>) -> Self {
+            Self { mvar }
+        }
+
+        fn metric_delta(&self, mvar_tag: &str, coords: &[f32]) -> f64 {
+            let mvar_tag = Tag::from_str(mvar_tag).unwrap();
+            let coords: Vec<F2Dot14> = coords
+                .iter()
+                .map(|coord| F2Dot14::from_f32(*coord))
+                .collect();
+            self.mvar.metric_delta(mvar_tag, &coords).unwrap().to_f64()
+        }
+    }
+
+    #[test]
+    fn sparse_global_metrics() {
+        let regular = NormalizedLocation::for_pos(&[("wght", 0.0)]);
+        let medium = NormalizedLocation::for_pos(&[("wght", 0.5)]);
+        let bold = NormalizedLocation::for_pos(&[("wght", 1.0)]);
+        let mut builder = new_mvar_builder(
+            vec![&regular, &medium, &bold],
+            vec![axis("wght", 400.0, 400.0, 700.0)],
+        );
+        // 'xhgt' defines a value for all three locations
+        add_sources(
+            &mut builder,
+            "xhgt",
+            &[(&regular, 500.0), (&medium, 530.0), (&bold, 550.0)],
+        );
+        // 'strs' is sparse: defines a value for regular and bold, not medium
+        add_sources(&mut builder, "strs", &[(&regular, 50.0), (&bold, 100.0)]);
+
+        let Some(mvar) = builder.build() else {
+            panic!("no MVAR?!");
+        };
+
+        let bytes = dump_table(&mvar).unwrap();
+        let mvar = read_mvar::Mvar::read(FontData::new(&bytes)).unwrap();
+
+        assert_eq!(mvar.value_records().len(), 2);
+        assert_eq!(mvar.value_records()[0].value_tag(), Tag::new(b"strs"));
+        assert_eq!(mvar.value_records()[1].value_tag(), Tag::new(b"xhgt"));
+
+        let mvar = MvarReader::new(mvar);
+        assert_eq!(mvar.metric_delta("xhgt", &[0.0]), 0.0);
+        assert_eq!(mvar.metric_delta("xhgt", &[0.5]), 30.0); // not 25.0
+        assert_eq!(mvar.metric_delta("xhgt", &[1.0]), 50.0);
+
+        assert_eq!(mvar.metric_delta("strs", &[0.0]), 0.0);
+        assert_eq!(mvar.metric_delta("strs", &[0.5]), 25.0); // interpolated
+        assert_eq!(mvar.metric_delta("strs", &[1.0]), 50.0);
+    }
+}