[skrifa] return overlap status from scaler (#583)

Changes `Scaler::outline` to return a new `ScalerMetrics` struct that contains the overlap flag. This also has optional fields to represent metrics that may be adjusted by hinting.

Based on #582 which must be merged first.
Fixes #581

read-fonts/src/tables/glyf.rs

@@ -958,50 +958,54 @@ mod tests {
         );
     }
 
-    #[test]
-    fn simple_glyph_overlapping_contour_flag() {
-        let font = FontRef::new(font_test_data::VAZIRMATN_VAR).unwrap();
+    // Test helper to enumerate all TrueType glyphs in the given font
+    fn all_glyphs(font_data: &[u8]) -> impl Iterator<Item = Option<Glyph>> {
+        let font = FontRef::new(font_data).unwrap();
         let loca = font.loca(None).unwrap();
         let glyf = font.glyf().unwrap();
         let glyph_count = font.maxp().unwrap().num_glyphs();
-        for gid in 0..glyph_count {
-            let glyph = match loca.get_glyf(GlyphId::new(gid), &glyf) {
-                Ok(Some(Glyph::Simple(glyph))) => glyph,
-                _ => continue,
-            };
-            if gid == 3 {
-                // Only GID 3 has the overlap bit set
-                assert!(glyph.has_overlapping_contours())
-            } else {
-                assert!(!glyph.has_overlapping_contours())
-            }
-        }
+        (0..glyph_count).map(move |gid| loca.get_glyf(GlyphId::new(gid), &glyf).unwrap())
     }
 
     #[test]
-    fn composite_overlapping_contour_flag() {
-        let font = FontRef::new(font_test_data::VAZIRMATN_VAR).unwrap();
-        let loca = font.loca(None).unwrap();
-        let glyf = font.glyf().unwrap();
-        let glyph_count = font.maxp().unwrap().num_glyphs();
-        for gid in 0..glyph_count {
-            let glyph = match loca.get_glyf(GlyphId::new(gid), &glyf) {
-                Ok(Some(Glyph::Composite(glyph))) => glyph,
-                _ => continue,
-            };
-            // Only GID 2, component 1 has the overlap bit set
-            for (component_ix, component) in glyph.components().enumerate() {
-                if gid == 2 && component_ix == 1 {
-                    assert!(component
-                        .flags
-                        .contains(CompositeGlyphFlags::OVERLAP_COMPOUND))
-                } else {
-                    assert!(!component
-                        .flags
-                        .contains(CompositeGlyphFlags::OVERLAP_COMPOUND))
-                }
-            }
-        }
+    fn simple_glyph_overlapping_contour_flag() {
+        let gids_with_overlap: Vec<_> = all_glyphs(font_test_data::VAZIRMATN_VAR)
+            .enumerate()
+            .filter_map(|(gid, glyph)| match glyph {
+                Some(Glyph::Simple(glyph)) if glyph.has_overlapping_contours() => Some(gid),
+                _ => None,
+            })
+            .collect();
+        // Only GID 3 has the overlap bit set
+        let expected_gids_with_overlap = vec![3];
+        assert_eq!(expected_gids_with_overlap, gids_with_overlap);
+    }
+
+    #[test]
+    fn composite_glyph_overlapping_contour_flag() {
+        let gids_components_with_overlap: Vec<_> = all_glyphs(font_test_data::VAZIRMATN_VAR)
+            .enumerate()
+            .filter_map(|(gid, glyph)| match glyph {
+                Some(Glyph::Composite(glyph)) => Some((gid, glyph)),
+                _ => None,
+            })
+            .flat_map(|(gid, glyph)| {
+                glyph
+                    .components()
+                    .enumerate()
+                    .filter_map(move |(comp_ix, comp)| {
+                        comp.flags
+                            .contains(CompositeGlyphFlags::OVERLAP_COMPOUND)
+                            .then_some((gid, comp_ix))
+                    })
+            })
+            .collect();
+        // Only GID 2, component 1 has the overlap bit set
+        let expected_gids_components_with_overlap = vec![(2, 1)];
+        assert_eq!(
+            expected_gids_components_with_overlap,
+            gids_components_with_overlap
+        );
     }
 
     #[test]

skrifa/src/scale/glyf/scaler.rs

@@ -746,11 +746,18 @@ mod tests {
         let font = FontRef::new(font_test_data::VAZIRMATN_VAR).unwrap();
         let scaler = Scaler::new(&font).unwrap();
         let glyph_count = font.maxp().unwrap().num_glyphs();
-        for gid in 0..glyph_count {
-            let glyph = scaler.glyph(GlyphId::new(gid), false).unwrap();
-            // GID 2 is a composite glyph with the overlap bit on a component
-            // GID 3 is a simple glyph with the overlap bit on the first flag
-            assert_eq!(glyph.has_overlaps, gid == 2 || gid == 3);
-        }
+        // GID 2 is a composite glyph with the overlap bit on a component
+        // GID 3 is a simple glyph with the overlap bit on the first flag
+        let expected_gids_with_overlap = vec![2, 3];
+        assert_eq!(
+            expected_gids_with_overlap,
+            (0..glyph_count)
+                .filter_map(|gid| scaler
+                    .glyph(GlyphId::new(gid), false)
+                    .unwrap()
+                    .has_overlaps
+                    .then_some(gid))
+                .collect::<Vec<_>>()
+        );
     }
 }

skrifa/src/scale/mod.rs

@@ -155,7 +155,7 @@ mod scaler;
 pub use read_fonts::types::Pen;
 
 pub use error::{Error, Result};
-pub use scaler::{Scaler, ScalerBuilder};
+pub use scaler::{Scaler, ScalerBuilder, ScalerMetrics};
 
 use super::{
     font::UniqueId,
@@ -217,7 +217,7 @@ impl Context {
 #[cfg(test)]
 mod tests {
     use super::{Context, Size};
-    use read_fonts::{scaler_test, FontRef};
+    use read_fonts::{scaler_test, types::GlyphId, FontRef, TableProvider};
 
     #[test]
     fn vazirmatin_var() {
@@ -246,6 +246,31 @@ mod tests {
         );
     }
 
+    #[test]
+    fn overlap_flags() {
+        let font = FontRef::new(font_test_data::VAZIRMATN_VAR).unwrap();
+        let mut cx = Context::new();
+        let mut path = scaler_test::Path {
+            elements: vec![],
+            is_cff: false,
+        };
+        let mut scaler = cx.new_scaler().build(&font);
+        let glyph_count = font.maxp().unwrap().num_glyphs();
+        // GID 2 is a composite glyph with the overlap bit on a component
+        // GID 3 is a simple glyph with the overlap bit on the first flag
+        let expected_gids_with_overlap = vec![2, 3];
+        assert_eq!(
+            expected_gids_with_overlap,
+            (0..glyph_count)
+                .filter_map(|gid| scaler
+                    .outline(GlyphId::new(gid), &mut path)
+                    .unwrap()
+                    .has_overlaps
+                    .then_some(gid))
+                .collect::<Vec<_>>()
+        );
+    }
+
     fn compare_glyphs(font_data: &[u8], expected_outlines: &str, is_cff: bool) {
         let font = FontRef::new(font_data).unwrap();
         let outlines = scaler_test::parse_glyph_outlines(expected_outlines);

skrifa/src/scale/scaler.rs

@@ -11,6 +11,20 @@ use read_fonts::{
     TableProvider,
 };
 
+/// Information and adjusted metrics generated while scaling a glyph.
+#[derive(Copy, Clone, Default, Debug)]
+pub struct ScalerMetrics {
+    /// True if the underlying glyph contains flags indicating the
+    /// presence of overlapping contours or components.
+    pub has_overlaps: bool,
+    /// If present, an adjusted left side bearing value generated by the
+    /// scaler.
+    pub adjusted_lsb: Option<f32>,
+    /// If present, an adjusted advance width value generated by the
+    /// scaler.
+    pub adjusted_advance_width: Option<f32>,
+}
+
 /// Builder for configuring a glyph scaler.
 ///
 /// See the [module level documentation](crate::scale#building-a-scaler)
@@ -206,7 +220,7 @@ impl<'a> Scaler<'a> {
 
     /// Loads a simple outline for the specified glyph identifier and invokes the functions
     /// in the given pen for the sequence of path commands that define the outline.
-    pub fn outline(&mut self, glyph_id: GlyphId, pen: &mut impl Pen) -> Result<()> {
+    pub fn outline(&mut self, glyph_id: GlyphId, pen: &mut impl Pen) -> Result<ScalerMetrics> {
         if let Some(outlines) = &mut self.outlines {
             outlines.outline(glyph_id, self.size, self.coords, pen)
         } else {
@@ -230,7 +244,7 @@ impl<'a> Outlines<'a> {
         size: f32,
         coords: &'a [NormalizedCoord],
         pen: &mut impl Pen,
-    ) -> Result<()> {
+    ) -> Result<ScalerMetrics> {
         match self {
             Self::TrueType(scaler, buf) => {
                 let glyph = scaler.glyph(glyph_id, false)?;
@@ -242,14 +256,21 @@ impl<'a> Outlines<'a> {
                     .memory_from_buffer(&mut buf[..])
                     .ok_or(Error::InsufficientMemory)?;
                 let outline = scaler.outline(memory, &glyph, size, coords)?;
-                Ok(outline.to_path(pen)?)
+                outline.to_path(pen)?;
+                Ok(ScalerMetrics {
+                    has_overlaps: glyph.has_overlaps,
+                    ..Default::default()
+                })
             }
             Self::PostScript(scaler, subfont) => {
                 let subfont_index = scaler.subfont_index(glyph_id);
                 if subfont_index != subfont.index() {
                     *subfont = scaler.subfont(subfont_index, size, coords)?;
                 }
-                Ok(scaler.outline(subfont, glyph_id, coords, false, pen)?)
+                scaler.outline(subfont, glyph_id, coords, false, pen)?;
+                // CFF does not have overlap flags and hinting never adjusts
+                // horizontal metrics
+                Ok(ScalerMetrics::default())
             }
         }
     }