Capture overlap bits from TrueType glyphs

First step in addressing #581. Captures simple and composite overlap bits and plumbs them through internally.

Still needs to be exposed in skrifa public API. This will be done in a follow up.

font-test-data/test_data/ttx/vazirmatn_var_trimmed.ttx

@@ -126,7 +126,7 @@ and is intended to contain examples of many variable font tables -->
 
     <TTGlyph name="Agrave" xMin="29" yMin="0" xMax="1310" yMax="1847">
       <component glyphName="A" x="0" y="0" flags="0x204"/>
-      <component glyphName="grave" x="303" y="311" flags="0x4"/>
+      <component glyphName="grave" x="303" y="311" flags="0x404"/>
       <instructions>
         <assembly>
           PUSHB[ ]	/* 2 values pushed */
@@ -142,7 +142,7 @@ and is intended to contain examples of many variable font tables -->
 
     <TTGlyph name="grave" xMin="57" yMin="1242" xMax="474" yMax="1536">
       <contour>
-        <pt x="281" y="1536" on="1"/>
+        <pt x="281" y="1536" on="1" overlap="1"/>
         <pt x="474" y="1242" on="1"/>
         <pt x="315" y="1242" on="1"/>
         <pt x="57" y="1536" on="1"/>

read-fonts/src/tables/glyf.rs

@@ -145,6 +145,17 @@ impl<'a> SimpleGlyph<'a> {
             .unwrap_or(0)
     }
 
+    /// Returns true if the contours in the simple glyph may overlap.
+    pub fn has_overlapping_contours(&self) -> bool {
+        // Checks the first flag for the OVERLAP_SIMPLE bit.
+        // Spec says: "When used, it must be set on the first flag byte for
+        // the glyph."
+        FontData::new(self.glyph_data())
+            .read_at::<SimpleGlyphFlags>(0)
+            .map(|flag| flag.contains(SimpleGlyphFlags::OVERLAP_SIMPLE))
+            .unwrap_or_default()
+    }
+
     /// Reads points and flags into the provided buffers.
     ///
     /// Drops all flag bits except on-curve. The lengths of the buffers must be
@@ -479,8 +490,10 @@ impl<'a> CompositeGlyph<'a> {
 
     /// Returns an iterator that yields the glyph identifier of each component
     /// in the composite glyph.
-    pub fn component_glyphs(&self) -> impl Iterator<Item = GlyphId> + 'a + Clone {
-        ComponentGlyphIdIter {
+    pub fn component_glyphs_and_flags(
+        &self,
+    ) -> impl Iterator<Item = (GlyphId, CompositeGlyphFlags)> + 'a + Clone {
+        ComponentGlyphIdFlagsIter {
             cur_flags: CompositeGlyphFlags::empty(),
             done: false,
             cursor: FontData::new(self.component_data()).cursor(),
@@ -490,7 +503,7 @@ impl<'a> CompositeGlyph<'a> {
     /// Returns the component count and TrueType interpreter instructions
     /// in a single pass.
     pub fn count_and_instructions(&self) -> (usize, Option<&'a [u8]>) {
-        let mut iter = ComponentGlyphIdIter {
+        let mut iter = ComponentGlyphIdFlagsIter {
             cur_flags: CompositeGlyphFlags::empty(),
             done: false,
             cursor: FontData::new(self.component_data()).cursor(),
@@ -586,14 +599,14 @@ impl Iterator for ComponentIter<'_> {
 /// Significantly faster in cases where we're just processing the glyph
 /// tree, counting components or accessing instructions.
 #[derive(Clone)]
-struct ComponentGlyphIdIter<'a> {
+struct ComponentGlyphIdFlagsIter<'a> {
     cur_flags: CompositeGlyphFlags,
     done: bool,
     cursor: Cursor<'a>,
 }
 
-impl Iterator for ComponentGlyphIdIter<'_> {
-    type Item = GlyphId;
+impl Iterator for ComponentGlyphIdFlagsIter<'_> {
+    type Item = (GlyphId, CompositeGlyphFlags);
 
     fn next(&mut self) -> Option<Self::Item> {
         if self.done {
@@ -616,7 +629,7 @@ impl Iterator for ComponentGlyphIdIter<'_> {
             self.cursor.advance_by(8);
         }
         self.done = !flags.contains(CompositeGlyphFlags::MORE_COMPONENTS);
-        Some(glyph)
+        Some((glyph, flags))
     }
 }
 
@@ -945,6 +958,52 @@ mod tests {
         );
     }
 
+    #[test]
+    fn simple_glyph_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::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())
+            }
+        }
+    }
+
+    #[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))
+                }
+            }
+        }
+    }
+
     #[test]
     fn compute_anchor_flags() {
         let anchor = Anchor::Offset { x: -128, y: 127 };

skrifa/src/scale/glyf/glyph.rs

@@ -41,6 +41,8 @@ pub struct ScalerGlyph<'a> {
     pub has_hinting: bool,
     /// True if the glyph requires variation delta processing.
     pub has_variations: bool,
+    /// True if the glyph contains any simple or compound overlap flags.
+    pub has_overlaps: bool,
 }
 
 impl<'a> ScalerGlyph<'a> {

skrifa/src/scale/glyf/mem.rs

@@ -155,6 +155,7 @@ mod tests {
             max_component_delta_stack: 4,
             has_hinting: false,
             has_variations: true,
+            has_overlaps: false,
         };
         let required_size = outline_info.required_buffer_size();
         let mut buf = vec![0u8; required_size];
@@ -185,6 +186,7 @@ mod tests {
             max_component_delta_stack: 4,
             has_hinting: false,
             has_variations: true,
+            has_overlaps: false,
         };
         // Required size adds 4 bytes slop to account for internal alignment
         // requirements. So subtract 5 to force a failure.

skrifa/src/scale/glyf/scaler.rs

@@ -112,12 +112,14 @@ impl<'a> Scaler<'a> {
                 info.contours += simple.end_pts_of_contours().len();
                 info.has_hinting = info.has_hinting || simple.instruction_length() != 0;
                 info.max_other_points = info.max_other_points.max(num_points_with_phantom);
+                info.has_overlaps |= simple.has_overlapping_contours();
             }
             Glyph::Composite(composite) => {
                 let (mut count, instructions) = composite.count_and_instructions();
                 count += PHANTOM_POINT_COUNT;
                 let point_base = info.points;
-                for component in composite.component_glyphs() {
+                for (component, flags) in composite.component_glyphs_and_flags() {
+                    info.has_overlaps |= flags.contains(CompositeGlyphFlags::OVERLAP_COMPOUND);
                     let component_glyph = self.loca.get_glyf(component, &self.glyf)?;
                     let Some(component_glyph) = component_glyph else {
                         continue;
@@ -733,3 +735,22 @@ impl<'a, H> ScalerInstance<'a, H> {
         self.phantom[3].y = self.phantom[2].y - F26Dot6::from_bits(vadvance);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use read_fonts::{FontRef, TableProvider};
+
+    #[test]
+    fn overlap_flags() {
+        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 overflag bit on the first flag
+            assert_eq!(glyph.has_overlaps, gid == 2 || gid == 3);
+        }
+    }
+}