Merge #583: M7: keep ligatures under small tracking

crates/text/src/features.rs

@@ -3,13 +3,25 @@
 use harfrust::{Feature, Tag};
 use vectorcraft_doc::CharStyle;
 
+/// Tracking (in 1/1000 em) beyond which standard ligatures are dropped automatically.
+///
+/// Letterspaced type should not ligate: an `fi` glyph keeps its letters tight while everything
+/// around it opens up, which reads as a blot. Small tracking adjustments made to fit a line
+/// (a few thousandths either way) are not letterspacing and keep the ligatures, so only
+/// `|tracking|` above this threshold suppresses them. An explicit `liga` / `clig` in the
+/// character's own features always wins.
+pub const LIGATURE_TRACKING_LIMIT: f64 = 50.0;
+
 /// OpenType features applied during shaping. Kerning, `case` and ligature suppression are driven
 /// by the character style (`kerning`, `all_caps`, `tracking`); the rest are layout-wide options.
+/// Standard ligatures are dropped when `|tracking|` exceeds [`LIGATURE_TRACKING_LIMIT`], unless
+/// the character's features turn `liga` (or `clig`) on explicitly.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct OtFeatures {
     /// Vertical glyph alternates (`vert`), set by the writing direction.
     pub vertical: bool,
-    /// Standard ligatures (`liga`, `clig`). Suppressed automatically when tracking is non-zero.
+    /// Standard ligatures (`liga`, `clig`). Suppressed automatically when `|tracking|` exceeds
+    /// [`LIGATURE_TRACKING_LIMIT`], unless the character turns them on explicitly.
     pub ligatures: bool,
     /// Contextual alternates (`calt`).
     pub contextual: bool,
@@ -117,7 +129,7 @@ impl OtFeatures {
         if st.kerning.is_some() {
             v.push(f(b"kern", false));
         }
-        let liga = s.ligatures && st.tracking.abs() < 1e-9;
+        let liga = s.ligatures && (!ligatures_suppressed_by(st.tracking) || explicit_ligatures(&st.features));
         if !liga {
             v.push(f(b"liga", false));
             v.push(f(b"clig", false));
@@ -150,6 +162,27 @@ impl OtFeatures {
     }
 }
 
+/// Does `tracking` (1/1000 em) count as letterspacing that drops standard ligatures?
+pub fn ligatures_suppressed_by(tracking: f64) -> bool {
+    // A non-finite value is not a real letterspacing request; keep the default.
+    tracking.is_finite() && tracking.abs() > LIGATURE_TRACKING_LIMIT
+}
+
+/// Do `features` turn standard ligatures on explicitly (`liga`, `+liga`, `clig`, `+clig`)? The last
+/// ligature tag wins, matching [`OtFeatures::with_tags`]. Absence of a tag means "default", which
+/// [`OtFeatures::to_tags`] never writes, so this is distinguishable from an explicit request.
+pub fn explicit_ligatures<S: AsRef<str>>(features: &[S]) -> bool {
+    features
+        .iter()
+        .rev()
+        .find_map(|t| match t.as_ref() {
+            "liga" | "+liga" | "clig" | "+clig" => Some(true),
+            "-liga" | "-clig" => Some(false),
+            _ => None,
+        })
+        .unwrap_or(false)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

crates/text/src/lib.rs

@@ -23,7 +23,7 @@ pub mod test_fonts;
 pub mod thread;
 
 pub use craft_fonts::{CRAFT_FONTS, CraftFont};
-pub use features::OtFeatures;
+pub use features::{LIGATURE_TRACKING_LIMIT, OtFeatures, explicit_ligatures, ligatures_suppressed_by};
 pub use fontdb::{FALLBACK_FAMILY, FontClass, FontDb, FontFace, FontMatch, FontTraits, style_weight, system_font_dirs};
 use kurbo::{Affine, BezPath, Point, Rect, Vec2};
 pub use layout::{layout, layout_with};

crates/text/src/tests_typo.rs

@@ -370,9 +370,22 @@ fn opentype_ligature_switches() {
     assert_eq!(layout(db(), &t).glyphs.len(), 1, "fi ligature by default");
     let off = LayoutOptions { features: OtFeatures { ligatures: false, ..Default::default() }, ..Default::default() };
     assert_eq!(layout_with(db(), &t, &off).glyphs.len(), 2);
-    // Tracking suppresses ligatures (as letterspaced type should).
-    let tracked = TextObject::point(Point::ZERO, "fi", CharStyle { tracking: 100.0, ..serif });
-    assert_eq!(layout(db(), &tracked).glyphs.len(), 2);
+    let fi = |tracking: f64, features: &[&str]| {
+        let st = CharStyle { tracking, features: features.iter().map(|s| s.to_string()).collect(), ..serif.clone() };
+        layout(db(), &TextObject::point(Point::ZERO, "fi", st)).glyphs.len()
+    };
+    // Small tracking adjustments (fitting a line) keep the ligature.
+    assert_eq!(fi(-5.0, &[]), 1, "tracking -5 keeps fi");
+    assert_eq!(fi(20.0, &[]), 1, "tracking +20 keeps fi");
+    assert_eq!(fi(LIGATURE_TRACKING_LIMIT, &[]), 1, "the limit itself keeps fi");
+    // Real letterspacing suppresses ligatures (as letterspaced type should)...
+    assert_eq!(fi(100.0, &[]), 2, "tracking 100 drops fi");
+    assert_eq!(fi(-100.0, &[]), 2, "tracking -100 drops fi");
+    // ...unless the character turns them on explicitly.
+    assert_eq!(fi(100.0, &["liga"]), 1, "explicit liga wins over tracking");
+    assert_eq!(fi(100.0, &["+liga"]), 1);
+    assert_eq!(fi(100.0, &["liga", "-liga"]), 2, "the last tag wins");
+    assert_eq!(fi(0.0, &["-liga"]), 2, "explicit off still turns it off");
     let f = OtFeatures::from_tags(["dlig", "-liga", "smcp", "onum", "bogus"]);
     assert!(f.discretionary_ligatures && !f.ligatures && f.small_caps && f.oldstyle_figures && !f.fractions);
 }

crates/ui-egui/src/panels/opentype.rs

@@ -63,7 +63,7 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
             *field(&mut next) = !on;
         }
     }
-    if st.tracking.abs() > 1e-9 && cur.ligatures {
+    if cur.ligatures && vectorcraft_text::ligatures_suppressed_by(st.tracking) && !vectorcraft_text::explicit_ligatures(&st.features) {
         widgets::dim_label(ui, tl!("Ligatures are off while the text is tracked."));
     }
     if next != cur {