Type: Kerning has an Optical option that spaces characters by their shapes

Squashed from #4825 onto main.

crates/doc/src/tests.rs

@@ -388,6 +388,18 @@ fn char_position_round_trips() {
     assert!((scale - 0.583).abs() < 1e-9 && (shift + 3.33).abs() < 1e-9);
 }
 
+#[test]
+fn optical_kerning_round_trips_and_old_style_defaults_to_auto() {
+    let st = CharStyle { optical_kerning: true, kerning: Some(120.0), ..CharStyle::default() };
+    let value = serde_json::to_value(&st).unwrap();
+    assert_eq!(value["opticalKerning"], true);
+    assert_eq!(serde_json::from_value::<CharStyle>(value.clone()).unwrap(), st);
+    let mut old = value;
+    old.as_object_mut().unwrap().remove("opticalKerning");
+    assert!(!serde_json::from_value::<CharStyle>(old).unwrap().optical_kerning);
+    assert!(serde_json::to_value(CharStyle::default()).unwrap().get("opticalKerning").is_none());
+}
+
 #[test]
 fn units() {
     assert_eq!(Unit::Inches.parse("1"), Some(72.0));

crates/doc/src/text/char_style.rs

@@ -25,9 +25,12 @@ pub struct CharStyle {
     /// Tracking in 1/1000 em.
     #[serde(default)]
     pub tracking: f64,
-    /// Kerning: None = Auto (metrics), Some(v) = manual in 1/1000 em.
+    /// Manual pair spacing in 1/1000 em; None uses the automatic mode.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub kerning: Option<f64>,
+    /// Use glyph shapes for automatic pair spacing; false uses font metrics.
+    #[serde(default, rename = "opticalKerning", skip_serializing_if = "crate::skip::is_default")]
+    pub optical_kerning: bool,
     #[serde(default)]
     pub baseline_shift: f64,
     #[serde(default = "hundred")]
@@ -266,6 +269,7 @@ impl Default for CharStyle {
             leading: None,
             tracking: 0.0,
             kerning: None,
+            optical_kerning: false,
             baseline_shift: 0.0,
             h_scale: 100.0,
             v_scale: 100.0,

crates/doc/src/text/tabs_lists.rs

@@ -223,6 +223,7 @@ impl ParaList {
             position: CharPosition::Normal,
             tracking: 0.0,
             kerning: None,
+            optical_kerning: first.optical_kerning,
             tate_chu_yoko: false,
             underline: false,
             strikethrough: false,