Add tracking to font function

3 files changed, 28 insertions, 1 deletions

diff --git a/src/geom/em.rs b/src/geom/em.rs
index 6d2f3aca..caddbcdd 100644
--- a/src/geom/em.rs
+++ b/src/geom/em.rs
@@ -3,7 +3,7 @@ use super::*;
 /// A length that is relative to the font size.
 ///
 /// `1em` is the same as the font size.
-#[derive(Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[derive(Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
 #[derive(Serialize, Deserialize)]
 pub struct Em(N64);
 
diff --git a/src/library/text.rs b/src/library/text.rs
index 914122a1..f2c19516 100644
--- a/src/library/text.rs
+++ b/src/library/text.rs
@@ -163,6 +163,7 @@ pub fn font(ctx: &mut EvalContext, args: &mut Args) -> TypResult<Value> {
     let fallback = args.named("fallback")?;
     let style = args.named("style")?;
     let weight = args.named("weight")?;
+    let tracking = args.named::<f64>("tracking")?;
     let stretch = args.named("stretch")?;
     let size = args.named::<Linear>("size")?.or_else(|| args.find());
     let top_edge = args.named("top-edge")?;
@@ -203,6 +204,10 @@ pub fn font(ctx: &mut EvalContext, args: &mut Args) -> TypResult<Value> {
             text.fill = Paint::Color(fill);
         }
 
+        if let Some(tracking) = tracking {
+            text.tracking = Em::new(tracking);
+        }
+
         set!(text.variant.style => style);
         set!(text.variant.weight => weight);
         set!(text.variant.stretch => stretch);
@@ -259,6 +264,8 @@ pub fn shape<'a>(
         );
     }
 
+    track_segment(&mut glyphs, style.tracking);
+
     let (size, baseline) = measure(ctx, &glyphs, style);
     ShapedText {
         text,
@@ -555,6 +562,23 @@ fn shape_segment<'a>(
     }
 }
 
+/// Apply tracking to a slice of shaped glyphs.
+fn track_segment(glyphs: &mut [ShapedGlyph], tracking: Em) {
+    if tracking.is_zero() {
+        return;
+    }
+
+    let mut glyphs = glyphs.iter_mut().peekable();
+    while let Some(glyph) = glyphs.next() {
+        if glyphs
+            .peek()
+            .map_or(false, |next| glyph.text_index != next.text_index)
+        {
+            glyph.x_advance += tracking;
+        }
+    }
+}
+
 /// Measure the size and baseline of a run of shaped glyphs with the given
 /// properties.
 fn measure(
diff --git a/src/style/mod.rs b/src/style/mod.rs
index 60734716..bb662032 100644
--- a/src/style/mod.rs
+++ b/src/style/mod.rs
@@ -140,6 +140,8 @@ pub struct TextStyle {
     pub families: Rc<FamilyStyle>,
     /// OpenType features.
     pub features: Rc<FontFeatures>,
+    /// The amount of space that should be added between character.
+    pub tracking: Em,
     /// Whether 300 extra font weight should be added to what is defined by the
     /// `variant`.
     pub strong: bool,
@@ -222,6 +224,7 @@ impl Default for TextStyle {
             fill: Paint::Color(Color::Rgba(RgbaColor::BLACK)),
             families: Rc::new(FamilyStyle::default()),
             features: Rc::new(FontFeatures::default()),
+            tracking: Em::zero(),
             strong: false,
             emph: false,
             monospace: false,