1 files changed, 45 insertions, 0 deletions
diff --git a/src/geom/path.rs b/src/geom/path.rs
index 836be1b4..721cc20b 100644
--- a/src/geom/path.rs
+++ b/src/geom/path.rs
@@ -71,3 +71,48 @@ impl Path {
         self.0.push(PathElement::ClosePath);
     }
 }
+
+/// Get the control points for a bezier curve that describes a circular arc
+/// of this angle with the given radius.
+pub fn bezier_arc(
+    angle: Angle,
+    radius: Length,
+    rotate: bool,
+    mirror_x: bool,
+    mirror_y: bool,
+) -> [Point; 4] {
+    let end = Point::new(angle.cos() * radius - radius, angle.sin() * radius);
+    let center = Point::new(-radius, Length::zero());
+
+    let mut ts = if mirror_y {
+        Transform::mirror_y()
+    } else {
+        Transform::identity()
+    };
+
+    if mirror_x {
+        ts = ts.pre_concat(Transform::mirror_x());
+    }
+
+    if rotate {
+        ts = ts.pre_concat(Transform::rotate(Angle::deg(90.0)));
+    }
+
+    let a = center * -1.0;
+    let b = end - center;
+
+    let q1 = a.x.to_raw() * a.x.to_raw() + a.y.to_raw() * a.y.to_raw();
+    let q2 = q1 + a.x.to_raw() * b.x.to_raw() + a.y.to_raw() * b.y.to_raw();
+    let k2 = (4.0 / 3.0) * ((2.0 * q1 * q2).sqrt() - q2)
+        / (a.x.to_raw() * b.y.to_raw() - a.y.to_raw() * b.x.to_raw());
+
+    let control_1 = Point::new(center.x + a.x - k2 * a.y, center.y + a.y + k2 * a.x);
+    let control_2 = Point::new(center.x + b.x + k2 * b.y, center.y + b.y - k2 * b.x);
+
+    [
+        Point::zero(),
+        control_1.transform(ts),
+        control_2.transform(ts),
+        end.transform(ts),
+    ]
+}