Fix cubic Bézier interpolation.

CHANGELOG.md

@@ -1,6 +1,13 @@
+# 2.0.1
+
+> Tue Sep 24th 2019
+
+- Fix the cubic Bézier curve interpolation. The “output” tangent is now taken by mirroring the
+  next key’s tangent around its control point.
+
 # 2.0.0
 
-> Mon Sep 24th 2019
+> Mon Sep 23rd 2019
 
 ## Major changes
 

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "splines"
-version = "2.0.0"
+version = "2.0.1"
 license = "BSD-3-Clause"
 authors = ["Dimitri Sabadie <dimitri.sabadie@gmail.com>"]
 description = "Spline interpolation made easy"

src/interpolate.rs

@@ -240,7 +240,10 @@ where V: Linear<T>,
   let one_t_3 = one_t_2 * one_t;
   let three = T::one() + T::one() + T::one();
 
-  a.outer_mul(one_t_3) + u.outer_mul(three * one_t_2 * t) + v.outer_mul(three * one_t * t * t) + b.outer_mul(t * t * t)
+  // mirror the “output” tangent based on the next key “input” tangent
+  let v_ = b + b - v;
+
+  a.outer_mul(one_t_3) + u.outer_mul(three * one_t_2 * t) + v_.outer_mul(three * one_t * t * t) + b.outer_mul(t * t * t)
 }
 
 macro_rules! impl_interpolate_simple {