From 516d2f21d7f2bd3158229982ebce4fe178fd3fd8 Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Mon, 26 Sep 2022 17:33:34 -0700
Subject: [PATCH] [Impeller] Add lerps (flutter/engine#36430)

---
 .../impeller/geometry/geometry_unittests.cc   | 21 +++++++++++++++++++
 engine/src/flutter/impeller/geometry/point.h  |  4 ++++
 engine/src/flutter/impeller/geometry/vector.h | 10 ++++++++-
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/engine/src/flutter/impeller/geometry/geometry_unittests.cc b/engine/src/flutter/impeller/geometry/geometry_unittests.cc
index 47ef5878530..f2fed24cf4d 100644
--- a/engine/src/flutter/impeller/geometry/geometry_unittests.cc
+++ b/engine/src/flutter/impeller/geometry/geometry_unittests.cc
@@ -965,6 +965,27 @@ TEST(GeometryTest, PointAngleTo) {
   }
 }
 
+TEST(GeometryTest, PointLerp) {
+  Point p(1, 2);
+  Point result = p.Lerp({5, 10}, 0.75);
+  Point expected(4, 8);
+  ASSERT_POINT_NEAR(result, expected);
+}
+
+TEST(GeometryTest, Vector3Lerp) {
+  Vector3 p(1, 2, 3);
+  Vector3 result = p.Lerp({5, 10, 15}, 0.75);
+  Vector3 expected(4, 8, 12);
+  ASSERT_VECTOR3_NEAR(result, expected);
+}
+
+TEST(GeometryTest, Vector4Lerp) {
+  Vector4 p(1, 2, 3, 4);
+  Vector4 result = p.Lerp({5, 10, 15, 20}, 0.75);
+  Vector4 expected(4, 8, 12, 16);
+  ASSERT_VECTOR4_NEAR(result, expected);
+}
+
 TEST(GeometryTest, CanUseVector3AssignmentOperators) {
   {
     Vector3 p(1, 2, 4);
diff --git a/engine/src/flutter/impeller/geometry/point.h b/engine/src/flutter/impeller/geometry/point.h
index 04b5461dbe0..42b97e2ca70 100644
--- a/engine/src/flutter/impeller/geometry/point.h
+++ b/engine/src/flutter/impeller/geometry/point.h
@@ -208,6 +208,10 @@ struct TPoint {
     return Radians{std::atan2(this->Cross(p), this->Dot(p))};
   }
 
+  constexpr TPoint Lerp(const TPoint& p, Scalar t) const {
+    return *this + (p - *this) * t;
+  }
+
   constexpr bool IsZero() const { return x == 0 && y == 0; }
 };
 
diff --git a/engine/src/flutter/impeller/geometry/vector.h b/engine/src/flutter/impeller/geometry/vector.h
index 2456b896b83..10066340a5b 100644
--- a/engine/src/flutter/impeller/geometry/vector.h
+++ b/engine/src/flutter/impeller/geometry/vector.h
@@ -140,6 +140,10 @@ struct Vector3 {
     return Vector3(x / scale, y / scale, z / scale);
   }
 
+  constexpr Vector3 Lerp(const Vector3& v, Scalar t) const {
+    return *this + (v - *this) * t;
+  }
+
   /**
    *  Make a linear combination of two vectors and return the result.
    *
@@ -224,10 +228,14 @@ struct Vector4 {
     return Vector4(x - v.x, y - v.y, z - v.z, w - v.w);
   }
 
-  constexpr Vector4 operator*(float f) const {
+  constexpr Vector4 operator*(Scalar f) const {
     return Vector4(x * f, y * f, z * f, w * f);
   }
 
+  constexpr Vector4 Lerp(const Vector4& v, Scalar t) const {
+    return *this + (v - *this) * t;
+  }
+
   std::string ToString() const;
 };