Reverted 447 (erincatto#655)

The old interpolation method was better. Modified unit tests to ensure this doesn't change. https://fgiesen.wordpress.com/2012/08/15/linear-interpolation-past-present-and-future/
moi-forki · Oct 12, 2020 · ee4e1c7 · ee4e1c7
1 parent e772b71
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diff --git a/include/box2d/b2_math.h b/include/box2d/b2_math.h
@@ -681,10 +681,11 @@ inline bool b2IsPowerOfTwo(uint32 x)
 	return result;
 }
 
+// https://fgiesen.wordpress.com/2012/08/15/linear-interpolation-past-present-and-future/
 inline void b2Sweep::GetTransform(b2Transform* xf, float beta) const
 {
-	xf->p = c0 + beta * (c - c0);
-	float angle = a0 + beta * (a - a0);
+	xf->p = (1.0f - beta) * c0 + beta * c;
+	float angle = (1.0f - beta) * a0 + beta * a;
 	xf->q.Set(angle);
 
 	// Shift to origin

diff --git a/unit-test/math_test.cpp b/unit-test/math_test.cpp
@@ -31,16 +31,24 @@ DOCTEST_TEST_CASE("math test")
 		// From issue #447
 		b2Sweep sweep;
 		sweep.localCenter.SetZero();
-		sweep.c0.Set(3.0f, 4.0f);
-		sweep.c.Set(3.0f, 4.0f);
-		sweep.a0 = 0.0f;
-		sweep.a = 0.0f;
+		sweep.c0.Set(-2.0f, 4.0f);
+		sweep.c.Set(3.0f, 8.0f);
+		sweep.a0 = 0.5f;
+		sweep.a = 5.0f;
 		sweep.alpha0 = 0.0f;
 
 		b2Transform transform;
-		sweep.GetTransform(&transform, 0.6f);
 
+		sweep.GetTransform(&transform, 0.0f);
 		DOCTEST_REQUIRE_EQ(transform.p.x, sweep.c0.x);
 		DOCTEST_REQUIRE_EQ(transform.p.y, sweep.c0.y);
+		DOCTEST_REQUIRE_EQ(transform.q.c, cosf(sweep.a0));
+		DOCTEST_REQUIRE_EQ(transform.q.s, sinf(sweep.a0));
+
+		sweep.GetTransform(&transform, 1.0f);
+		DOCTEST_REQUIRE_EQ(transform.p.x, sweep.c.x);
+		DOCTEST_REQUIRE_EQ(transform.p.y, sweep.c.y);
+		DOCTEST_REQUIRE_EQ(transform.q.c, cosf(sweep.a));
+		DOCTEST_REQUIRE_EQ(transform.q.s, sinf(sweep.a));
 	}
 }