Fix matrix interpolation between rotations of 180 degrees

ui/gfx/geometry/quaternion_unittest.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #define _USE_MATH_DEFINES  // For VC++ to get M_PI. This has to be first.
 
 #include <cmath>
 
 #include "base/macros.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 124 matching lines @@
   }
 }
 
 TEST(QuatTest, SlerpOppositeAngles) {
   Vector3dF axis(1, 1, 1);
   double start_radians = -M_PI_2;
   double stop_radians = M_PI_2;
   Quaternion start(axis, start_radians);
   Quaternion stop(axis, stop_radians);
 
-  // When quaternions are pointed in the fully opposite direction, we take the
-  // interpolated quaternion to be the first. This is arbitrary, but if we
-  // change this policy, this test should fail.
-  Quaternion expected = start;
+  // When quaternions are pointed in the fully opposite direction, this is
+  // ambiguous, so we rotate as per https://www.w3.org/TR/css-transforms-1/
+  Quaternion expected(axis, 0);
 
-  for (size_t i = 0; i < 100; ++i) {
-    float t = static_cast<float>(i) / 100.0f;
-    Quaternion interpolated = start.Slerp(stop, t);
-    CompareQuaternions(expected, interpolated);
-  }
+  Quaternion interpolated = start.Slerp(stop, 0.5f);
+  EXPECT_NEAR(expected.x(), interpolated.x(), kEpsilon);
+  EXPECT_NEAR(expected.y(), interpolated.y(), kEpsilon);
+  EXPECT_NEAR(expected.z(), interpolated.z(), kEpsilon);
+  EXPECT_NEAR(expected.w(), interpolated.w(), kEpsilon);
 }
 
 }  // namespace gfx