Fix matrix interpolation between rotations of 180 degrees

ui/gfx/geometry/quaternion.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.
 
 #include "ui/gfx/geometry/quaternion.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/strings/stringprintf.h"
@@ skipping 48 matching lines @@
 Quaternion Quaternion::Slerp(const Quaternion& q, double t) const {
   double dot = x_ * q.x_ + y_ * q.y_ + z_ * q.z_ + w_ * q.w_;
 
   // Clamp dot to -1.0 <= dot <= 1.0.
   dot = std::min(std::max(dot, -1.0), 1.0);
 
   // Quaternions are facing the same direction.
   if (std::abs(dot - 1.0) < kEpsilon || std::abs(dot + 1.0) < kEpsilon)
     return *this;
 
-  // TODO(vmpstr): In case the dot is 0, the vectors are exactly opposite
-  // of each other. In this case, it's technically not correct to just pick one
-  // of the vectors, we instead need to pick how to interpolate. However, the
-  // spec isn't clear on this. If we don't handle the -1 case explicitly, it
-  // results in inf and nans however, which is worse. See crbug.com/506543 for
-  // more discussion.
-  if (std::abs(dot) < kEpsilon)
-    return *this;
-
   double denom = std::sqrt(1.0 - dot * dot);
   double theta = std::acos(dot);
   double w = std::sin(t * theta) * (1.0 / denom);
 
   double s1 = std::cos(t * theta) - dot * w;
   double s2 = w;
 
   return (s1 * *this) + (s2 * q);
 }
 
 Quaternion Quaternion::Lerp(const Quaternion& q, double t) const {
   return (((1.0 - t) * *this) + (t * q)).Normalized();
 }
 
 Quaternion Quaternion::Normalized() const {
   double length = x_ * x_ + y_ * y_ + z_ * z_ + w_ * w_;
   if (length < kEpsilon)
     return *this;
   return *this / sqrt(length);
 }
 
 std::string Quaternion::ToString() const {
   return base::StringPrintf("[%f %f %f %f]", x_, y_, z_, w_);
 }
 
 }  // namespace gfx