Add RELATIVE_ORIENTATION sensor implementation on macOS to //device/generic_sensor

device/generic_sensor/relative_orientation_fusion_algorithm_using_accelerometer.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 "device/generic_sensor/relative_orientation_fusion_algorithm_using_accelerometer.h"
+
+#include <stdint.h>
+
+#include <cmath>
+
+#include "device/generic_sensor/generic_sensor_consts.h"
+
+namespace {
+
+void ComputeQuaternionFromEulerAngles(double alpha,
+                                      double beta,
+                                      double gamma,
+                                      double* x,
+                                      double* y,
+                                      double* z,
+                                      double* w) {
+  double cx = std::cos(beta / 2);
+  double cy = std::cos(gamma / 2);
+  double cz = std::cos(alpha / 2);
+  double sx = std::sin(beta / 2);
+  double sy = std::sin(gamma / 2);
+  double sz = std::sin(alpha / 2);
+
+  *x = sx * cy * cz - cx * sy * sz;
+  *y = cx * sy * cz + sx * cy * sz;
+  *z = cx * cy * sz + sx * sy * cz;
+  *w = cx * cy * cz - sx * sy * sz;
+}
+
+}  // namespace
+
+namespace device {
+
+RelativeOrientationFusionAlgorithmUsingAccelerometer::
+    RelativeOrientationFusionAlgorithmUsingAccelerometer() {}
+
+RelativeOrientationFusionAlgorithmUsingAccelerometer::
+    ~RelativeOrientationFusionAlgorithmUsingAccelerometer() {}
+
+void RelativeOrientationFusionAlgorithmUsingAccelerometer::
+    GetRelativeOrientationData(double* x, double* y, double* z, double* w) {
+  // Transform the accelerometer values to W3C draft angles.
+  //
+  // Accelerometer values are just dot products of the sensor axes
+  // by the gravity vector 'g' with the result for the z axis inverted.
+  //
+  // To understand this transformation calculate the 3rd row of the z-x-y
+  // Euler angles rotation matrix (because of the 'g' vector, only 3rd row
+  // affects to the result). Note that z-x-y matrix means R = Ry * Rx * Rz.
+  // Then, assume alpha = 0 and you get this:
+  //
+  // x_acc = sin(gamma)
+  // y_acc = - cos(gamma) * sin(beta)
+  // z_acc = cos(beta) * cos(gamma)
+  //
+  // After that the rest is just a bit of trigonometry.
+  //
+  // Also note that alpha can't be provided but it's assumed to be always zero.
+  // This is necessary in order to provide enough information to solve
+  // the equations.
+  //
+  double alpha = 0.0;
+  double beta = std::atan2(-acceleration_y_, acceleration_z_);
+  double gamma = std::asin(acceleration_x_ / kMeanGravity);

[timvolodine, 2017/06/21 18:22:12]

strictly speaking this is actually not real sensor fusion, just the use of
accelerometer.. ;)

[juncai, 2017/07/20 00:22:16]

Yes, the sensor fusion concept is general here.

+
+  ComputeQuaternionFromEulerAngles(alpha, beta, gamma, x, y, z, w);

[timvolodine, 2017/06/21 18:22:12]

looking ahead e.g. when providing RELATIVE_ORIENTATION_ANGLE it would make sense
to avoid computing the quaternion representation unless explicitly needed..

[juncai, 2017/07/20 00:22:16]

In the new CL:
https://chromium-review.googlesource.com/c/578253/
This conversion is avoided.

+}
+
+}  // namespace device