Android-四元数-控制VR设备的旋转

首先参考维基百科了解四元数的基本概念：

https://zh.wikipedia.org/wiki/%E5%9B%9B%E5%85%83%E6%95%B0%E4%B8%8E%E7%A9%BA%E9%97%B4%E6%97%8B%E8%BD%AC

参考

http://www.2cto.com/kf/201210/162858.html

http://blog.csdn.net/flowingflying/article/details/43451351

http://www.4byte.cn/question/502095/get-quaternion-from-android-gyroscope.html

Android中对传感器类型的定义：

#define SENSOR_TYPE_ACCELEROMETER 1 //加速度

#define SENSOR_TYPE_MAGNETIC_FIELD 2 //磁力

#define SENSOR_TYPE_ORIENTATION 3 //方向

#define SENSOR_TYPE_GYROSCOPE 4 //陀螺仪

#define SENSOR_TYPE_LIGHT 5 //光线感应

#define SENSOR_TYPE_PRESSURE 6 //压力

#define SENSOR_TYPE_TEMPERATURE 7 //温度

#define SENSOR_TYPE_PROXIMITY 8 //接近

#define SENSOR_TYPE_GRAVITY 9 //重力

#define SENSOR_TYPE_LINEAR_ACCELERATION 10//线性加速度

#define SENSOR_TYPE_ROTATION_VECTOR 11//旋转矢量

native层Sensor.h中没有定义旋转矢量的枚举变量，仅有：

/*
 * Sensor types
 * (keep in sync with hardware/sensor.h)
 */

enum {
    ASENSOR_TYPE_ACCELEROMETER      = 1,
    ASENSOR_TYPE_MAGNETIC_FIELD     = 2,
    ASENSOR_TYPE_GYROSCOPE          = 4,
    ASENSOR_TYPE_LIGHT              = 5,
    ASENSOR_TYPE_PROXIMITY          = 8
};

因此这里用整型11做参数获取旋转矢量：

	appState.engine.sensorManager = ASensorManager_getInstance();
	appState.engine.accelerometerSensor = ASensorManager_getDefaultSensor(
			appState.engine.sensorManager, 11);
	appState.engine.sensorEventQueue = ASensorManager_createEventQueue(
			appState.engine.sensorManager, app->looper, LOOPER_ID_USER, NULL,
			NULL);

再模拟SensorManager.java中的getQuaternionFromVector函数转换成四元数即可(不是很严格，为简单，去掉了rv长度为4的情形)

void getQuaternionFromVector(float Q[], float rv[]) {
	ALOGV("rv(%f,%f,%f)",rv[0],rv[1],rv[2]);
	Q[0] = 1 - rv[0] * rv[0] - rv[1] * rv[1] - rv[2] * rv[2];
	Q[0] = (Q[0] > 0) ? (float) sqrt(Q[0]) : 0;
	Q[1] = rv[0];
	Q[2] = rv[1];
	Q[3] = rv[2];
	ALOGV("Q(%f,%f,%f,%f)",Q[0],Q[1],Q[2],Q[3]);
}

转换出来的长度为4的数组Q即为所需四元数。

ps：记得，横竖屏切换的时候传-z，而非z（rotation[2] = -event.vector.z;）