android: why getrotationmatrix return false?

问题

I want to get the orientation of my phone and I have using this code that I find lot of people using it. this is the code

    public void onSensorChanged(SensorEvent event) {
    //if the data sensor is unreliabel
    if(event.accuracy == SensorManager.SENSOR_STATUS_UNRELIABLE)
        return;

    //gets the value
    switch (event.sensor.getType()) {
    case Sensor.TYPE_ACCELEROMETER:
        gravity = event.values.clone();
        break;

    case Sensor.TYPE_MAGNETIC_FIELD:
        geomag = event.values.clone();
        break;
    }
    getOrientation();
}

private void getOrientation(){

    //if gravity n geomag have value, find the rotation matrix
            if(gravity != null && geomag != null){

                //check the rotation matrix found
                boolean success = SensorManager.getRotationMatrix(inR, I, gravity, geomag);

                if(success){
                    SensorManager.getOrientation(inR, orientVals);
                    azimuth = Math.toDegrees(orientVals[0]);

                    TextView azi = (TextView) findViewById(R.id.textAzi);
                    azi.setText("azi : " + azimuth);

                }
                TextView cek = (TextView) findViewById(R.id.cek);
                cek.setText("rotation: "+success);
            }   

but why the getrotationmatrix always return to false? where is the problem?

回答1:

I've try again and get the solution but I change the code to become like below

    public void onSensorChanged(SensorEvent event) {
    // TODO Auto-generated method stub
    switch (event.sensor.getType()) {
    case Sensor.TYPE_ACCELEROMETER:
        for(int i=0; i<3; i++){
            accelValues[i] =  event.values[i];
        }
        if(compassValues[0] != 0)
            ready = true;

        break;

    case Sensor.TYPE_MAGNETIC_FIELD:
        for(int i=0; i<3; i++){
            compassValues[i] = event.values[i];
        }
        if(accelValues[2] != 0)
            ready = true;

        break;
    }

    if(!ready)
        return;

    boolean cek = SensorManager.getRotationMatrix(inR, inclineMatrix, accelValues, compassValues);

    if(cek){
        SensorManager.getOrientation(inR, prefValues);
        mInclination = SensorManager.getInclination(inclineMatrix);

        //display every 30th values
        if(counter++ % 30 == 0){
            //do your code, what you want to do to the result
            counter = 1;
        }
    }

and everything work well

回答2:

Make sure that you have registered your listeners to listen to the accelerometer, magmetic field and gyro (optional). You do this in the onResume method. If you forget to do this you will get "false" return value when calling getRotationMatrix.

I hope this helps!

   protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

   mSensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);

   // do more staff here..
   }


   protected void onResume() {
    super.onResume();

    mSensorManager.registerListener(
            this, 
            mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
             SensorManager.SENSOR_DELAY_NORMAL );
          mSensorManager.registerListener(
            this, 
            mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD), 
            SensorManager.SENSOR_DELAY_NORMAL );
          mSensorManager.registerListener(
                this, 
                mSensorManager.getDefaultSensor(Sensor.TYPE_GYROSCOPE), 
                SensorManager.SENSOR_DELAY_NORMAL );


}

protected void onPause() {
    super.onPause();
    mSensorManager.unregisterListener(this);
}

回答3:

The problem is with this part of code:

if(event.accuracy == SensorManager.SENSOR_STATUS_UNRELIABLE)
    return;

For accelerometer event.accuracy return SensorManager.SENSOR_STATUS_UNRELIABLE that is why gravity is always null.

来源：https://stackoverflow.com/questions/12720284/android-why-getrotationmatrix-return-false