How to use Android sensor event to determine if device is facing up or down

Question

I have quite a simple requirement. Assuming a device is standing on its end, perpendicular to the ground, and it is tilted, all I need to determine is whether the phone is tilted forward or back (screen more toward the ground or more toward the ceiling).

I know how to read values from the various sensors and I figure that using sensor TYPE_ROTATION_VECTOR is the way forward. All I'm missing is the maths know-how to determine forward or back from the three values it returns.

I've read all related threads on SO without enlightenment, any help very much appreciated.

Answer 1

float[] rotationMatrix = new float[9];
float[] inclinationMatrix = new float[9];
float[] accelerometer; // values from sensor
float[] magnetic; // values from sensor
SensorManager.getRotationMatrix(rotationMatrix, inclinationMatrix, accelerometer, magnetic) 
int inclination = (int) Math.round(Math.toDegrees(Math.acos(rotationMatrix[8])));

if (inclination < 90)
{
      // face up
}

if (inclination > 90)
{
       // face down
}

Answer 2

The X axis is horizontal and points to the right, the Y axis is vertical and points up and the Z axis points towards the outside of the front face of the screen. In this system, coordinates behind the screen have negative Z values.

The reference coordinate system is defined as a direct orthonormal basis, where:

X is defined as the vector product Y.Z (It is tangential to the ground at the device's current location and roughly points East).
Y is tangential to the ground at the device's current location and points towards magnetic north.
Z points towards the sky and is perpendicular to the ground.

In your case try this,

if(Round(y,4) < 8.0){
           Log.d("sensor", "=====UP====");


        }

        else if(Round(y,4) < -8.0){
            Log.d("sensor", "=====DOWN====");

        }