Using orientation information to get down direction


I am working on an app that involves drawing a line on my android screen based on its orientation and could use some help or pointers.

The line is drawn in the following way: If the phone is held flat then then the line shrinks and becomes a dot and as the phone is tilted and orientatated the line becomes bigger - ie the phone stood up and the line points down and is max magnitude of 9.8 and held flat it is a small dot. Crucialy no matter what angle the phone is held at the arrow allways points down- ie the line of gravity.

Now I figured out how to calculate the yaw pitch and roll angles of the phones but mathematically I am a bit lost on how to derive the vector of this line from this information - any pointers would be most welcome.



Ok so I figured this out with a lot of help from the Replica Island source and in turn an Nvidia paper.

Once you have the pitch, roll, yaw from the TYPE_ORIENTATION sensor reading:

@Override public void onSensorChanged(SensorEvent event) { synchronized (this) { m_orientationInput[0] = x; m_orientationInput[1] = y; m_orientationInput[2] = z; canonicalOrientationToScreenOrientation(m_rotationIndex, m_orientationInput, m_orientationOutput); // Now we have screen space rotations around xyz. final float horizontalMotion = m_orientationOutput[0] / 90.0f; final float verticalMotion = m_orientationOutput[1] / 90.0f; // send details to renderer.... } }

Here is the canonicalOrientationToScreenOrientation function:

// From NVIDIA http://developer.download.nvidia.com/tegra/docs/tegra_android_accelerometer_v5f.pdf private void canonicalOrientationToScreenOrientation(int displayRotation, float[] canVec, float[] screenVec) { final int axisSwap[][] = { { 1, -1, 0, 1 }, // ROTATION_0 {-1, -1, 1, 0 }, // ROTATION_90 {-1, 1, 0, 1 }, // ROTATION_180 { 1, 1, 1, 0 } // ROTATION_270 }; final int[] as = axisSwap[displayRotation]; screenVec[0] = (float)as[0] * canVec[ as[2] ]; screenVec[1] = (float)as[1] * canVec[ as[3] ]; screenVec[2] = canVec[2]; }


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