/// <summary>
/// Changes the camera based on the tilt of the Windows Phone.
/// </summary>
/// <param name="deltaTime">The elapsed time since the last call of <see cref="Update"/>.</param>
private void TiltCamera(TimeSpan deltaTime)
{
// (Note: We use DigitalRune Mathematics instead of the XNA math types - mainly because
// DigitalRune Mathematics provides a 3x3 matrix to describe rotations and the QuaternionF
// provides a nice helper function to create a rotation from two given vectors.
//
// Please note that DigitalRune Mathematics uses column vectors whereas XNA uses row vectors.
// When using Vector3F and Matrix33F we need to multiple them in this order: v' = M * v.
// When using Vector3 and Matrix we need to multiple them in this order: v' = v * M.)
// Get accelerometer value transformed into world space.
Vector3F accelerometerVector = new Vector3F(
-InputService.AccelerometerValue.Y,
InputService.AccelerometerValue.Z,
-InputService.AccelerometerValue.X);
// Run the accelerometer signal through a low-pass filter to remove noise and jitter.
Vector3F currentGravityDirection = _lowPassFilter.Filter(accelerometerVector, (float)deltaTime.TotalSeconds);
Matrix33F cameraTilt;
if (!currentGravityDirection.IsNumericallyZero)
{
// We have some valid sensor readings.
// Let's compute the tilt of the camera. When the phone is lying flat on a table the camera
// looks down onto the scene. When the phone is tilted we want to rotate the position of
// the camera. QuaternionF contains a useful helper function that creates a rotation from
// two given directions - exactly what we need here.
cameraTilt = QuaternionF.CreateRotation(currentGravityDirection, new Vector3F(0, -1, 0)).ToRotationMatrix33();
}
else
{
// Current acceleration is nearly (0, 0, 0). We cannot infer any useful direction from
// this vector. Reset the camera tilt.
cameraTilt = Matrix33F.Identity;
}
// ----- Set up the view matrix (= the position and orientation of the camera).
Vector3F cameraTarget = new Vector3F(0, 2, 0); // That's were the camera is looking at.
Vector3F cameraPosition = new Vector3F(0, _cameraDistance, 0); // That's the default position of the camera.
Vector3F cameraUpVector = new Vector3F(0, 0, -1); // That's the up-vector of the camera.
// Apply the camera tilt to the position and orientation.
cameraPosition = cameraTilt * cameraPosition;
cameraUpVector = cameraTilt * cameraUpVector;
// Keep the camera above the ground.
cameraPosition.Y = Math.Max(0.5f, cameraPosition.Y);
// Create the view matrix from these points.
GraphicsScreen.CameraNode.View = Matrix44F.CreateLookAt(cameraPosition, cameraTarget, cameraUpVector);
// Save the camera position because it is required for hit-testing in DragBodies().
_cameraPosition = cameraPosition;
}