1. Topics Covered

• Programming the accelerometer

• Basic solution structure of this tutorial is similar to the solution structure of Pinch Zoom Touch Panel Tutorial (e.g., reference to the SimpleLibrary).
• Basic main source code file of this tutorial is structured similar to the main source code file structure of Pinch Zoom Touch Panel Tutorial.

2. Demo Program Behavior

• The basketball "falling" according to gravitational pull
• The large orange arrows showing the current downward acceleration magnitudes in the x and  y directions.

The following is an example screen shot of what you may see.

The following discuss the implementation details.

3. Solution Structure and External Environment

The source code structure is similar to the structure of touch panel pinch zoom tutorial where we refer to the SimpleLibrary for image and font output. In addition:

• Source code files: The implementation are contain in two files:
  • AccelerometerDemo.cs: implements the XNA framework functions and supports user interaction.
  • AccelerometerDemo_Compute.cs: implements event service routines for the accelerometer.
  In our discussion, we will first look at AccelerometerDemo.cs followed by examining the implementation of the event service routines.
   
• External Resources: there are the usual background music and sound effects, including two image objects: the basketball and the background maze images. Other supporting images we will not be discussing include the support for visualization of the accelerometer states (the large orange arrows), and the display for coordinate markers: upper-left red square representing the origin, the W marker representing (W, 0), and the H marker representing the (0, H) positions. Please refer to the Orientation Tutorial for discussions on the coordinate markers.

4. Implementation Details:

4a. AccelerometerDemo.cs: Implements XNA framework functions and supports user interaction

The above listing shows a source code structure similar to to the structure from the Pinch Zoom Touch Panel Tutorial. The main difference is that in this case, at Label E, we have defined local variables to support the programming with the accelerometer:

1. mAccMeter: this object is our interface to the accelerometer sensors. We will initialize this object, and through it, activate the sensors, and receive data.
2. MShowAcceleration: this object is capable of showing the magnitude and size of vectors as arrows in the middle of the screen. This class is meant to help us visualize the accelerometer state and is completely independent from programming with the accelerometer. We will not go into the details of this class.

The above listing shows that during initialization we must:

• Instantiate the accelerometer object,
• Register a event service routine (AccMeterChange()) to service accelerometer reading changes, and
• Attempt to "start" the accelerometer sampling.

We can consider the accelerometer being sampled by a separate thread. When interesting readings occurs, this sampling thread will send us an event asynchronously by calling the AccMeterChange() function (the details of this function is discussed later in this tutorial here).

The above listing shows that, since accelerometer readings occurs asynchronously, during the regular synchronous update there is actually nothing much to do. Here at Label U1, we simple invoke the mBall's update function. If mBall has non zero velocity or acceleration, it's position will be updated accordingly.

4b. AccelerometerDemo_Compute.cs: This file implements the services for the asynchronous events from the accelerometer sampling thread.

The above listing shows, as in all cases of XNA asynchronous event services, two functions are defined to service the asynchronous events from the sampling thread.

This function shows, accelerometer sampled values are returned as three floating point values (X, Y, Z), each in the range between -2 to 2. Normal gravitation pull is -1. In this case, we simply use the scaled (X,Y) values as the acceleration of the mBall to set the ball in motion.

This is the actual event service routine for the accelerometer reading change events. Notice that since this routine is invoked asynchronously by an external thread, it does not make any changes to any of the instance variables. Instead, this routine creates a lamba (function call) and request a thread-safe invocation of the function.

5. Self Test and Exercises

• If gravitational pull is -1, why does the accelerometer range supports to values of -2? Try jerking the phone of give the phone a sudden shake to see the orange arrow size grow very large as a result. Remember, F=ma, mass of the phone is constant, if you apply a large force to the phone, you will get a large acceleration reading.
• Why can't I call mBall.SetAcceleration() directly from AccMeterChange() service routine? Answer, AccMeterChange() is invoked asynchronously from the accelerometer sampling thread. It is very possible that the AccMeterChange() function is called at the very same time when mBall.Update() is being called from our own Update() function. In this case, we end up having two threads both trying to modify mBall's acceleration values. We will  end up with inconsistent results.
• If we replace the control of the mBall x-movement from using the X readings from the accelerometer to using the z-readings, how will I direct the ball to move leftward? Answer: leftward, is -x direction, to get negative values from the z-accelerometer, I must flip my phone upside-down.
• Practice Project: give the ball a constant speed (e.g., 3) and use the accelerometer reading to control the direction of the ball. In this case, the ball should never speed up or slow down, just roll around reacting to how the phone is tilted.

References

• Documentations:
  • Accelerometer for WP7 overview: http://msdn.microsoft.com/en-us/library/ff431793%28v=VS.92%29.aspx
  • Accelerometer namespace (Microsoft.Device.Sensors): http://msdn.microsoft.com/en-us/library/ff403003%28v=VS.92%29.aspx
   
• Credits:
  • Background music: http://incompetech.com/m/c/royalty-free/index.html.
  • Clip arts and icons: http://office.microsoft.com/en-us/images/images-clip-art-photos-sounds-animations-FX101741979.aspx?pid=CL100570201033

	This work is supported in part by Microsoft Research under the Computer Gaming Curriculum in Computer Science RFP, Award Number 15871 and 16531, and Microsoft Higher Education.
9/15/2010