Augmented Reality#2: Is that picture level?

by CameronM 14. October 2011 08:33

A while ago I wrote a post about creating a simple spirit-level using the Accelerometer in Windows Phone 7. In the Mango release a number of things have changed with the way the Accelerometer API is accessed and how its’ results are returned. There has also been one huge new feature released in Mango that takes the simple spirit-level app to a whole new level. Of course I am speaking about the VideoBrush control, which enables access to the live video feed from the device.

As I pondered potential uses of Augmented Reality, my mind drifted back to the spirit-level application and I began to think about how useful it would be to be able to stand back from an object and determine if it was level. I mean, you have to admit it is easier to stand back from your newly hung photo frame and check that it is level than to simply guess.  Ok, you may be able to place your trusty spirit-level (or WP7) against a photo frame and test that it is level, but what about when you are standing in the Louvre and suddenly realise that the Mona Lisa looks a little crocked? I don’t think those burly security guards are going to let you walk up and put your phone on top of the frame.

The first thing you will notice when dealing with the Accelerometer in Mango is that there is no ReadingChanged event. According to MSDN, this has been replaced by the CurrentValueChanged event. The next thing you’ll notice is that the CurrentValueChanged event expects a SensorReadingEventArgs parameter instead of the AccelerometerReadingEventArgs parameter we utilised in our code for Windows Phone 7. In the Mango release, Microsoft have decided to standardise the way developers access the ever increasing range of sensors – including the Accelerometer, Compass, Gyroscope and Motion.

//display the video feed

PhotoCamera m_camera = new PhotoCamera();



//start the Accelerometer

sensor.CurrentValueChanged +=new EventHandler<SensorReadingEventArgs<AccelerometerReading>>(sensor_CurrentValueChanged);


Thankfully, all of these changes result in only a handful of changes to the original Windows Phone 7.0 code. One thing the Mango 7.5 code takes advantage is, especially when dealing with 3D objects, is the XNA framework. This is evident by that fact that the EventArgs returned by the CurrentValueChanged event no longer refer to X,Y,Z, but to a XNA object called Vector3 (Microsoft.Xna.Framework.Vector3). Again, a small change to our previous code enables us to visualise in which direction the device is tilted, just like a real spirit-level.

void sensor_CurrentValueChanged(object sender, SensorReadingEventArgs<AccelerometerReading> e)


    Dispatcher.BeginInvoke(() => MyReadingChanged(e));


void MyReadingChanged(SensorReadingEventArgs<AccelerometerReading> e)


    //use the XNA Vector3 object to access X,Y,Z

    Microsoft.Xna.Framework.Vector3 moved = e.SensorReading.Acceleration;

    BallTransform.X = -e.SensorReading.Acceleration.Y * (Track.Width - Ball.Width);


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Windows Phone 7 | WP7