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Windows Phone and XNA for Fun, Games, Profit and Physics. Rob Miles| Microsoft MVP | University of Hull Twitter: @robmiles. Agenda. Getting started with XNA Sorting out orientation Using the touch panel gesture support

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windows phone and xna for fun games profit and physics

Windows Phone and XNA for Fun, Games, Profit and Physics

Rob Miles| Microsoft MVP | University of

Twitter: @robmiles

  • Getting started with XNA
  • Sorting out orientation
  • Using the touch panel gesture support
  • Adding a touch of Physics with the Farseer Physics Engine
  • Using the Windows Phone accelerometer
  • The Windows Phone Marketplace
quick overview of xna
Quick Overview of XNA
  • The XNA Framework provides everything you need to get started writing games:
  • Full Content Management (integrated into Visual Studio)
  • Support for 2D Sprite-based gameplay
  • Support for 3D games
  • Common behaviours across the Windows PC, Xbox 360 and Windows Phone
    • One game engine can run on all platforms
  • Well factored object model
how games work
How Games Work
  • Every game that has ever been written has these fundamental behaviours:
  • Initialise all the resources at the start
    • fetch all textures, models, scripts etc
  • Repeatedly run the game loop:
    • Update the game world
      • read the controllers, update the state and position of game elements
    • Draw the game world
      • render the game elements on the viewing device
methods in an xna game
Methods in an XNA game
  • The XNA Game class contains methods that will provide these behaviours
  • Initialise all the resources at the start
    • The Initialize and LoadContent methods
  • Repeatedly run the game loop:
    • Update the game world
      • The Update method
    • Draw the game world
      • The Draw method
getting started with xna
Getting Started with XNA
  • When you create a new XNA game project you are provided with empty versions of the game methods
  • Creating an XNA game is a matter of filling in these methods to get the game behaviours that are required
  • We are going to start by getting some cheese moving around the display
  • Then we are going to start rolling it around to score points
  • This is the latest cheese game in a long running series:
windows phone and orientation
Windows Phone and Orientation
  • By default a Windows Phone XNA game assumes it is running in “landscape” mode with the screen on the left of the controls
  • I want to change this, because I want to send the cheese down the long axis of the phone when it is rolled
  • I can select orientation when the game starts
orientation and display
Orientation and display
  • // Tell XNA we can only support Portrait orientation
  • // Can support a range of orientations by ORing
  • // together orientation values
  • graphics.SupportedOrientations = DisplayOrientation.Portrait;
  • // Set up hardware scaling of the display area
  • graphics.PreferredBackBufferWidth = 480;
  • graphics.PreferredBackBufferHeight = 800;// Want a full screen display
  • graphics.IsFullScreen = true;
orientation management
Orientation management
  • An XNA game can specify the orientations it can support
  • The game can bind a method to the event which is fired when the orientation changes
    • Games can animate the transition to the new orientation if they wish
  • When orientation changes the origin for the draw position is moved to the top left hand corner of the viewport
  • The frame of reference for accelerometer readings (of which more later) is always as for the phone held in portrait mode
size and the scaler
Size and the Scaler
  • An XNA game can also request a specific size of back buffer for the display
  • The game will be given this resolution irrespective of the actual device
    • The Graphics Processor (GPU) performs this scaling automatically and interpolates to remove any jagged edges
  • This is a great way to get performance boost by rendering to a lower resolution than the screen itself
    • In a fast moving game the lower resolution is not noticeable
creating a game world
Creating a Game World
  • An XNA game uses a number of variables to represent the state of the game itself
    • The Update method will update their values
    • The Draw method will produce a display that reflects their value
  • In the case of Cheese Roller the game must manage the position and movement of the cheese on the screen
  • We use a Texture2D value and a Rectangle to hold the position and a Vector2 to give the direction of movement
  • We also have a Friction value to slow the velocity over time
using gestures
Using Gestures
  • We are going to use the touch panel to allow the player to “flick” the cheese around
  • For the ultimate in control you can get direct access to touch events from the panel
    • Up to four events can be tracked at one time
    • Each event is uniquely identified throughout its lifetime
  • However, XNA games can also use built in gesture recognition
  • Register an interest in a particular gesture type and then get notification when one has been performed
supported gestures
Supported Gestures
  • The touch panel can detect a number of different gestures including
    • Tap
    • DoubleTap
    • Hold
    • HorizontalDrag, VerticalDrag and FreeDrag
    • Pinch
    • Flick
  • The Cheese Roller game is going to use the Flick gesture
registering an interest in a gesture
Registering an interest in a gesture
  • The game must select those gestures that it wants to use
  • This can be done once at the start of the game
  • I do it in the XNA Initialise method
  • A game can also query the TouchPanel about the number of points it can track simultaneously
  • TouchPanel.EnabledGestures = GestureType.Flick;
retrieving gestures
Retrieving Gestures
  • The Flick gesture returns a Delta value which gives the speed of the flick
    • The divide factor of 100 is something I came up with by playtesting
    • The smaller the value, the more powerful the effect of the flick
  • while (TouchPanel.IsGestureAvailable)
  • {
  • GestureSample gesture = TouchPanel.ReadGesture();
  • if (gesture.GestureType == GestureType.Flick)
  • cheeseSpeed = Vector2.Divide(gesture.Delta, 100);
  • }
improving cheese roller
Improving Cheese Roller
  • The obvious improvement to Cheese Roller is to add more cheese
  • If we do this the physics becomes more complicated
  • Our simple model can no longer cope with all the collisions
  • Fortunately there are a lot of engines out there that can help
  • One of these is the Farseer engine
  • It is a 2D physics engine that is very easy to use and works well on Windows Phone
the farseer physics engine
The FarSeer Physics Engine
  • The FarSeer Physics Engine is a Codeplex based project you can download from here:

  • Version 3.0 has just been released but there is not a version of this that is suitable for Windows Phone just yet
  • I have used Version 2.0 for my Windows Phone “Destruction Golf” game
farseer objects
Farseer Objects
  • Objects in the Farseer 2.n universe are made up of a Body and a Geometry
  • We create instances of these and add them to the Farseer simulator
  • When the XNA game updates we also update the simulator so that it moves the objects around
  • We create the simulator at the start of the program
the simulator
The Simulator
  • When we create the simulator we also give it a vector that indicates the direction of gravity
    • A vector is a way of expressing a direction
    • We can create it by giving it x and y direction values
  • We can change this to make gravity work in any direction we like, or with any amount of strength, which might be fun

//The physics simulator is always needed. // We supply a gravity of 250 units

PhysicsSimulator simulator = newPhysicsSimulator(newVector2(0, 250));

the shelf data
The shelf data
  • The shelf is the simplest object on the screen
  • It has a texture that is used to draw it, a Farseer body and a Farseer geometry
  • It also has an origin value, which is the “centre” of the object


/// Texture of the shelf the target sits on






the shelf body
The shelf body
  • This creates a shelf body and places it in the world
  • Note that I’ve scaled the texture to fit the display world
  • I have also made the shelf static, so that it can’t move around in the world

shelfTexture = Content.Load(@"images\shelf");

shelfOrigin = newVector2(shelfTexture.Width / 2.0f,

shelfTexture.Height / 2.0f);

shelfBody = BodyFactory.Instance.CreateRectangleBody(

shelfTexture.Width, shelfTexture.Height, 1);

shelfBody.IsStatic = true;

shelfBody.Position = newVector2(550, 400);


the shelf geometry
The shelf geometry
  • This creates a shelf geometry and places it in the world
  • This sets out how much bounce (RestitutionCoefficient) we get from the shelf and how slippery (FrictionCoefficient) we want it to be
  • Playing with these values is fun

shelfGeom = GeomFactory.Instance.CreateRectangleGeom(shelfBody,shelfTexture.Width, shelfTexture.Height);

shelfGeom.RestitutionCoefficient = 0.3f;

shelfGeom.FrictionCoefficient = 0.5f;


drawing the shelf
Drawing the shelf
  • We use a more advanced version of the Draw method that lets us specify a rotation value for the texture
    • The shelf won’t rotate, but other objects will
  • Note that we get these values out of the geometry for the shape
  • The draw operations for all the game objects look the same

spriteBatch.Draw(shelfTexture, shelfGeom.Position, null,Color.White, shelfGeom.Rotation, shelfOrigin, 1, SpriteEffects.None, 1);

the target data
The Target data
  • The target is the house we are aiming at
  • It has the same information stored about it as the shelf
  • We will place the target on the shelf when the game starts
  • The only difference between the target and the shelf is that the target is allowed to move
  • The aim of the game is to knock the target off the shelf
the target mass
The Target mass
  • Bodies in Farseer are given mass
  • The higher the mass the harder they are to get moving, and the more damage they do when they hit
  • The mass of the shelf is not an issue, since it is fixed in position
  • I’ve given the target a mass of 3
  • This is a value you might want to fine tune

targetBody = BodyFactory.Instance.CreateRectangleBody(targetTexture.Width, targetTexture.Height, 3);

ball position
Ball Position
  • The ball geometry is based on a circle rather than a rectangle
  • When create a circle you give the radius and the number of edges
    • This is because the circle is actually an approximation
  • The more edges, the more accurate the approximation but the more work it is to calculate

ballGeom = GeomFactory.Instance.CreateCircleGeom(ballBody,ballTexture.Width/2, 50);

ballGeom.RestitutionCoefficient= 0.3f;

ballGeom.FrictionCoefficient = 0.5f;

ball launching
Ball Launching
  • We use Windows Phone gesture support to fire the ball
  • The Flick gesture provides a Delta vector that we apply as an impulse to the ball object

while (TouchPanel.IsGestureAvailable)


GestureSample gesture = TouchPanel.ReadGesture();

if (gesture.GestureType == GestureType.Flick)


ballBody.ApplyImpulse(Vector2.Divide(gesture.Delta, 1.5f));



detecting collisions
Detecting Collisions
  • Farseer will detect collisions for us
  • We can bind an event handler to the collision event
  • When the collision occurs the handler method is called automatically
  • The above code binds a method to the collision event

ballGeom.OnCollision += newCollisionEventHandler(ballHits);

collision handler
Collision handler
  • Called whenever a collision is detected
  • Checks to see if the ball has hit the shelf or the target

boolballHits(Geom geometry1, Geom geometry2, ContactListcontactList)


if (ballSoundActive) returntrue;if (geometry1 == shelfGeom || geometry2 == shelfGeom)


ballSoundActive = true;




collision handler sound management
Collision handler sound management
  • Only plays the sound if the ball has hit the house
  • Only plays the sound once

boolballHits(Geom geometry1, Geom geometry2, ContactListcontactList)


if (ballSoundActive) returntrue;if (geometry1 == shelfGeom || geometry2 == shelfGeom)


ballSoundActive = true;




updating farseer
Updating Farseer
  • We need to add this method call to the Update method to keep the Farseer simulation running
  • The time factor controls the rate at which the engine updates
  • You can tune this if you have problems with items moving so fast they go inside or through each other
  • For the purpose of my program the value above works fine

simulator.Update(gameTime.ElapsedGameTime.Milliseconds * .0005f);

scrolling background
Scrolling Background
  • We use a transformation matrix for the draw operation so that we can scroll the screen to follow the ball
  • We can get the position out of the ball geometry so our program can tell where the ball is

if (ballGeom.Position.X + scrollMargin > width)


// need to scroll the screen

xOffset = width - (ballGeom.Position.X + scrollMargin);


transformation matrix
Transformation Matrix
  • A transformation matrix is a lump of maths that can be applied to a drawing operation to change the way it looks
    • Scaling
    • Rotation
    • Translation
  • We just want to translate our view when we draw it
  • XNA will create a translation matrix for us

Matrix transformation;transformation =

Matrix.CreateTranslation(xOffset, 0, 0);

drawing with a transformation matrix
Drawing with a Transformation Matrix
  • We can ask SpriteBatch to use this matrix to transform the drawing operation
  • All the drawing performed will be moved according to the matrix supplied
  • We can follow this with an “untransformed” draw if we wish


null, null, null, null, null, transformation);

// transformed drawing


using the phone accelerometer
Using the phone accelerometer
  • You can also use the accelerometer to control the behaviour of objects in your game
  • The Accelerometer can measure acceleration in X, Y and Z
  • You can use just the X and Y values to turn it into a replacement for a gamepad
  • The values that are returned are in the same range
    • -1 to +1 in each axis
adding the sensors library
Adding the Sensors library
  • The reason why the accelerometer is event driven is that XNA actually uses the same sensor interface as Silverlight
  • This means that you need to include the appropriate sensor library into your program to obtain accelerometer readings
  • You need to add Microsoft.Devices.Sensors to your solution to bring in the library
xna 4 0 accelerometer
XNA 4.0 Accelerometer
  • Unlike other XNA input devices the accelerometer in XNA 4.0 is event driven
  • The accelerometer generates events when new readings are available
  • You must bind a method to the event
  • The method can store the settings for later use
creating an accelerometer
Creating an Accelerometer
  • The above code runs in the Initialise method to set up the accelerometer
  • It creates a new Accelerometer, binds an event handler to it and then starts it running
  • Accelerometer accel = newAccelerometer();
  • accel.ReadingChanged +=
  • newEventHandler
  • (accel_ReadingChanged);
  • accel.Start();
reading the accelerometer
Reading The Accelerometer
  • objectaccelLock = newobject();
  • voidaccel_ReadingChanged(object sender,AccelerometerReadingEventArgs e)
  • {
  • lock (accelLock)
  • {
  • accelReading.X = (float)e.X;
  • accelReading.Y = (float)e.Y;
  • accelReading.Z = (float)e.Z;
  • }
  • }
using the accelerometer
Using The Accelerometer
  • lock (accelLock)
  • {
  • cheeseAcceleration.X = accelReading.X * acceleratorPower;
  • cheeseAcceleration.Y = -accelReading.Y * acceleratorPower;
  • }
  • This code uses the accelerometer values stored by the event handler to manage the cheese movement
  • Note that I use a lock to stop the event handler and the Update method from fighting over the values
tipping games
“Tipping” games
  • The accelerometer makes it very easy to create “tipping” games, which simulate gravity moving items around the screen
  • The further you tip the device the greater the force acting on the object
  • We could use the accelerometer values as forces that drive the Farseer physics engine directly
games and applications
Games and Applications
  • Programs are loaded onto the phone from the Marketplace over the air (WiFi or 3G) or via the Zune application
  • This is the only way to put programs on a “locked” device
  • Registered developers can unlock a device for testing
joining the marketplace
Joining the Marketplace
  • If you want to take your games to market you will need to join the Windows Phone Marketplace as a developer
    • This is managed via your Windows Live identity
    • It costs $99 per year
    • Students can get free membership via DreamSpark
  • Joining the Marketplace lets you publish games and unlock Windows Phone devices for testing
    • Developers can unlock up to three devices
    • Students can unlock just one
free and trial applications
Free and Trial Applications
  • You can give away up to 100 applications each year you are member of the Marketplace
    • Any further free applications will cost $20 each for validation
  • You can also make available “trial” versions of a game or application
  • Customers can purchase an upgrade from within the application
submitting applications
Submitting Applications
  • You are guided through the submission process for your application
  • The application itself is submitted as a single “xap” file
  • This is an archive with a manifest which is generated by Visual Studio when the game is built
  • It contains all the game content and resources
application approval
Application Approval
  • The application process is “semi-automatic”
    • Checks for application pre-requisites and code behaviours along with a review of the program itself
  • Each developer has a dashboard that keeps them informed of the progress of each submission
    • You can submit upgrades at no cost
  • The submission guidelines give a lot of help to ensure that your submissions have the best chance of succeeding
  • You must read these before sending a game for approval
making money
Making Money
  • Marketplace publishers get 70% of the price paid for the games
  • This is paid once you have earned at least $200
  • If you are not from the US you will have to fill in a US Tax Waiver form and fax it to the Windows Marketplace team
  • You can still submit applications for sale before you have sorted this out
what we have learnt
What we have learnt
  • The Windows Phone platform provides a mix of resources that can be used to create compelling games
  • The Touch sensor and accelerometer inputs are easy to use in XNA programs
  • By adding a physics engine we can make our XNA games even more interesting
  • Publishing is easy and cheap
  • There has never been a better time to create mobile games
what to do next
What to do next
  • Download the developer tools from:

  • Download the sample code from:
  • Watch the Jumpstart videos 
  • Register as a developer
  • Download the FarSeer engine from:
  • Start making money and having fun!

Don’t forget to fill out your eval forms!

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