Windows Phone and XNA for Fun, Games, Profit and PhysicsRob Miles| Microsoft MVP | University of Hullwww.robmiles.comTwitter: @robmiles

AGENDA

• 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

• 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

• 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

• 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

• 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:• Behold CHEESE ROLLER!

DEMODemo 1: Drawing Cheese

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

// Tell XNA we can only support Portrait orientation// Can support a range of orientations by ORing // together orientation valuesgraphics.SupportedOrientations = DisplayOrientation.Portrait;

// Set up hardware scaling of the display areagraphics.PreferredBackBufferWidth = 480;graphics.PreferredBackBufferHeight = 800;

// Want a full screen displaygraphics.IsFullScreen = true;

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

• 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

DEMODemo 2: Orientation and Scale

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

• 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

• 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

• 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

• 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);}

DEMODemo 3: Rolling Cheese

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 is a Codeplex based project you can download from here:

http://farseerphysics.codeplex.com/

• 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

• 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

• 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 unitsPhysicsSimulator simulator = new PhysicsSimulator(new Vector2(0, 250));

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

/// <summary>/// Texture of the shelf the target sits on/// </summary>Texture2D shelfTexture;Body shelfBody;Geom shelfGeom;Vector2 shelfOrigin;

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<Texture2D>(@"images\shelf");shelfOrigin = new Vector2(shelfTexture.Width / 2.0f, shelfTexture.Height / 2.0f);shelfBody = BodyFactory.Instance.CreateRectangleBody( shelfTexture.Width, shelfTexture.Height, 1);shelfBody.IsStatic = true;shelfBody.Position = new Vector2(550, 400);simulator.Add(shelfBody);

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;simulator.Add(shelfGeom);

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 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

• 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

• 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

• 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

• 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 += new CollisionEventHandler(ballHits);

COLLISION HANDLER

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

bool ballHits(Geom geometry1, Geom geometry2, ContactList contactList){ if (ballSoundActive) return true; if (geometry1 == shelfGeom || geometry2 == shelfGeom) return true; ballSoundActive = true; bangSound.Play(); return true;}

COLLISION HANDLER SOUND MANAGEMENT

• Only plays the sound if the ball has hit the house• Only plays the sound once

bool ballHits(Geom geometry1, Geom geometry2, ContactList contactList){ if (ballSoundActive) return true; if (geometry1 == shelfGeom || geometry2 == shelfGeom) return true; ballSoundActive = true; bangSound.Play(); return true;}

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

• 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

• 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

• 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

spriteBatch.Begin(SpriteSortMode.Immediate, null, null, null, null, null, transformation);// transformed drawing

spriteBatch.End();

DEMODemo 4: Destruction Golf

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

• 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

• 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

• 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 = new Accelerometer();accel.ReadingChanged += new EventHandler<AccelerometerReadingEventArgs> (accel_ReadingChanged);

accel.Start();

READING THE ACCELEROMETERobject accelLock = new object();

void accel_ReadingChanged(object sender, AccelerometerReadingEventArgs e){ lock (accelLock) { accelReading.X = (float)e.X; accelReading.Y = (float)e.Y; accelReading.Z = (float)e.Z; }}

USING THE ACCELEROMETERlock (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

• 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

DEMODemo 5: Cheese Lander

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

• 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

• 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

• 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

• 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

• 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

• 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

• Download the developer tools from: http://create.msdn.com

• Download the sample code from: http://robmiles.com

• Watch the Jumpstart videos • Register as a developer• Download the FarSeer engine from:

http://farseerphysics.codeplex.com/ • Start making money and having fun!

Questions?

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