XNA - Pong Clone - Reflecting ball when it hits a wall?

Question

I'm trying to make the ball bounce off of the top and bottom 'Walls' of my UI when creating a 2D Pong Clone. This is my Game.cs

public void CheckBallPosition()
{
    if (ball.Position.Y == 0 || ball.Position.Y >= graphics.PreferredBackBufferHeight)
        ball.Move(true);
    else
        ball.Move(false);

    if (ball.Position.X < 0 || ball.Position.X >= graphics.PreferredBackBufferWidth)
        ball.Reset();
}

At the moment I'm using this in my Ball.cs

    public void Move(bool IsCollidingWithWall)
    {
        if (IsCollidingWithWall)
        {
            Vector2 normal = new Vector2(0, 1);
            Direction = Vector2.Reflect(Direction,normal);
            this.Position += Direction;
            Console.WriteLine("WALL COLLISION");
        }
        else
            this.Position += Direction;
    }

It works, but I'm using a manually typed Normal and I want to know how to calculate the normal of the top and bottom parts of the screen?

Answer 1

Well, this is how I would handle it

public void CheckBallPositionAndMove()
{
    if (ball.Position.Y <= 0 || ball.Position.Y >= graphics.PreferredBackBufferHeight)
        ball.HandleWallCollision();

    ball.Move();

    if (ball.Position.X < 0 || ball.Position.X >= graphics.PreferredBackBufferWidth)
        ball.Reset();
}

//In Ball.cs:
private void HandleWallCollision(Vector2 normal)
{
    Direction.Y *= -1; //Reflection about either normal is the same as multiplying y-vector by -1
}

private void Move()
{
    this.Position += Direction;
}

---

Note however that using this "discrete" collision detection, you wait until after the ball has moved past the top/bottom of the screen to detect a collision; collisions that occur "between" frames may be noticably off, especially if the ball is moving fast. This is especially a problem if you are using this collision-detection method to detect collision with a paddle, since, if the ball is moving fast enough, it is possible for the ball to move right through the paddle!

The solution to this problem is to use what is known as Continuous Collision Detection. CCD is usually significantly more complex than discrete collision detection; fortunately, pong is simple enough that doing CCD would only be slightly more complex. However, you'd still need a solid grasp of high-school algebra to solve the equations.

If you are still interested, there is a good explaination of CCD in this lecture, and this GameDev article goes a bit more in-depth. There are also many questions relating to it on SO.

Answer 2

You could change boolean IsCollidingWithWall with some enum like:

enum CollideType
{
    None,
    Vertical,
    Horizontal
}

and check this type when creating normal.

Answer 3

Each of the boundaries in your world is a line. One side of the line is solid and the other is not. The normal you are trying to compute is one part of the equation for that line. It points toward the non-solid side of the line. The other part of the line equation is the distance from the line to the origin. The equation for the line can be found from two points on that line. You can define these two points based on the coordinates in your game space where you want a wall.

The normal is computed by rotating the line segment defined by the two points 90 degrees and then Normalizing.

public static Vector2 ComputeNormal(Vector2 point1, Vector2 point2)
{
    Vector2 normal = new Vector2();
    normal.X = point2.Y - point1.Y;
    normal.Y = point1.X - point2.X;

    normal.Normalize();

    return normal;
}

You are using the preferred back buffer width and height to define your world space so your would use these to define the points used to compute the normals.

float left = 0.0f;
float right = graphics.PreferredBackBufferWidth;
float top = 0.0f;
float bottom = graphics.PreferredBackBufferHeight;

Vector2 topNormal = ComputeNormal(new Vector2(left, top), new Vector2(right, top));
Vector2 bottomNormal = ComputeNormal(new Vector2(right, bottom), new Vector2(left, bottom));

Note that the points must be given in clockwise order so that the normal points in the correct direction.

The following XNA 4.0 program demonstrates these concepts in use:

using System;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;

namespace WindowsGame
{
    public class Ball
    {
        const int DIAMETER = 40;
        const float RADIUS = DIAMETER * 0.5f;
        const float MASS = 0.25f;
        const int PIXELS = DIAMETER * DIAMETER;

        static readonly uint WHITE = Color.White.PackedValue;
        static readonly uint BLACK = new Color(0, 0, 0, 0).PackedValue;

        Texture2D m_texture;
        Vector2 m_position;
        Vector2 m_velocity;

        public Ball(GraphicsDevice graphicsDevice)
        {
            m_texture = new Texture2D(graphicsDevice, DIAMETER, DIAMETER);

            uint[] data = new uint[PIXELS];

            for (int i = 0; i < DIAMETER; i++)
            {
                float iPosition = i - RADIUS;

                for (int j = 0; j < DIAMETER; j++)
                {
                    data[i * DIAMETER + j] = new Vector2(iPosition, j - RADIUS).Length() <= RADIUS ? WHITE : BLACK;
                }
            }

            m_texture.SetData<uint>(data);
        }

        public float Radius
        {
            get
            {
                return RADIUS;
            }
        }

        public Vector2 Position
        {
            get
            {
                return m_position;
            }
        }

        public Vector2 Velocity
        {
            get
            {
                return m_velocity;
            }

            set
            {
                m_velocity = value;
            }
        }

        public void ApplyImpulse(Vector2 impulse)
        {
            Vector2 acceleration = impulse / MASS;
            m_velocity += acceleration;
        }

        public void Update(float dt)
        {
            m_position += m_velocity;   // Euler integration - innaccurate and unstable but it will do for this simulation
        }

        public void Draw(SpriteBatch spriteBatch)
        {
            spriteBatch.Draw(m_texture, DrawRectangle, Color.White);
        }

        private Rectangle DrawRectangle
        {
            get
            {
                int x = (int)Math.Round(m_position.X - RADIUS);
                int y = (int)Math.Round(m_position.Y - RADIUS);

                return new Rectangle(x, y, DIAMETER, DIAMETER);
            }
        }
    }

    public class Boundary
    {
        private Vector2 m_point1;
        private Vector2 m_point2;
        private Vector2 m_normal;
        private float m_distance;

        public Boundary(Vector2 point1, Vector2 point2)
        {
            m_point1 = point1;
            m_point2 = point2;

            m_normal = new Vector2();
            m_normal.X = point2.Y - point1.Y;
            m_normal.Y = point1.X - point2.X;

            m_distance = point2.X * point1.Y - point1.X * point2.Y;

            float invLength = 1.0f / m_normal.Length();

            m_normal *= invLength;
            m_distance *= invLength;
        }

        public Vector2 Normal
        {
            get
            {
                return m_normal;
            }
        }

        public void PerformCollision(Ball ball)
        {
            float distanceToBallCenter = DistanceToPoint(ball.Position);

            if (distanceToBallCenter <= ball.Radius)
            {
                ResolveCollision(ball);
            }
        }

        public void ResolveCollision(Ball ball)
        {
            ball.Velocity = Vector2.Reflect(ball.Velocity, m_normal);
        }

        private float DistanceToPoint(Vector2 point)
        {
            return 
                m_normal.X * point.X + 
                m_normal.Y * point.Y + 
                m_distance;
        }
    }

    public class World
    {
        Boundary m_left;
        Boundary m_right;
        Boundary m_top;
        Boundary m_bottom;

        public World(float left, float right, float top, float bottom)
        {
            m_top = new Boundary(new Vector2(right, top), new Vector2(left, top));
            m_right = new Boundary(new Vector2(right, bottom), new Vector2(right, top));
            m_bottom = new Boundary(new Vector2(left, bottom), new Vector2(right, bottom));
            m_left = new Boundary(new Vector2(left, top), new Vector2(left, bottom));
        }

        public void PerformCollision(Ball ball)
        {
            m_top.PerformCollision(ball);
            m_right.PerformCollision(ball);
            m_bottom.PerformCollision(ball);
            m_left.PerformCollision(ball);
        }
    }

    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        Matrix viewMatrix;
        Matrix inverseViewMatrix;
        Ball ball;
        World world;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
            IsMouseVisible = true;
        }

        protected override void Initialize()
        {
            spriteBatch = new SpriteBatch(GraphicsDevice);

            ball = new Ball(GraphicsDevice);

            float right = Window.ClientBounds.Width * 0.5f;
            float left = -right;
            float bottom = Window.ClientBounds.Height * 0.5f;
            float top = -bottom;

            world = new World(left, right, top, bottom);

            viewMatrix = Matrix.CreateTranslation(Window.ClientBounds.Width * 0.5f, Window.ClientBounds.Height * 0.5f, 0.0f);
            inverseViewMatrix = Matrix.Invert(viewMatrix);

            base.Initialize();
        }

        private void ProcessUserInput()
        {
            MouseState mouseState = Mouse.GetState();

            Vector2 mousePositionClient = new Vector2((float)mouseState.X, (float)mouseState.Y);
            Vector2 mousePositionWorld = Vector2.Transform(mousePositionClient, inverseViewMatrix);

            if (mousePositionWorld != ball.Position)
            {
                Vector2 impulse = mousePositionWorld - ball.Position;
                impulse *= 1.0f / impulse.LengthSquared();
                ball.ApplyImpulse(-impulse);
            }
        }

        protected override void Update(GameTime gameTime)
        {
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();

            float dt = (float)gameTime.ElapsedGameTime.TotalSeconds;

            ProcessUserInput();

            ball.Update(dt);
            world.PerformCollision(ball);

            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);

            spriteBatch.Begin(SpriteSortMode.Deferred, null, null, null, null, null, viewMatrix);

            ball.Draw(spriteBatch);

            spriteBatch.End();

            base.Draw(gameTime);
        }
    }
}

Answer 4

Wouldn't you just take the position of the ball minus the position of the wall and then normalize that vector to get what you needed without hardcoding it?

Vector2 normal = Position - WallPosition;
normal.Normalize();

The rest of your code should just work the same.