2D Collision to Windows boundaries...

XNA Game Studio Express

Nukie

Hi there,

I'm a beginner with XNA, and for the sake of personal knowledge i'm trying to create a simple breakout clone (loving the cliche of this). Anyway, this far i'm going ok, but now I have a problem with collision detection between the ball and the windows boundaries.

I also know what is causing this problem, but I dont know how to solve it.

The code has been set up that the ball is stick to the paddle untill you release the ball by hitting space. At this point the ball will move by a pre-set speed (given in the class of the ball) along the Y axis. This works without problems. However, the direction the ball is moving on on the X axis is set pick a random direction. This ransomness is causing the problem with the collisions. When the ball collide with the top or bottom of the screen (the Y axis) everything works perfect and the ball, will bounce back. But when the ball hits one of the side walls (the X axis) the ball sticks against the wall and wont move anymore.

The problem here is the randomness given to the X axis of the bal. If I start the game without thise random code running the ball will bounce against the walls with no problem. But doing this means the ball will not get a random direction at the point it is realeased from the playerpaddle, so the whole point of the game will be gone.

This is the code in the Ball.cs clas file:

Code Snippet

public Vector2 pos = new Vector2();

public float speed;

public int direction = 0;

public Ball()

{

pos = new Vector2(0f, 0f);

speed = 5;

Random rand = new Random();

direction = rand.Next(-5, 5);

}

And this is the code in the Game.cs class file that is used to check the collision:

Code Snippet

// Handles the balls speed and direction after the ball has been released form the paddle

void BallUpdate()

{

if (keyReleased)

{

ball.pos.Y -= ball.speed;

ball.pos.X -= ball.direction;

}

int MinX = 0;

int MaxX = graphics.GraphicsDevice.Viewport.Width - t_ball.Width;

int MinY = 0;

int MaxY = graphics.GraphicsDevice.Viewport.Height - t_ball.Height;

if (ball.pos.X > MaxX)

{

ball.speed *= -1;

ball.pos.X = MaxX;

}

else if (ball.pos.X < MinX)

{

ball.speed *= -1;

ball.pos.X = MinX;

}

if (ball.pos.Y > MaxY)

{

ball.speed *= -1;

ball.pos.Y = MaxY;

}

else if (ball.pos.Y < MinY)

{

ball.speed *= -1;

ball.pos.Y = MinY;

}

}


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Re: XNA Game Studio Express 2D Collision to Windows boundaries...

Shawn Hargreaves - MSFT

When the ball bounces off a vertical wall, you need to negate the X component of its velocity.

You're already doing that correctly for Y changes. Instead of thinking of it as a speed and direction, think of it as xspeed and yspeed. When you hit the top or bottom of the screen, negate the yspeed, or when you hit the left or right, negate xspeed.





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Re: XNA Game Studio Express 2D Collision to Windows boundaries...

Nukie

Doing that doesnt help a bit. The ball still sticks to the side.

Ball class now looks like this:

Code Snippet

{

class Ball

{

public Vector2 pos = new Vector2();

public float yspeed;

public float xspeed;

public Ball()

{

pos = new Vector2(0f, 0f);

yspeed = 5;

Random rand = new Random();

xspeed = rand.Next(-5, 5);

}

}

}

I changed the names in Game.cs accoarding to this.




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Re: XNA Game Studio Express 2D Collision to Windows boundaries...

frogman

I think your on the right track. Firstly I changed the speed to a vector

Code Snippet

public class Ball

{

public Vector2 pos;

public Vector2 speed;

public Ball()

{

pos = new Vector2(50.0f, 50.0f);

Random rand = new Random();

speed.X = rand.Next(-5, 5);

speed.Y = rand.Next(-5, 5);

}

}

Then I just tweaked some logic concerning position and speed

Code Snippet

void BallUpdate()

{

//'t_ball' is the ball texture

int MinX = 0;

int MaxX = graphics.GraphicsDevice.Viewport.Width - t_ball.Width;

int MinY = 0;

int MaxY = graphics.GraphicsDevice.Viewport.Height - t_ball.Height;

// Add the balls speed to its current position to get its

// new position

ball.pos.Y += ball.speed.Y;

ball.pos.X += ball.speed.X;

// If the ball goes beyond any of the windows edges then

// 'bounce'/reverse its speed and cap its potion to the edge

if (ball.pos.X > MaxX)

{

ball.speed.X *= -1;

ball.pos.X *= MaxX;

}

else if (ball.pos.X < MinX)

{

ball.speed.X *= -1;

ball.pos.X = MinX;

}

if (ball.pos.Y > MaxY)

{

ball.speed.Y *= -1;

ball.pos.Y = MaxY;

}

else if (ball.pos.Y < MinY)

{

ball.speed.Y *= -1;

ball.pos.Y = MinY;

}

}





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