Asteroids fly physics

Hey guys, I'm trying to program the game Asteroids and I totally stuck at the original flight behavior.
I mean, the spaceship will fly a little bit to the old position if you chance the direction and after having
no speed in the old direction, it'll fly to the new one. But I really don't know how to realize this.

I hope someone of you can give me some advice :).

I'm not sure if I fully understand your question, but I will attempt to answer it by providing some simple motion logic that should work for asteroids. You could use vectors or just floats or some such. I'll use the latter for this example.

Given these fields:
float x, y; // position of the asteroid (x, y)
float speed, heading; // speed is pixels per update, heading is the angle in radians (0 to 2pi)

Every time your game updates, the position should be changed according to the speed and heading, like so:

x += speed * Math.cos(heading);
y += speed * Math.sin(heading);


As for the "original flight behavior," I would start the asteroid at a random position and random direction:

// init
x = Math.random() * WIDTH;
y = Math.random() * HEIGHT;
heading = Math.random() * 2 * Math.PI
speed = ?; //whatever you want to start the speed at


You may want to add some logic so that the initial location isn't overlapping the player at start, and for when the asteroids move beyond the edges of the screen they "wrap around" to the other side, and perhaps some acceleration.

First of all, thank you, but the asteroids aren't the problem, it's the movement of the player spaceship.

Ah, sorry about that. The logic for a spaceship moving is pretty much the same as the asteroid, though. It would probably help a lot if you posted a some of your code.

This is what my code looks like. Currently, I haven't any spaceship or asteroid objects, i just wanted to know, how to let the spaceship fly like in the original game. The other problem is my Timer class.... I don't really know how to program a Timer class,... so... I think my solution is really bad, I hope you can help me.

Start.java

Code:

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import org.newdawn.slick.AppGameContainer;
import org.newdawn.slick.BasicGame;
import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.Image;
import org.newdawn.slick.Input;
import org.newdawn.slick.SlickException;
import org.newdawn.slick.geom.Vector2f;

public class Start extends BasicGame {
        private final static int        WINDOW_WIDTH        = 800;
        private final static int        WINDOW_HEIGHT        = 600;

        private Image                                spaceship;
        private Vector2f                        spaceshipPosition;
        private Vector2f                        viewDirection;
       
        private Vector2f                        currentDirection;
       
        private float                                speed;

        public Start() {
                super( "Asteroids" );

        }

        public static void main( String[] args ) throws SlickException {
                AppGameContainer gc = new AppGameContainer( new Start() );
                gc.setDisplayMode( WINDOW_WIDTH, WINDOW_HEIGHT, false );
                gc.setShowFPS( false );
                gc.start();
        }

        @Override
        public void init( GameContainer container ) throws SlickException {
                speed = 0.0f;

                spaceship = new Image( "res/spaceship.png" );

                float startxPosition = WINDOW_WIDTH / 2 - spaceship.getWidth() / 2;
                float startyPosition = WINDOW_HEIGHT / 2 - spaceship.getHeight() / 2;

                spaceshipPosition = new Vector2f( startxPosition, startyPosition );
                viewDirection = new Vector2f();
               
                currentDirection = new Vector2f();
               
                currentDirection.x = (float) ( Math.sin( Math.toRadians( spaceship.getRotation() ) ) * speed );
                currentDirection.y = (float) ( Math.cos( Math.toRadians( spaceship.getRotation() ) ) * speed );
        }

        @Override
        public void render( GameContainer container, Graphics g ) throws SlickException {
                spaceship.draw( spaceshipPosition.x, spaceshipPosition.y );
        }

        @Override
        public void update( GameContainer container, int delta ) throws SlickException {
                viewDirection.x = (float) ( Math.sin( spaceship.getRotation() ) );
                viewDirection.y = (float) ( Math.cos( spaceship.getRotation() ) );

                Input input = container.getInput();

                if ( input.isKeyDown( Input.KEY_UP ) ) {
                        accelerate();

                        spaceshipPosition.x += (float) ( Math.sin( Math.toRadians( spaceship.getRotation() ) ) * delta * speed );
                        spaceshipPosition.y -= (float) ( Math.cos( Math.toRadians( spaceship.getRotation() ) ) * delta * speed );
                }

                if ( !input.isKeyDown( Input.KEY_UP ) ) {
                        slowDown();

                        spaceshipPosition.x += (float) ( Math.sin( Math.toRadians( spaceship.getRotation() ) ) * delta * speed );
                        spaceshipPosition.y -= (float) ( Math.cos( Math.toRadians( spaceship.getRotation() ) ) * delta * speed );
                }

                if ( input.isKeyDown( Input.KEY_RIGHT ) ) {
                        spaceship.rotate( 4.0f );
                }

                if ( input.isKeyDown( Input.KEY_LEFT ) ) {
                        spaceship.rotate( -4.0f );
                }

        }


        private void accelerate() {
                Timer timer = new Timer( 2000000 );
                timer.decreaseTime( 1 );

                speed += 0.01f;
                if ( speed > 0.3f ) {
                        speed = 0.5f;
                }
        }

        private void slowDown() {
                Timer timer = new Timer( 500000 );
                timer.decreaseTime( 0.1f );

                speed -= 0.01f;
                if ( speed < 0.0f ) {
                        speed = 0.0f;
                }
        }
}

---

Timer.java

Code:

---

public class Timer {
        private float currentTime;
       
        public Timer(float time) {
                this.currentTime = time;
        }
       
        public void decreaseTime(float timeDelay) {
                for(float i = currentTime; currentTime > 0; i--) {
                        currentTime -= timeDelay;
                }
        }
}

---

I recoded the whole thing and hope, it's better to understand now.

GameObject.java

Code:

---

import org.newdawn.slick.Image;
import org.newdawn.slick.Input;
import org.newdawn.slick.geom.Vector2f;

public abstract class GameObject {
        protected Vector2f        currentPosition;
        protected Image                objectImage;

        public GameObject( Image objectImage ) {
                this.objectImage = objectImage;
               
                this.objectImage.setCenterOfRotation( this.objectImage.getWidth() / 2, this.objectImage.getHeight() / 2 );
        }

        public Image getImage() {
                return this.objectImage;
        }

        public void setStartPosition( Vector2f position ) {
                this.currentPosition = position;
        }
       
        public Vector2f getCurrentPosition() {
                return this.currentPosition;
        }
       
        public void setCurrentPosition( float x, float y ) {
                this.currentPosition.x = x;
                this.currentPosition.y = y;
        }
       
        public abstract boolean collide( GameObject gameObject );
        public abstract void move( Input input, int delta );
}

---

Spaceship.java

Code:

---

import org.newdawn.slick.Image;
import org.newdawn.slick.Input;

public class Spaceship extends GameObject {
        private final float MAX_SPEED = 1.0f;
        private float spaceshipSpeed;
       
        private double oldSinRotation, oldCosRotation;
       
        public Spaceship( Image spaceshipImage ) {
                super( spaceshipImage );
               
                this.spaceshipSpeed = 0.0f;
        }

        @Override
        public void move( Input input, int delta ) {
                if( input.isKeyDown( Input.KEY_UP ) ) {
                        accelerate();
                        this.currentPosition.x += (float) ( Math.sin( Math.toRadians( this.objectImage.getRotation() ) ) * delta * this.spaceshipSpeed );
                        this.currentPosition.y -= (float) ( Math.cos( Math.toRadians( this.objectImage.getRotation() ) ) * delta * this.spaceshipSpeed );
                        this.changeScreenSide();
                       
                        this.oldSinRotation = Math.sin( Math.toRadians( this.objectImage.getRotation() ) );
                        this.oldCosRotation = Math.cos( Math.toRadians( this.objectImage.getRotation() ) );
                       
                } else {
                        this.decelerate();
                        this.currentPosition.x += (float) ( this.oldSinRotation * delta * this.spaceshipSpeed );
                        this.currentPosition.y -= (float) ( this.oldCosRotation * delta * this.spaceshipSpeed );
                        this.changeScreenSide();
                }
               
                if( input.isKeyDown( Input.KEY_RIGHT ) ) {
                        this.objectImage.rotate( 0.4f );
                }
               
                if( input.isKeyDown( Input.KEY_LEFT ) ) {
                        this.objectImage.rotate( -0.4f );
                }
        }

        @Override
        public boolean collide( GameObject gameObject ) {
                return false;
        }
       
        private void changeScreenSide() {
                if( this.getCurrentPosition().x < -49.16f ) {
                        this.setCurrentPosition( 782.19f, this.getCurrentPosition().y );
                }
               
                if( this.getCurrentPosition().x > 782.81f ) {
                        this.setCurrentPosition( -49.18f, this.getCurrentPosition().y );
                }
               
                if( this.getCurrentPosition().y < - 45.29f ) {
                        this.setCurrentPosition( this.getCurrentPosition().x, 584.06f );
                }
               
                if( this.getCurrentPosition().y > 584.07f ) {
                        this.setCurrentPosition( this.getCurrentPosition().x, -45.30f );
                }
        }
       
        private void accelerate() {
                if( this.spaceshipSpeed >= this.MAX_SPEED ) {
                        this.spaceshipSpeed = this.MAX_SPEED;
                }
               
                this.spaceshipSpeed += 0.0009f;
        }
       
        private void decelerate() {
                if( this.spaceshipSpeed <= 0 ) {
                        this.spaceshipSpeed = 0;
                }
               
                this.spaceshipSpeed -= 0.0009f;
        }
}

---

Game.java

Code:

---

import org.newdawn.slick.AppGameContainer;
import org.newdawn.slick.BasicGame;
import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.Image;
import org.newdawn.slick.Input;
import org.newdawn.slick.SlickException;
import org.newdawn.slick.geom.Vector2f;

public class Game extends BasicGame {

        private final static int        GAME_WIDTH        = 800;
        private final static int        GAME_HEIGHT        = 600;

        private Spaceship                        spaceship;

        public Game( String title ) {
                super( title );
        }
       
        private static void startGame() throws SlickException {
                AppGameContainer game = new AppGameContainer( new Game( "Asteroids" ) );
                game.setDisplayMode( Game.GAME_WIDTH, Game.GAME_HEIGHT, false );
                game.setShowFPS( false );
                game.start();
        }
       
        public static void main(String[] args) throws SlickException {
                Game.startGame();
        }

        @Override
        public void init( GameContainer container ) throws SlickException {               
                this.spaceship = new Spaceship( new Image( "res/spaceship.png" ) );
               
                float spaceshipxStartPosition = Game.GAME_WIDTH / 2 - this.spaceship.getImage().getWidth() / 2;
                float spaceshipyStartPosition = Game.GAME_HEIGHT / 2 - this.spaceship.getImage().getHeight() / 2;
               
                this.spaceship.setStartPosition( new Vector2f( spaceshipxStartPosition, spaceshipyStartPosition ) );
        }

        @Override
        public void render( GameContainer container, Graphics g ) throws SlickException {
                this.spaceship.getImage().draw( this.spaceship.getCurrentPosition().x, this.spaceship.getCurrentPosition().y);
        }

        @Override
        public void update( GameContainer container, int delta ) throws SlickException {
                Input input = container.getInput();
               
                this.spaceship.move( input, delta );
        }

}

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I hope someone can help me with the flying problem