Limiting FPS for my game

Question

Alright, i'm making a small game, and I need to limit my FPS, because, when I play on my really fast computer, I have around 850 FPS, and the game will go like, REALLY fast, and when I switch to my older computer, it goes alot slower, so I will need to limit my FPS to get this right. How do I limit my FPS? My main game loop:

    public void startGame(){
    initialize();
    while(true){
        drawScreen();
        drawBuffer();
        plyMove();

        //FPS counter
        now=System.currentTimeMillis(); 
        framesCount++; 
        if(now-framesTimer>1000){
            framesTimer=now; 
            framesCountAvg=framesCount; 
            framesCount=0; 
        }

        try{
            Thread.sleep(14);
        }catch(Exception ex){}
    }
}

How I draw the screen, and draw all of the other things, players, the ball, etc. The game is a pong remake, btw.

    public void drawBuffer(){
    Graphics2D g = buffer.createGraphics();
    g.setColor(Color.BLACK);
    g.fillRect(0,0,600,500);
    g.setColor(Color.GREEN);
    g.fillRect(ply1.getX(),ply1.getY(),ply1.getWidth(),ply1.getHeight());
    g.setColor(Color.RED);
    g.fillRect(ply2.getX(),ply2.getY(),ply2.getWidth(),ply2.getHeight());
    g.setColor(Color.WHITE);
    g.fillOval(ball1.getX(),ball1.getY(),ball1.getWidth(),ball1.getHeight());
    g.drawString("" + framesCountAvg,10,10);
}

public void drawScreen(){
    Graphics2D g = (Graphics2D)this.getGraphics();
    g.drawImage(buffer,0,0,this);
    Toolkit.getDefaultToolkit().sync();
    g.dispose();
}

Answer 1

It sounds like your display it tied to your game engine. Make sure the two are disconnected. You want the game playing at the same speed no matter what the framerate is. If your fast computer repaints the screen faster that is ok if the engine is causing the game to play at a constant speed.

Answer 2

Rather than limiting your FPS, make it so that the game doesn't go really fast when the fps is high.

Presumably you have code that does certain things each frame e.g. moves the character forward if a button is pressed. What you want to do instead is move the character forward an amount dependent on the amount of time that has passed since the previous frame.

Answer 3

As aforementioned you need to separate your display loop from your update loop somewhere at the core of you game there is probably something like this

while (1)
{
    drawscene();
    update();
}

and in update you are advancing the time by a fixed amount e.g 0.17 sec. If you drawing at exactly 60fps then the animations are running in realtime, at 850fps everything would be sped by a factor 14 (0.17*850) to prevent this you should make your update time dependent. e.g.

elapsed = 0;
while (1)
{
   start = time();
   update(elapsed); 
   drawscene();
   sleep(sleeptime);
   end = time();

   elapsed = end - start;
}

Answer 4

A description of what I meant in my comment. Inside your method plyMove(); I suspect there is something like

plyMove() {
    ply1.x += amountX;
    ply1.y += amountY;
}

Instead make the movement depending on the time elapsed

plyMove(double timeElapsed) {
    ply1.x += amountX * timeElapsed;
    ply1.y += amountY * timeElapsed;
}

where you calculate timeElapsed from the time difference inside your main loop and scale it accordingly to get a decent movement.

double timePrev = System.currentTimeMillis();
while(true) {
    double timeNow = System.currentTimeMillis();
    double elapsed = timeNow - timePrev;

    drawScreen();
    drawBuffer();
    plyMove(0.001 * elapsed);

    timePrev = timeNow;
}

Answer 5

If you really want a fixed timeframe game you could do this:

float timer = 0;
float prevTime= System.currentTimeMillis();
while(true)
{
    draw();

    float currentTime = System.currentTimeMillis();
    timer += currentTime - prevTime;

    while (timer > UPDATE_TIME) // To make sure that it still works properly when drawing takes too long
    {
        timer -= UPDATE_TIME;
        update();
    }
}

Where UPDATE_TIME is at what interval you want to update at in milliseconds.

Answer 6

One way would be using

try {
    Thread.sleep(20);
} catch(InterruptedException e) {}

at the end of the main loop. Adjust the number to your needs.