Java Game Loop Explanation

Question

Recently, I've been getting into Java Game programming. I have learned Java before, but this is my first time with Graphics, Game Loops etc.

I followed this tutorial for a Game Loop, and it's working pretty well.

There is nothing wrong with it, but I just can't figure out why I am thread.sleeping the code, and whats the point of it.

If anyone could explain what start, elapsed and running do, and why I am doing thread.sleep(wait) then I would be super appreciative!

GAME LOOP (Remember, it works, I just don't know why):

while (running) {

        start = System.nanoTime();

        update();
        draw();
        drawToScreen();

        elapsed = System.nanoTime() - start;

        wait = targetTime - elapsed / 1000000;
        if (wait < 0) {
            wait = 5;
        }

        try {
            Thread.sleep(wait);
        } catch (Exception e) {
            Game.logger.log("ERROR! Printing Stacktrace...");

            e.printStackTrace();
        }

    }

}

Sibbo · Accepted Answer · 2013-12-24 00:24:58Z

3

Well, running is a flag that can be set to false to terminate the game.

start and elapsed are used to measure the time one round in the loop took. You are waiting to not make the game run too fast. If you are moving elements with a set speed, then having the game loop run too fast makes them also move too fast. That can lead to a bad user experience, since the user might not be fast enough to play the game.

Additionally, any animation doesn't run smooth anymore without the waiting. It will speed up and slow down depending on how fast your code executes.

Just continue with the tutorial, and when are at the point where you make something move, remove the waiting and see what happens.

EDIT

The code makes a round in the loop take about targetTime. It's unit is milliseconds. So to calculate the frame rate, just divide 1000 / targetTime.

edited Dec 24, 2013 at 0:24

answered Dec 24, 2013 at 0:17

Sibbo

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1 Comment

SirTrashyton Over a year ago

Okay, I understand. But how would I calculate the FPS in that?

Flight Odyssey · Accepted Answer · 2013-12-24 00:21:30Z

1

start is the time before the game logic runs. elapsed is the amount of time that it takes for the game logic to run. running is a boolean (true/false) variable that determines whether the game should continue. Thread.sleep(wait) is a method that delays for a certain amount of time.

The goal is to keep the amount of time between frames roughly constant by delaying, so that the animations don't run faster or slower depending on how fast your computer processor is running.

answered Dec 24, 2013 at 0:21

Flight Odyssey

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3yakuya · Accepted Answer · 2013-12-24 00:39:01Z

1

It appears the code aims to be executed targetTime seconds. So you count how much time has already passed (elapsed), and then calculate how much to wait (divide it to get your remaining time in miliseconds, required for methon sleep(wait)).

One purpose of such waiting is often forcing some time between repainting to the screen.

To get your FPS, one way would be to slightly change your code:

long prevStart = 0; //just initialize. First FPS is of course wrong, next get fine.
double FPS = 0;

while (running) {
        start = System.nanoTime();
        FPS = 1 / double(start - prevStart);
        prevStart = start;

        update();
        draw();
        drawToScreen();

        elapsed = System.nanoTime() - start;

        wait = targetTime - elapsed / 1000000;
        if (wait < 0) {
            wait = 5;
        }

        try {
            Thread.sleep(wait);
        } catch (Exception e) {
            Game.logger.log("ERROR! Printing Stacktrace...");

            e.printStackTrace();
        }

    }

edited Dec 24, 2013 at 0:39

answered Dec 24, 2013 at 0:15

3yakuya

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7 Comments

SirTrashyton Over a year ago

I understand, but how would I calculate the FPS? Is it just targetTime?

3yakuya Over a year ago

Why do you want to calculate FPS? You could get it by dividing one second by your target time, more or less.

SirTrashyton Over a year ago

Just so I can keep a record of it, so if there's something making the game slower I can find it.

3yakuya Over a year ago

Well, if you have your targetTime, which is how much time you want this part of code to run, and then divide a second by it + timeOfCodeExecution you get an approximation of your FPS (as this is how many Times your repaint method will be called). Alternatively, you could measure how much does your code "exactly" take to execute (store System.nanoTime() result in some variable and in the next loop substract it from the current System.nanoTime()) and then 1second/result would be your FPS.

SirTrashyton Over a year ago

Hm. Would it be more efficient to just increment a variable and print/reset that per second, or Subtract System.nanoTime? Do you think you could show me what you talked about in code since it was a little hard to understand.

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