Killer Game Programming in Java

FPS and Sleeping for Varying Times

A weakness of the animation loop is that its execution speed is unconstrained. On a slow machine, it may loop 20 times per second; the same code on a fast machine may loop 80 times, making the game progress four times faster and perhaps making it unplayable. The loop's execution speed should be about the same on all platforms.

A popular measure of how fast an animation progresses is frames per second (FPS). For GamePanel, a frame corresponds to a single pass through the update-render-sleep loop inside run(). Therefore, the desired 100 FPS imply that each iteration of the loop should take 1000/100 == 10 ms. This iteration time is stored in the period variable in GamePanel.

The use of active rendering makes it possible to time the update and render stages of each iteration. Subtracting this value from period gives the sleep time required to maintain the desired FPS. For instance, 100 FPS mean a period of 10 ms, and if the update/render steps take 6 ms, then sleep() should be called for 4 ms. Of course, this is different on each platform, so must be calculated at runtime.

The following modified run() method includes timing code and the sleep time calculation:

    public void run()
    /* Repeatedly: update, render, sleep so loop takes close
       to period ms */
    {
      long beforeTime, timeDiff, sleepTime;

      beforeTime = System.currentTimeMillis();

      running = true;
      while(running) {
        gameUpdate();
        gameRender();
        paintScreen();

        timeDiff = System.currentTimeMillis() - beforeTime; ...