Question

i have a problem with deltaTime ive been using. My Code is the following:

public class Time {
    public static float deltaTime = 0;                      
    public static long frameCount = 0;                      
    public static float fixedTime = 0;                     
    public static float fixedDeltaTime = 0.1f;      
    public static float maxDeltaTime = 0.25f;       

}

And now in my MainThread.java in my run() function:

 while (running) {
        canvas = null;
        try {
            canvas = this.surfaceHolder.lockCanvas();
            synchronized (surfaceHolder) {
                float newTime = System.nanoTime() / 1000000000.0f;
                Time.deltaTime = frameTime = newTime - currentTime;

                if(frameTime > Time.maxDeltaTime)
                        frameTime = Time.maxDeltaTime;

                currentTime = newTime;

                accumulator += frameTime;

                while(accumulator > Time.fixedDeltaTime)
                { 
                        this.gamePanel.update();   // where the player and my enemy gets updated                         
                        accumulator -= Time.fixedDeltaTime;
                }

                this.gamePanel.render(canvas);
                //Perform all non-physics-related updates here


                ++Time.frameCount;

                framesSkipped = 0;  // resetting the frames skipped

                timeDiff = System.currentTimeMillis() - beginTime;
                // calculate sleep time
                sleepTime = (int)(FRAME_PERIOD - timeDiff);

                    if (sleepTime > 0) {
                            // if sleepTime > 0 we're OK
                            try {
                                    Thread.sleep(sleepTime);    
                            } catch (InterruptedException e) {}
                    }

                    while (sleepTime < 0 && framesSkipped < MAX_FRAME_SKIPS) {
                            // we need to catch up
                            this.gamePanel.update(); // update without rendering
                            sleepTime += FRAME_PERIOD;  // add frame period to check if in next frame
                            framesSkipped++;
                    }

                    if (framesSkipped > 0) {

                            Log.d(TAG, "Skipped:" + framesSkipped);
                    }
                            // for statistics
                            framesSkippedPerStatCycle += framesSkipped;
                            // calling the routine to store the gathered statistics
                            storeStats();
                }
            } finally {
                // in case of an exception the surface is not left in
                // an inconsistent state
                    if (canvas != null) {
                    surfaceHolder.unlockCanvasAndPost(canvas);
                    }
            } //end finally

    } 

}

in the gamePanel.update() ive got the update calls for the player and my enemy.
Now my Problem is, that at the start of my game the deltaTime is extremly high und thus my movement is very fast, because i have the following in my enemy class in the update method:

                   x += vX * Time.deltaTime; // velocity * deltaTime
                   y -= vY * Time.deltaTime;
                   //Log.d(TAG, "Time:"+Time.deltaTime+" x: "+x+" y: "+y);
                   vY -= gravity;

Am i doing it right, or is something wrong with my structure?
Thanks for any help.

Answer 1

You need to initialize currentTime before the start of the loop. You don’t show how it’s being initialized, but my guess it starts at 0, so on the first loop iteration, deltaTime is being set to the current time as returned by System.nanoTime().

P.S. Is there some reason you are using System.nanoTime() in one place and System.currentTimeMillis() in another? Also, consider sticking to computing everything in milliseconds (or nanoseconds) with long values and eliminating the floating point calculations.