Java & OpenGL; Rendering Artifacts when using delta to calculate frame independent movement

Question

So, I've been making a small demo program in java using opengl just as an exercise. All was going well until I implemented getTime(); and getDelta(); functions to calculate how much time each frame takes. I then applied it to all my movement calculations by multiplying their speed with getDelta(); and it works well, only that when frames randomly slightly drop and getDelta(); returns a slightly larger value, my calculations start outputting really weird values which are still in the range of the movement the calculation calculates but incorrect, that happens for a few frames until the framerate goes up to normal (250FPS in this case) and the movement continues normally and even from the exact position. I have no idea what could cause this since the getDelta(); seems to function properly?

Heres some code:

private void movementCalculations(double delta) {

    sinInc += 0.0015;
    sinInc2 += 0.010;

    xMove[0] = xSpeed * (float) Math.sin(Math.toDegrees(sinInc / 100 * delta)) * 7500;
    yMove[0] = ySpeed * (float) Math.sin(Math.toDegrees(sinInc / 200 * Math.PI * delta)) * 2500;
    zMove[0] = zSpeed * (float) Math.sin(Math.toDegrees(sinInc / 200 * delta)) * 1250;

    for (int i = 1; i<16; i++) {
        xMove[i] = xSpeed * (float) Math.sin(sinInc2 / 128 * Math.PI * i/2 * delta) * WIDTH * 16;
        yMove[i] = ySpeed * (float) Math.sin(sinInc2 / 64 * Math.PI * i * delta) * HEIGHT * 4;
    }

    xMove[16] = xSpeed * (float) Math.sin(sinInc2 / 64 * Math.PI * 2 * delta) * WIDTH * 8;
    yMove[16] = ySpeed * (float) Math.sin(sinInc2 / 32 * Math.PI * 2 * delta) * HEIGHT * 4;

    xMove[17] = xSpeed * (float) Math.sin(sinInc2 / 32 * Math.PI * 2 * delta) * HEIGHT * 10;
}

In the following debug output, Delta 4.0 is the normal value for 250FPS, then FPS drops and suddenly xMove[17] returns a nearly as large but NEGATIVE value?

Last frame in milliseconds: 231884072 Delta: 4.0 xMove[17]: 164.297 Last frame in milliseconds: 231884077 Delta: 5.0 xMove[17]: -160.83817 Last frame in milliseconds: 231884080 Delta: 3.0 xMove[17]: -0.7198944 Last frame in milliseconds: 231884084 Delta: 4.0 xMove[17]: 162.12277

And if not that's enough, here's a working JAR exectuable of the application which has everything for running it inside the JAR so you can see yourself: http://harha.us.to/files/files_public/Starfield_Exe.jar

I just can't figure out what's wrong or is there even anything wrong in my code at all?

Answer 1

You're currently multiplying sinInc and sinInc2 with delta, so what's framerate-independent is the... rotation speed?

You probably want to multiply the movement speeds ({x|y|z}Speed), though it's hard to be sure.

You should start by moving the delta multiplication up to the common factor instead of pasting * delta everywhere.

sinInc += (0.0015 * delta);

will make it clear that the sine increase will be framerate-independent, .0015 units per unit time.

As for movement, I suspect you're doing someXCoordinate[i] += xMove[i] somewhere. If so, then that's the place to put delta:

obj.xPosition += (obj.xVelocity * delta);