LibGdx physics independent from framerate

Question

I am working on a simple platform games like Super Mario. I am using Java with the LibGdx engine. I have a problem with physics being independent of the framerate. In my game the character can do a jump, the jump height is apparently dependent of the framerate.

On my desktop the game runs fine, it runs at 60 frames per second. I also tried the game on a tablet on which it ran at a lower fps. What happened was that the character could jump much higher than when I jumped on the desktop version.

I have already read some articles about fixing the timestep, I do understand it, but not enough to apply it to this situation. I just seem to be missing something.

Here is the physics part of the code:

protected void applyPhysics(Rectangle rect) {
    float deltaTime = Gdx.graphics.getDeltaTime();
    if (deltaTime == 0) return;
    stateTime += deltaTime;

    velocity.add(0, world.getGravity());

    if (Math.abs(velocity.x) < 1) {
        velocity.x = 0;
        if (grounded && controlsEnabled) {
            state = State.Standing;
        }
    }

    velocity.scl(deltaTime); //1 multiply by delta time so we know how far we go in this frame

    if(collisionX(rect)) collisionXAction();
    rect.x = this.getX();
    collisionY(rect);

    this.setPosition(this.getX() + velocity.x, this.getY() +velocity.y); //2
    velocity.scl(1 / deltaTime); //3 unscale the velocity by the inverse delta time and set the latest position

    velocity.x *= damping;

    dieByFalling();
}

A jump() function gets called and adds a variable jump_velocity = 40 to the velocity.y.

The velocity is used in the collision detection.

Charbel Charbel · Accepted Answer · 2014-02-06T17:21:50

I think your problem is here:

velocity.add(0, world.getGravity());

You also need to scale the gravity when modifying the velocity. Try:

velocity.add(0, world.getGravity() * deltaTime);

On a seperate note, trying using box2D, which can handle these for you :)

LibGdx physics independent from framerate

1 Answers