How to check if a body has almost stopped moving in libgdx + box2d

Question

So, I have a player body + fixture etc, it is essentially a ball that bounces around.

I want to detect when it is 'pretty much' finished moving.

At the moment I do this:

public Boolean isStopped() {
    return body.getLinearVelocity().x <= 0.3f && body.getLinearVelocity().y <= 0.3f;
}

This mostly works, the problem being when the player hits something, there's a split second where its velocity is 0, so this returns true. What I really wanted is to just return true when it is basically finished. Preferably within a range that I can set to whatever I like as I tweak the physics of my game world.

I can't use a check on whether it is sleeping or not as that comes too late, it doesn't sleep until after it has stopped having forces act upon it, I need just before.

I could just store how long it has been stopped/a count of stopped steps, but I was hoping there would be a nice pre existing method that I missed.

Any ideas?

Answer 1

You can keep track of recent movement and update it by mixing in a little of the current speed each time step:

float speedNow = body.getLinearVelocity().len();
recentSpeed = 0.1 * speedNow + 0.9 * recentSpeed;
if ( recentSpeed < someThreshold )
    ... do something ...

You would need to set recentSpeed to a suitably high value to begin with, otherwise it might be below the threshold in the first time step.

Answer 2

Seeing how you've determined that your false positives are caused by the body making contact with another, why not add a couple of lines in your ContactListener's beginContact method, storing the body's current speed in its user data? Then you can check that speed in your isStopped method. If there is a stored speed and the current speed isn't greater, this means the body is in the process of bouncing off whatever it hit: ignore. If there is a stored speed and the current speed is greater, the ball has bounced and is proceeding in some new direction: clear the stored speed. If there is no stored speed and the current speed is below your threshold, you've detected the sought situation.

In your ContactListener:

public void beginContact(Contact contact) {
    Body a = contact.getFixtureA().getBody();
    Body b = contact.getFixtureB().getBody();

    if (a == mBall) {
        a.setUserData(a.getLinearVelocity().len());
    } else if (b == mBall) {
        b.setUserData(b.getLinearVelocity().len());
    }
}

And in your isStopped check:

public Boolean isStopped() {
    float storedSpd = (Float) body.getUserData();
    float currentSpd = body.getLinearVelocity().len();

    if ((storedSpd > Float.MIN_VALUE) && (currentSpd > storedSpd)) {
        body.setUserData(Float.MIN_VALUE);
        return false;
    } else {
        return (currentSpd < THRESHOLD);
    }
}

This is untested, but you get the idea. Also, remember to initially set the user data to Float.MIN_VALUE.

Answer 3

In the end I have simply passed the delta from each render call to the isStopped() method.

public Boolean isStopped(float delta) {
    boolean isMoving = (
            Math.abs(body.getLinearVelocity().x) >= 0.25f || Math.abs(body.getLinearVelocity().y) >= 0.25f);
    if(isMoving) {
        timeStopped = 0f;
        return false;
    } else {
        timeStopped += delta;
        return timeStopped >= 0.3f;
    }
}

timeStopped is just a class property that starts off as zero. This does return true for the beginning of the game (before the user has made a move) but in my application that is absolutely fine. Besides which, it is true to say it has stopped in that circumstance.

I'd still love to see a way to do this without storing extra crap, since I'm guessing box2d must have this information somewhere in order to figure out if a body with zero velocity has no force acting upon or if it is just changing direction after an impact.