a couple useful Rigidbody extension methods

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On a Discord server, I was asked how to do a damped spring torque. As in, you have a world rotation you want your Rigidbody to turn towards. I had just used these old methods I had laying around earlier in the day, so I thought I’d share here as well.

    body.ApplySpringForceToward(targetPosition, spring, damping,
        ForceMode.Acceleration);
    body.ApplySpringTorqueToward(targetRotation, spring, damping,
        ForceMode.Acceleration);

If using ForceMode.Acceleration, a spring of 500 and a damping of 20 gives a slightly jiggly but responsive motion for bodies regardless of mass.

    public static void ApplySpringForceToward(this Rigidbody body,
        Vector3 targetPosition, float strength, float damping,
        ForceMode mode=ForceMode.Force)
    {
        if (body.isKinematic)
            return;

        Vector3 difference = (body.position - targetPosition);
        Vector3 direction = difference.normalized;
        float force = strength * difference.magnitude;
        body.AddForce(-force * direction - body.velocity * damping,
            mode);
    }

    public static void ApplySpringTorqueToward(this Rigidbody body,
        Quaternion targetRotation, float strength, float damping,
        ForceMode mode=ForceMode.Force)
    {
        if (body.isKinematic)
            return;

        Quaternion difference = ShortestRotation(
            targetRotation, body.rotation);

        difference.ToAngleAxis(out float degrees, out Vector3 axis);
        axis.Normalize();

        float radians = degrees * Mathf.Deg2Rad;
        body.AddTorque(
            (axis * radians * strength) - (body.angularVelocity * damping),
            mode);
    }

    public static Quaternion ShortestRotation(Quaternion a, Quaternion b)
    {
        if (Quaternion.Dot(a, b) < 0)
            return a * Quaternion.Inverse(b.Flipped());
        return a * Quaternion.Inverse(b);
    }
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2

That first seconds of the video snippet was all…

public ApplySpringForceToReachLowEarthOrbit()

amirite? :slight_smile:

Have you played with PD filters for this stuff? They’re not a be-all-end-all solution but with good constants they can give really solid single-axis control. I use 'em for the tilty-tilt as well as the yaw on my Jetpack Kurt game.

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Unfortunately, it was my boneheaded mistake to “pull object until it aligns” but get the direction of the vector subtraction wrong. Some days, Quaternions are easier than Vectors. Made me laugh though.