Extension methods for working with Configurable Joints for Unity

ConfigurableJointExtensions.cs

using UnityEngine;

public static class ConfigurableJointExtensions
{
	/// <summary>
	/// Sets a joint's targetRotation to match a given local rotation.
	/// The joint transform's local rotation must be cached on Start and passed into this method.
	/// </summary>
	public static void SetTargetRotationLocal (this ConfigurableJoint joint, Quaternion targetLocalRotation, Quaternion startLocalRotation)
	{
		if (joint.configuredInWorldSpace) {
			Debug.LogError ("SetTargetRotationLocal should not be used with joints that are configured in world space. For world space joints, use SetTargetRotation.", joint);
		}
		SetTargetRotationInternal (joint, targetLocalRotation, startLocalRotation, Space.Self);
	}
	
	/// <summary>
	/// Sets a joint's targetRotation to match a given world rotation.
	/// The joint transform's world rotation must be cached on Start and passed into this method.
	/// </summary>
	public static void SetTargetRotation (this ConfigurableJoint joint, Quaternion targetWorldRotation, Quaternion startWorldRotation)
	{
		if (!joint.configuredInWorldSpace) {
			Debug.LogError ("SetTargetRotation must be used with joints that are configured in world space. For local space joints, use SetTargetRotationLocal.", joint);
		}
		SetTargetRotationInternal (joint, targetWorldRotation, startWorldRotation, Space.World);
	}
	
	static void SetTargetRotationInternal (ConfigurableJoint joint, Quaternion targetRotation, Quaternion startRotation, Space space)
	{
		// Calculate the rotation expressed by the joint's axis and secondary axis
		var right = joint.axis;
		var forward = Vector3.Cross (joint.axis, joint.secondaryAxis).normalized;
		var up = Vector3.Cross (forward, right).normalized;
		Quaternion worldToJointSpace = Quaternion.LookRotation (forward, up);
		
		// Transform into world space
		Quaternion resultRotation = Quaternion.Inverse (worldToJointSpace);
		
		// Counter-rotate and apply the new local rotation.
		// Joint space is the inverse of world space, so we need to invert our value
		if (space == Space.World) {
			resultRotation *= startRotation * Quaternion.Inverse (targetRotation);
		} else {
			resultRotation *= Quaternion.Inverse (targetRotation) * startRotation;
		}
		
		// Transform back into joint space
		resultRotation *= worldToJointSpace;
		
		// Set target rotation to our newly calculated rotation
		joint.targetRotation = resultRotation;
	}
	
	/// <summary>
	/// Adjust ConfigurableJoint settings to closely match CharacterJoint behaviour
	/// </summary>
	public static void SetupAsCharacterJoint (this ConfigurableJoint joint)
	{
		joint.xMotion = ConfigurableJointMotion.Locked;
		joint.yMotion = ConfigurableJointMotion.Locked;
		joint.zMotion = ConfigurableJointMotion.Locked;
		joint.angularXMotion = ConfigurableJointMotion.Limited;
		joint.angularYMotion = ConfigurableJointMotion.Limited;
		joint.angularZMotion = ConfigurableJointMotion.Limited;
		joint.breakForce = Mathf.Infinity;
		joint.breakTorque = Mathf.Infinity;
		
		joint.rotationDriveMode = RotationDriveMode.Slerp;
		var slerpDrive = joint.slerpDrive;
		slerpDrive.mode = JointDriveMode.Position;
		slerpDrive.maximumForce = Mathf.Infinity;
		joint.slerpDrive = slerpDrive;
	}
}

Thanks! It was very useful :D

Thank you for this, I use it a lot. Absolute nightmare using target rotation usually.

Used this a long while ago to set up 6DOF Joints in Godot. It worked there as well. Cheers, mate!

Thank you!

Used this a long while ago to set up 6DOF Joints in Godot. It worked there as well. Cheers, mate!

How exactly did you apply this to godot, if you remember? I'm trying to match spring equilibrium with a world space rotation using jolt in 4.4 and I just can't seem to figure out how it works.

Thank you!! I hate configurable joints so much, they are very confusing, but your extension works like a charm!!

Thanks! It was very useful :D

Thank you for your video about active ragdolls :D

That’s fantastic... bless you!