Replicate local movement between objects

Questions & Answers
1

Hi,
I’m trying to write a script to be added to a game object A that replicates the deltas (between frames) of local position and local rotation of another game object B.
For example when B moves to its right by say 1 unit, A moves to its right by 1 unit.
The same thing for rotation: when B rotates by 10 degrees on its left, A rotates by 10 degrees on its left.

In the script I keep the local position and rotation of the object in the previous frame and calculate the deltas in the current frame, I managed to make the rotation work, but I’m struggling with replicating the position.

public class MirrorLocalMovement : MonoBehaviour
{
	public Transform target;
		
	private Vector3 prevLocalPos;
	private Quaternion prevLocalRot;
	private bool firstUpdate = true;

	private void Update()
	{
		if (firstUpdate)
		{
			firstUpdate = false;
		}
		else
		{
			transform.Rotate((target.localRotation * Quaternion.Inverse(prevLocalRot)).eulerAngles);
			
	// Translation?

		}

		prevLocalPos = target.localPosition;
		prevLocalRot = target.localRotation;
	}
}

I tried with transform.Translate and transform.InverseTransformDirection but with no success.
How can I replicate the translation in local space between the two objects?

Thank you in advance

2

Here a possible solution (tempTransform is a temporary empty transform to be set in inspector, used to maintain the previous rotation and call InverseTransformDirection):

using System.Collections;
using System.Collections.Generic;
using UnityEngine;


public class MirrorBehaviour : MonoBehaviour
{
    public Transform target;
    public Transform tempTransform;

    private Vector3 prevPos;
    private Quaternion prevRot;
    private bool firstUpdate;


    private void Update()
    {
        if (firstUpdate)
        {
            firstUpdate = false;
        }
        else
        {
            Vector3 deltaPos = target.localPosition - prevPos;
            tempTransform.rotation = prevRot;
            Vector3 localDir = tempTransform.InverseTransformDirection(deltaPos);
            transform.Translate(localDir.x, localDir.y, localDir.z);

            Quaternion deltaRot = target.localRotation * Quaternion.Inverse(prevRot);
            Vector3 eulerDeltaRot = deltaRot.eulerAngles;
            transform.Rotate(eulerDeltaRot.x, eulerDeltaRot.y, eulerDeltaRot.z);
        }

        prevPos = target.localPosition;
        prevRot = target.localRotation;
    }
}