Why do I get a jitter/stutter when moving and following a target?

Question

I am using the script below to move a transform to quickly follow a target (the player), and come to a stop close to the player.

When I move the player around, the transform is following the player, keeping the 1.5 distance.

To ensure the transform can keep up with the player, I set the moveSpeed to 5, But when I do that, the transform jitters/stutters when moving.

If I set the moveSpeed to 1 for example, the transform will move smoothly but will not keep up with the player - the player has to stop moving and wait for it to catch up. But increasing the speed makes the stutter return.

Why is this stutter happening, and how can I make the movement smooth?

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using UnityEngine;
using UnityEngine.UI;

public class Follow : MonoBehaviour
{
    public Transform targetToFollow;
    public Text text;
    public Text text1;
    public float lookAtRotationSpeed;
    public float moveSpeed;

    private float minMoveSpeed = 0f;
    private Vector3 originPos;

    // Start is called before the first frame update
    void Start()
    {
        originPos = targetToFollow.position;
    }

    // Update is called once per frame
    void Update()
    {
        Vector3 lTargetDir = targetToFollow.position - transform.position;
        lTargetDir.y = 0.0f;
        transform.rotation = Quaternion.RotateTowards(transform.rotation,
            Quaternion.LookRotation(lTargetDir), Time.time * lookAtRotationSpeed);

        var distance = Vector3.Distance(transform.position, targetToFollow.position);

        text.text = "Transform Distance From Target " + distance.ToString();

        float ms = moveSpeed;

        if (distance > 5f)
        {
            ms = moveSpeed + 0.5f;
        }
        else if (distance < 1.5f)
        {
            ms = Mathf.Max(minMoveSpeed, ms - 0.3f);
        }
        else
        {
            //transform.position = Vector3.MoveTowards(transform.position, targetToFollow.position, Time.deltaTime * ms);
        }

        if (distance < 0.5f && originPos == targetToFollow.position)
        {
            ms = 0f;
        }

        if (distance > 1.5f)
            transform.position = Vector3.MoveTowards(transform.position, targetToFollow.position, Time.deltaTime * ms);

        originPos = targetToFollow.position;
    }
}

Answer 1

You have two hard cutoffs in your code:

if (distance > 5f)
{
    ms = moveSpeed + 0.5f;
}

This switches to a higher seed if the object is more than 5 units away.

if (distance > 1.5f)
    transform.position = Vector3.MoveTowards(transform.position, targetToFollow.position, Time.deltaTime * ms);

This moves the object only if the object is more than 1.5 units away.

Consider what happens when your distance is close to one of these thresholds.

• On frame 1, I'm 1.51 units away.

  That's more than 1.5, so I take a step of .16 units, now I'm 1.34 units away

• On frame 2, my target has moved .1 units further away, so now I'm 1.44 units from it.

  That's less than 1.5, so I don't move.

• On frame 3, my target has moved .1 units again, so now I'm 1.54 units from it.

  That's more than 1.5, so I take a step of .16 units, now I'm 1.38 units away.

• On frame 5, my target has moved .1 units again, so now I'm 1.48 units from it.

  That's less than 1.5, so I don't move.

You can see, instead of approaching at a slow rate to keep pace with the object I'm following, I stop and start. On alternating frames I find myself too far (so I move and overshoot) or too close (so I stop), creating a jerky, stuttering movement.

The same thing can happen more subtly at the 5 unit cutoff, where I alternate between taking big steps (because I'm just past 5 units) and small steps (because I'm just under 5 units).

You can solve the hard movement cutoff by aiming for the place where you would want to stop. That way you take a smaller move when you're very close, rather than overshooting.

float followRadius = 1.5f;

// Compute a position no further than followRadius away from our target.
Vector3 fromTarget = Vector3.ClampMagnitude(-lTargetDir.normalized, followRadius);    
Vector3 stopPoint = targetToFollow.position + fromTarget;

// Move as far as we can at our speed ms to reach the stopPoint, without overshooting.
transform.position = Vector3.MoveTowards(transform.position, stopPoint, Time.deltaTime * ms);

To solve the sharp speed change you get at the 5 unit mark, you can blend the speed gradually over a longer distance, something like...

float fastRadius = 5f;
float speedBoost = 0.5f;

float speedBlend= Mathf.Clamp01((distance - followRadius)/(fastRadius-followRadius));

ms = moveSpeed + speedBlend * speedBoost;

This has you moving at your normal move speed when close to the followRadius, and at your faster speed when at/beyond your fastRadius, with a smooth acceleration/deceleration in between.