Unity Follow The Leader Behavior

Question

I asked this question two years ago. Not ever having success, I abandoned the idea until recently.

I have since been able to semi-fix / replicate the mechanic. However, all the objects seem to jump to their next position, with some duplicating their "leader's" position.

The orange is the head, with the body parts being green.

As you can see from the commented out code below, I have tried multiple permutations to get the children to follow their leader smoothly with the distance between each body-part just being the circle colliders radius.

My thought was, if the "leader" has moved the distance of the radius, then the follower can move towards the leaders old position. This give the leader time to move.

But the only one that seems to semi work, is the un-commented one.

Can anyone see the problem?

FollowTheLeader.cs

public class FollowTheLeader : MonoBehaviour
{
    [Header("Head")]
    public GameObject bodyPart;
    public int bodyLength = 6;

    [Header("Move Speed")]
    [Range(0.25f, 2.0f)] public float moveMin = 0.5f;
    [Range(0.25f, 2.0f)] public float moveMax = 2.0f;

    [Header("Change Directions")]
    [Range(0.25f, 2.0f)] public float changeMin = 0.5f;
    [Range(0.25f, 2.0f)] public float changeMax = 2.0f;


    [SerializeField]
    private Vector2 oldPosition;
    public Vector2 OldPosition { get => oldPosition; set => oldPosition = value; }

    [SerializeField]
    private Vector2 moveDirection = new Vector2(0, -1);
    public Vector2 MoveDirection { get => moveDirection; set => moveDirection = value; }



    [Header("Child")]
    public int index;
    public bool isChild;
    public FollowTheLeader leader;
    public float leaderDistance;



    private CircleCollider2D m_collider2D;
    private Rigidbody2D body2d;

    private float moveSpeed;
    private float moveTimePassed;
    private float changeDirInterval;

    private void Awake()
    {
        m_collider2D = GetComponent<CircleCollider2D>();
        body2d = GetComponent<Rigidbody2D>();

        AddBodyParts();

        DefineDirection(moveDirection);
    }

    private void AddBodyParts()
    {
        if (isChild || bodyPart == null)
            return;

        //The head will generate its body parts. Each body part will have reference to the one before it.

        FollowTheLeader temp = this;

        for (int i = 1; i <= bodyLength; i++)
        {
            GameObject bp = Instantiate(bodyPart, transform);
            bp.transform.SetParent(null);
            //bp.transform.position = transform.position;
            bp.transform.position = new Vector2(i * m_collider2D.radius, 0);
            bp.name = $"Body {i}";

            FollowTheLeader c = bp.AddComponent<FollowTheLeader>();
            c.isChild = true;
            c.index = i;
            c.OldPosition = bp.transform.position;
            c.leader = temp;

            // cache the parent for the next body part 
            temp = c;
        }
    }

    private void Start()
    {
        OnNewDirection();
    }

    private void FixedUpdate()
    {
        //Store the old postion for the next child
        OldPosition = body2d.position;


        // If child
        if (isChild)
        {
            // Calculate the leaders distance
            leaderDistance = Vector2.Distance(OldPosition, leader.OldPosition);

            // We only want to move if the parent is as far away as the  m_collider2D.radius.
            if (leaderDistance < m_collider2D.radius)
                return;


            // BARELY ANY MOVEMENT
            //body2d.MovePosition(leader.OldPosition.normalized);
            //body2d.MovePosition(leader.OldPosition.normalized * moveSpeed);
            //body2d.MovePosition(leader.OldPosition.normalized * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(leader.OldPosition.normalized * parentDistance * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(leader.OldPosition.normalized * m_collider2D.radius * parentDistance * moveSpeed * Time.deltaTime);

            //FLYS ALL OVER THE PLACE
            //body2d.MovePosition(body2d.position + leader.OldPosition.normalized);
            //body2d.MovePosition(body2d.position + leader.OldPosition.normalized * moveSpeed);
            //body2d.MovePosition(body2d.position + leader.OldPosition.normalized * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(body2d.position + leader.OldPosition.normalized * parentDistance * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(body2d.position + leader.OldPosition.normalized * m_collider2D.radius * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(body2d.position + leader.OldPosition.normalized * m_collider2D.radius * parentDistance * moveSpeed * Time.deltaTime);


            // BARELY ANY MOVEMENT
            //body2d.MovePosition(leader.OldPosition * moveSpeed);
            //body2d.MovePosition(leader.OldPosition * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(leader.OldPosition * parentDistance * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(leader.OldPosition * m_collider2D.radius * parentDistance * moveSpeed * Time.deltaTime);

            //FLYS ALL OVER THE PLACE
            //body2d.MovePosition(body2d.position + leader.OldPosition);
            //body2d.MovePosition(body2d.position + leader.OldPosition * moveSpeed);
            //body2d.MovePosition(body2d.position + leader.OldPosition * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(body2d.position + leader.OldPosition * parentDistance * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(body2d.position + leader.OldPosition * m_collider2D.radius * moveSpeed * Time.deltaTime);
            //body2d.MovePosition(body2d.position + leader.OldPosition * m_collider2D.radius * parentDistance * moveSpeed * Time.deltaTime);

            // KINDA FOLLOWS BUT ALL SEEM TO JUMP INTO THE SAME POSITION AS SEEN IN THE GIF
            body2d.MovePosition(leader.OldPosition);


            return;
        } 

        // HEAD ONLY
        // Countdown to next direction change
        moveTimePassed += Time.deltaTime;

        if (moveTimePassed >= changeDirInterval)
        {
            OnNewDirection();
        }

        // Calculate the next position
        body2d.MovePosition(body2d.position + MoveDirection.normalized * moveSpeed * Time.deltaTime);

    }

    public void OnNewDirection()
    {
        moveTimePassed = 0;
        moveSpeed = Random.Range(moveMin, moveMax);
        changeDirInterval = Random.Range(changeMin, changeMax);

        RandomDirection();
    }

    private void RandomDirection()
    {
        switch (Random.Range(0, 4))
        {
            case 0:
                DefineDirection(Vector2.up);
                break;
            case 1:
                DefineDirection(Vector2.right);
                break;
            case 2:
                DefineDirection(Vector2.down);
                break;
            case 3:
                DefineDirection(Vector2.left);
                break;
            default:
                DefineDirection(Vector2.down);
                break;
        }
    }

    public void DefineDirection(Vector2 direction)
    {
        if (direction.Equals(Vector2.up))
        {
            MoveDirection = Vector2.up;
        }

        if (direction.Equals(Vector2.down))
        {
            MoveDirection = Vector2.down;

        }

        if (direction.Equals(Vector2.left))
        {
            MoveDirection = Vector2.left;
        }


        if (direction.Equals(Vector2.right))
        {
            MoveDirection = Vector2.right;
        }
    }
}

Is there a reason why you want to do this by code? Do the children have to move towards the exact parent location? Have you ever looked at the use of `Joints` (Unity one, not the one you smoke)? — Immorality, Aug 17 '19 at 17:32
@Immorality I want a specific move style, and Joints do not give me that style. — user-44651, Aug 17 '19 at 18:52

Iggy · Accepted Answer · 2019-08-17T21:47:26.693

Many different ways you can approach it but let me show you one way.

Snake - moves the leader forward, creates new points in the path, manages minions
Path - ring buffer with of all the points
Minion - follow the path based on the distance from the leader

Here's an example with gizmos showing:

Green is the leader
Red is the head of the path
Blue is the tail of the path

The snake is where the main logic is at.

The snake moves forward automatically. When the distance between the leader and the last point is greater than RADIUS we create a new point. We then move all of the minions along the path of points.

public class Snake : MonoBehaviour
{
    public const float RADIUS = 1f; // distance between minions
    public const float MOVE_SPEED = 1f; // movement speed

    public Vector2 dir = Vector2.up; // movement direction
    public float headDist = 0f; // distance from path 'head' to leader (used for lerp-ing between points)
    public Path path = new Path(1); // path points
    public List<Minion> minions = new List<Minion>(); // all minions

    public Minion Leader => minions[0];

    void Awake()
    {
        path.Add(this.transform.position);
        AddMinion(new Knight());
    }

    void AddMinion(Minion minion)
    {
        // Initialize a minion and give it an index (0,1,2) which is used as offset later on
        minion.Init(minions.Count);

        minions.Add(minion);
        minion.MoveOnPath(path, 0f);

        // Resize the capacity of the path if there are more minions in the snake than the path
        if (path.Capacity <= minions.Count) path.Resize();
    }

    void FixedUpdate()
    {
        MoveLeader();
        MoveMinions();
    }

    void MoveLeader()
    {
        // Move the first minion (leader) towards the 'dir'
        Leader.transform.position += ((Vector3)dir) * MOVE_SPEED * Time.deltaTime;

        // Measure the distance between the leader and the 'head' of that path
        Vector2 headToLeader = ((Vector2)Leader.transform.position) - path.Head().pos;

        // Cache the precise distance so we can reuse it when we offset each minion
        headDist = headToLeader.magnitude;

        // When the distance between the leader and the 'head' of the path hits the threshold, spawn a new point in the path
        if (headDist >= RADIUS)
        {
            // In case leader overshot, let's make sure all points are spaced exactly with 'RADIUS'
            float leaderOvershoot = headDist - RADIUS;
            Vector2 pushDir = headToLeader.normalized * leaderOvershoot;

            path.Add(((Vector2)Leader.transform.position) - pushDir);

            // Update head distance as there is a new point we have to measure from now
            headDist = (((Vector2)Leader.transform.position) - path.Head().pos).sqrMagnitude;
        }
    }

    void MoveMinions()
    {
        float headDistUnit = headDist / RADIUS;

        for (int i = 1; i < minions.Count; i++)
        {
            Minion minion = minions[i];

            // Move minion on the path
            minion.MoveOnPath(path, headDistUnit);

            // Extra push to avoid minions stepping on each other
            Vector2 prevToNext = minions[i - 1].transform.position - minion.transform.position;

            float distance = prevToNext.magnitude;
            if (distance < RADIUS)
            {
                float intersection = RADIUS - distance;
                minion.Push(-prevToNext.normalized * RADIUS * intersection);
            }
        }
    }
}

Path is a ring buffer, Head() gives you the newest point that was added, you can use Head(index) to get the head and offset it in a direction(+/-). Minions use it to fetch points that are just behind the head: path.Head(-1).

public class Path
{
    public Vector2[] Points { get; private set; }
    public int Capacity => Points.Length;

    int head;

    public Path(int capacity)
    {
        head = 0;
        Points = new Vector2[capacity];
    }

    public void Resize()
    {
        Vector2[] temp = new Vector2[Capacity * 2];

        for (int i = 0; i < temp.Length; i++)
        {
            temp[i] = i < Capacity ? Head(i + 1) : Tail();
        }

        head = Capacity - 1;

        Points = temp;
    }

    public void Add(Vector2 pos)
    {
        int prev = Mod(head, Capacity);

        Next();

        int next = Mod(head, Capacity);

        Points[next].pos = pos;
    }

    public Vector2 Head()
    {
        return Points[head];
    }

    public Vector2 Head(int index)
    {
        return Points[Mod(head + index, Capacity)];
    }

    public Vector2 Tail()
    {
        return Points[Mod(head + 1, Capacity)];
    }

    public Vector2 Tail(int index)
    {
        return Points[Mod(head + 1 + index, Capacity)];
    }

    void Next()
    {
        head++;
        head %= Capacity;
    }

    int Mod(int x, int m)
    {
        return (x % m + m) % m;
    }
}

A minion contains an index, which tells us the placement of the minion within the snake (first, second, third). We use this index to get the two points needed for interpolation. path.Head(-0) will give us the leader's point. path.Head(-1) will give us the first minion's point.

public class Minion : MonoBehaviour
{
    int index;

    public Init(int index)
    {
        this.index = index;
    }

    // Move the minion along the path
    public void MoveOnPath(Path path, float dist)
    {
        Vector2 prev = path.Head(-index);
        Vector2 next = path.Head(-index + 1);

        // Interpolate the position of the minion between the previous and the next point within the path. 'dist' is the distance between the 'head' of the path and the leader
        this.transform.position = Vector2.Lerp(prev.pos, next.pos, dist);
    }

    // Push the minion to avoid minions stepping on each other
    public void Push(Vector2 dir)
    {
        this.transform.position += (Vector3)dir;
    }
}

I've stripped out a lot of code to make the example simpler. I hope you get the basic idea and will be able to implement your own solution.

OMG, you are the best, im trying to create this like a month and you put this code that work perfectly, just some changes and read. It works thank you!!!!! — Eduardo Steffens, Jun 16 '21 at 01:25

score 0 · Answer 2 · answered Aug 17 '19 at 20:13

I tried recreating this scenario in 3D and got somewhat the same behaviour as yours. First, you want to use twice the radius, as each child would otherwise overlap half of the parent since it is the center of the circle that is being moved.

I used a different approach to move the children than you did. The result is a smooth, snake-like motion:

The code is simple enough:

First I rotate the child to point its forward axis at the leader's position
Second the length from the current position to the desired position is calculated. I subtract twice the radius, as the new position would otherwise result in overlapping spheres

I use the translate function to move the GameObject in the forward direction using the calculated magnitude.

    // KINDA FOLLOWS BUT ALL SEEM TO JUMP INTO THE SAME POSITION AS SEEN IN THE GIF
    //body2d.MovePosition(leader.OldPosition);

    transform.LookAt(leader.transform);
    float length = leaderDistance - (m_collider2D.radius * 2);
    transform.Translate(transform.forward *  length, Space.World);

    return;

The result is a smooth, predictable motion. You can even turn off kinematic on the rigidbody to enable collisions.

Hope that helped you.

Unity Follow The Leader Behavior

2 Answers2