ClosestPoint and ContactPoint Collision in Unity

Learn how to use GetClosestPoint() for colliders and ContactPoints for collisions to respond to collisions to specific points on your GameObject in Unity. Check out the YouTube video below!

Source Code

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

public class Ghost : MonoBehaviour
{
    public float moveSpeed = 2f;
    public float rotateSpeed = 50f;

    public Transform ground;

    public Transform face;
    public Transform rightHand;
    public Transform leftHand;

    public Material trailMaterial;
    public Material rightHandMaterial;
    public Material leftHandMaterial;

    private Rigidbody rb;
    private Collider groundCollider;

    // Start is called before the first frame update
    void Start()
    {
        rb = GetComponent<Rigidbody>();
        groundCollider = ground.GetComponent<Collider>();
    }

    // Update is called once per frame
    void Update()
    {
        ProcessMovement();

        Vector3 closestOnGround = groundCollider.ClosestPointOnBounds(transform.position);
        LeaveTrail(closestOnGround, .1f, trailMaterial);
    }

    private void ProcessMovement()
    {
        if (Input.GetKey(KeyCode.W))
        {
            rb.AddForce(transform.forward * Time.deltaTime * moveSpeed, ForceMode.VelocityChange);
        }
        if (Input.GetKey(KeyCode.S))
        {
            rb.AddForce(-transform.forward * Time.deltaTime * moveSpeed, ForceMode.VelocityChange);
        }
        if (Input.GetKey(KeyCode.A))
        {
            transform.Rotate(transform.up * Time.deltaTime * -rotateSpeed);
        }
        if (Input.GetKey(KeyCode.D))
        {
            transform.Rotate(transform.up * Time.deltaTime * rotateSpeed);
        }
    }

    /// <summary>
    /// Called when the ghost collides with a solid object
    /// </summary>
    /// <param name="collision">The collision information</param>
    private void OnCollisionStay(Collision collision)
    {
        // Make an empty list to hold contact points
        ContactPoint[] contacts = new ContactPoint[10];

        // Get the contact points for this collision
        int numContacts = collision.GetContacts(contacts);

        // Iterate through each contact point
        for (int i = 0; i < numContacts; i++)
        {
            // Test the distance from the contact point to the right hand
            if (Vector3.Distance(contacts[i].point, rightHand.position) < .2f)
            {
                LeaveTrail(contacts[i].point, 0.1f, rightHandMaterial);
            }

            // Test the distance from the contact point to the left hand
            if (Vector3.Distance(contacts[i].point, leftHand.position) < .2f)
            {
                LeaveTrail(contacts[i].point, 0.1f, leftHandMaterial);
            }
        }
    }

    /// <summary>
    /// Called when the ghost is inside a trigger
    /// </summary>
    /// <param name="other">The trigger collider of the other object</param>
    private void OnTriggerStay(Collider other)
    {
        // Find the closest point on the collider to the left hand
        Vector3 closestToLeftHand = other.ClosestPoint(leftHand.position);
        if (Vector3.Distance(closestToLeftHand, leftHand.position) < .03f)
        {
            LeaveTrail(closestToLeftHand, 0.1f, leftHandMaterial);
        }

        // Find the closest point on the collider to the right hand
        Vector3 closestToRightHand = other.ClosestPoint(rightHand.position);
        if (Vector3.Distance(closestToRightHand, rightHand.position) < .03f)
        {
            LeaveTrail(closestToRightHand, 0.1f, rightHandMaterial);
        }
    }

    /// <summary>
    /// Places a single sphere at a specific point in space, and sets it to auto-destroy
    /// </summary>
    /// <param name="point">The world point at which to spawn the sphere</param>
    /// <param name="scale">The local scale of the sphere</param>
    /// <param name="material">The material to apply to the sphere</param>
    private void LeaveTrail(Vector3 point, float scale, Material material)
    {
        GameObject sphere = GameObject.CreatePrimitive(PrimitiveType.Sphere);
        sphere.transform.localScale = Vector3.one * scale;
        sphere.transform.position = point;
        sphere.transform.parent = transform.parent;
        sphere.GetComponent<Collider>().enabled = false;
        sphere.GetComponent<Renderer>().material = material;
        Destroy(sphere, 10f);
    }
}