Game Development Series Part 6: Physics & Collision Detection

Rigidbodies & Forces

A Rigidbody is a component that enables physics simulation on a game object. Once added, the physics engine takes over—the object responds to gravity, forces, collisions, and momentum.

Physics Engine Architecture

Physics Engine Components:

┌─────────────────────────────────────────────────────────┐
│                    PHYSICS ENGINE                        │
│                                                          │
│   ┌──────────────┐   ┌──────────────┐   ┌────────────┐  │
│   │  COLLISION   │   │   PHYSICS    │   │   SOLVER   │  │
│   │  DETECTION   │──▶│  SIMULATION  │──▶│ (Resolve   │  │
│   │  (Who hit?)  │   │  (How move?) │   │ overlaps)  │  │
│   └──────────────┘   └──────────────┘   └────────────┘  │
│          │                  │                  │         │
│          ▼                  ▼                  ▼         │
│   ┌─────────────────────────────────────────────────┐   │
│   │              FIXED TIMESTEP UPDATE              │   │
│   │        (Every 0.02s = 50 physics updates/sec)   │   │
│   └─────────────────────────────────────────────────┘   │
└─────────────────────────────────────────────────────────┘

Popular Physics Engines:
  • PhysX (Unity, Unreal) - Full 3D physics
  • Box2D (Most 2D games) - Lightweight 2D physics
  • Havok (AAA games) - Industry standard
  • Bullet (Open source) - Used by Blender, many games

Rigidbody Properties

Applying Forces

public class PhysicsController : MonoBehaviour
{
    private Rigidbody rb;
    public float jumpForce = 500f;
    public float moveForce = 10f;
    
    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }
    
    void FixedUpdate()  // Use FixedUpdate for physics!
    {
        // Movement force (continuous)
        float horizontal = Input.GetAxis("Horizontal");
        rb.AddForce(Vector3.right * horizontal * moveForce);
        
        // Different force modes:
        // Force: Continuous push (affected by mass)
        rb.AddForce(Vector3.forward * 10f, ForceMode.Force);
        
        // Acceleration: Continuous push (ignores mass)
        rb.AddForce(Vector3.forward * 10f, ForceMode.Acceleration);
        
        // Impulse: Instant push (affected by mass) - for jumps!
        if (Input.GetButtonDown("Jump"))
            rb.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);
        
        // VelocityChange: Instant push (ignores mass)
        rb.AddForce(Vector3.up * 5f, ForceMode.VelocityChange);
    }
    
    // Direct velocity manipulation
    void Jump()
    {
        // Set velocity directly (overrides physics)
        rb.velocity = new Vector3(rb.velocity.x, 10f, rb.velocity.z);
    }
}

// 2D Equivalent
public class Physics2DController : MonoBehaviour
{
    private Rigidbody2D rb2d;
    
    void FixedUpdate()
    {
        rb2d.AddForce(Vector2.right * 10f);
        rb2d.AddForce(Vector2.up * 500f, ForceMode2D.Impulse);
    }
}

Colliders & Triggers

Colliders define the physical shape of an object for collision detection. They can be solid (blocking) or triggers (detection only).

Collider Types

Collider Types (2D and 3D):

  Box Collider           Sphere/Circle          Capsule
   ┌───────┐                  ●                   ╭───╮
   │       │               ╱     ╲                │   │
   │       │             ●         ●              │   │
   │       │               ╲     ╱                │   │
   └───────┘                  ●                   ╰───╯
   
  Cheap to compute     Cheapest check         Character shapes
  Good for crates,     Perfect for balls,     Ideal for players,
  walls, platforms     bullets, power-ups     NPCs, humanoids

  Mesh Collider         Polygon (2D)          Composite
     ╱╲                   ┌─╮                  ┌─────┐
    ╱  ╲                 ╱   ╲                 │  ●  │
   ╱    ╲               ╱     ╲                │     │
  ╱──────╲             └───────┘               └─────┘
  
  Matches exact mesh   Custom 2D shapes       Multiple simple
  EXPENSIVE! Use only  Use sparingly          colliders combined
  for static geometry

Collision vs Trigger

public class CollisionExample : MonoBehaviour
{
    // COLLISION events (solid objects)
    void OnCollisionEnter(Collision collision)
    {
        // Get collision details
        Debug.Log("Hit: " + collision.gameObject.name);
        Debug.Log("Impact force: " + collision.impulse.magnitude);
        
        // Get contact point
        ContactPoint contact = collision.contacts[0];
        Debug.Log("Hit at: " + contact.point);
        Debug.Log("Surface normal: " + contact.normal);
        
        // Check what we hit by tag
        if (collision.gameObject.CompareTag("Enemy"))
        {
            TakeDamage(10);
        }
    }
    
    void OnCollisionStay(Collision collision)
    {
        // Called every frame while touching
    }
    
    void OnCollisionExit(Collision collision)
    {
        // Called when no longer touching
    }
    
    // TRIGGER events (pass-through zones)
    void OnTriggerEnter(Collider other)
    {
        if (other.CompareTag("Coin"))
        {
            CollectCoin(other.gameObject);
            Destroy(other.gameObject);
        }
        
        if (other.CompareTag("Checkpoint"))
        {
            SetSpawnPoint(other.transform.position);
        }
    }
    
    void OnTriggerStay(Collider other)
    {
        // Inside damage zone? Take continuous damage
        if (other.CompareTag("Lava"))
            TakeDamage(5 * Time.deltaTime);
    }
    
    void OnTriggerExit(Collider other)
    {
        if (other.CompareTag("SafeZone"))
            EnableEnemyAttacks();
    }
}

Raycasting

Raycasting shoots an invisible line to detect what it hits. Essential for aiming, ground detection, line-of-sight, and interaction systems.

Raycasting Visualization:

  Standard Raycast:              Spherecast:
  
  Origin ●────────────────→ Hit    Origin ●═══════════════→ Hit
         │                              ○═══════════════○
         │ Single line                  ○═══════════════○
                                        Fat ray with radius
  
  Boxcast:                       Overlap Sphere:
  
  ┌─────┬─────┬─────┐                   ╱ ╲
  │     │     │     │──→ Hit           ●   ●
  └─────┴─────┴─────┘                 ╲   ╱
  Sweeps a box shape                   Get all in radius

Common Raycast Patterns

public class RaycastExamples : MonoBehaviour
{
    public float interactRange = 3f;
    public LayerMask interactableLayers;
    
    void Update()
    {
        // 1. BASIC RAYCAST - Shooting, clicking
        if (Input.GetMouseButtonDown(0))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            RaycastHit hit;
            
            if (Physics.Raycast(ray, out hit, 100f))
            {
                Debug.Log("Clicked on: " + hit.collider.name);
                Debug.Log("Hit point: " + hit.point);
                Debug.Log("Distance: " + hit.distance);
                
                // Spawn effect at hit point
                Instantiate(hitEffect, hit.point, Quaternion.LookRotation(hit.normal));
            }
        }
        
        // 2. GROUND CHECK - Is player grounded?
        bool isGrounded = Physics.Raycast(
            transform.position,     // Origin
            Vector3.down,           // Direction
            1.1f,                   // Max distance
            groundLayer             // Only check ground layer
        );
        
        // 3. INTERACTION - What's in front of player?
        Ray interactRay = new Ray(transform.position, transform.forward);
        RaycastHit interactHit;
        
        if (Physics.Raycast(interactRay, out interactHit, interactRange, interactableLayers))
        {
            // Show "Press E to interact"
            IInteractable interactable = interactHit.collider.GetComponent<IInteractable>();
            if (interactable != null && Input.GetKeyDown(KeyCode.E))
                interactable.Interact();
        }
        
        // 4. LINE OF SIGHT - Can enemy see player?
        Vector3 directionToPlayer = (player.position - transform.position).normalized;
        if (!Physics.Raycast(transform.position, directionToPlayer, 
            Vector3.Distance(transform.position, player.position), obstacleLayer))
        {
            // No obstacle in the way - player is visible!
            AttackPlayer();
        }
    }
    
    // 5. SPHERECAST - Wider detection (for melee attacks)
    void MeleeAttack()
    {
        RaycastHit hit;
        if (Physics.SphereCast(transform.position, 0.5f, transform.forward, 
            out hit, 2f, enemyLayer))
        {
            hit.collider.GetComponent<Health>().TakeDamage(25);
        }
    }
    
    // 6. OVERLAP SPHERE - Find all nearby objects
    void FindNearbyEnemies()
    {
        Collider[] enemies = Physics.OverlapSphere(transform.position, 10f, enemyLayer);
        foreach (Collider enemy in enemies)
        {
            enemy.GetComponent<Enemy>().AlertToPlayer();
        }
    }
    
    // 7. RAYCAST ALL - Hit multiple objects (piercing bullet)
    void PiercingShot()
    {
        RaycastHit[] hits = Physics.RaycastAll(
            transform.position, transform.forward, 50f);
        
        // Sort by distance
        System.Array.Sort(hits, (a, b) => a.distance.CompareTo(b.distance));
        
        foreach (RaycastHit hit in hits)
        {
            hit.collider.GetComponent<Health>()?.TakeDamage(10);
        }
    }
}

Physics Materials

Physics Materials define surface properties like friction (grip) and bounciness. They determine how objects slide and bounce off each other.

Material Properties

Common Physics Material Presets:

Material        Friction    Bounciness    Use Case
─────────────────────────────────────────────────────
Ice             0.05        0.0           Slippery floors
Metal           0.4         0.1           Machinery, armor
Wood            0.6         0.2           Crates, platforms
Rubber          0.8         0.8           Tires, balls
Bouncy Ball     0.3         1.0           Power-ups, toys
Sticky          1.0         0.0           Gum, climbing walls

// Creating physics materials at runtime
public class DynamicPhysicsMaterial : MonoBehaviour
{
    void Start()
    {
        // Create ice material
        PhysicMaterial iceMaterial = new PhysicMaterial("Ice");
        iceMaterial.dynamicFriction = 0.05f;
        iceMaterial.staticFriction = 0.05f;
        iceMaterial.bounciness = 0f;
        iceMaterial.frictionCombine = PhysicMaterialCombine.Minimum;
        
        GetComponent<Collider>().material = iceMaterial;
    }
    
    // Change material based on surface
    void OnCollisionEnter(Collision collision)
    {
        if (collision.gameObject.CompareTag("Ice"))
        {
            // Reduce player friction when on ice
            GetComponent<Collider>().material.dynamicFriction = 0.1f;
        }
    }
}

Joints & Constraints

Joints connect rigidbodies together with specific constraints. They enable chains, doors, ragdolls, vehicles, and more.

Joint Types:

Fixed Joint:              Hinge Joint:              Spring Joint:
  ●═══════●                  ●                         ●
  Welded together           ╱╲                      ∿∿∿∿∿
  No movement              ╱  ╲                        ●
                          ●    Rotates on axis     Stretchy connection

Configurable Joint:       Character Joint:          Distance Joint:
  ●─ ─ ─ ─●              Shoulder●─●Elbow            ●───────────●
  Custom limits              ╱   ╲                   Fixed distance
  Full control              ●Wrist                    apart

Common Joint Uses

// HINGE JOINT - Doors, levers, pendulums
public class DoorHinge : MonoBehaviour
{
    void Start()
    {
        HingeJoint hinge = gameObject.AddComponent<HingeJoint>();
        
        // Set the axis of rotation
        hinge.axis = Vector3.up;  // Rotate around Y
        
        // Limit rotation range
        JointLimits limits = hinge.limits;
        limits.min = 0;      // Closed position
        limits.max = 90;     // Max open angle
        hinge.limits = limits;
        hinge.useLimits = true;
        
        // Optional: Add spring to close automatically
        JointSpring spring = hinge.spring;
        spring.spring = 50f;
        spring.targetPosition = 0;  // Target = closed
        hinge.spring = spring;
        hinge.useSpring = true;
    }
}

// SPRING JOINT - Grappling hook, bungee
public class GrapplingHook : MonoBehaviour
{
    public LineRenderer rope;
    private SpringJoint spring;
    
    void Fire(Vector3 hitPoint)
    {
        spring = gameObject.AddComponent<SpringJoint>();
        spring.autoConfigureConnectedAnchor = false;
        spring.connectedAnchor = hitPoint;
        
        // Spring properties
        spring.spring = 50f;      // Stiffness
        spring.damper = 5f;       // Reduce bouncing
        spring.maxDistance = 10f; // Max rope length
        spring.minDistance = 1f;  // Min rope length
    }
}

// FIXED JOINT - Picking up objects
public class PickupSystem : MonoBehaviour
{
    private FixedJoint joint;
    
    void PickUp(Rigidbody obj)
    {
        obj.transform.position = holdPoint.position;
        joint = obj.gameObject.AddComponent<FixedJoint>();
        joint.connectedBody = playerRigidbody;
    }
    
    void Drop()
    {
        Destroy(joint);
    }
}

Physics Simulation

Understanding how the physics engine works helps you write better, more performant physics code.

The Physics Loop

Physics Simulation Loop (Each Fixed Update):

1. APPLY FORCES ────────────────────────────────────────────┐
   │  Gravity, AddForce(), explosions, wind               │
   ▼                                                       │
2. INTEGRATE VELOCITY ─────────────────────────────────────│
   │  velocity += acceleration * dt                        │
   │  position += velocity * dt                            │
   ▼                                                       │
3. BROAD PHASE COLLISION ──────────────────────────────────│
   │  Quick AABB check: "Might these collide?"             │
   │  Eliminates obviously non-colliding pairs             │
   ▼                                                       │
4. NARROW PHASE COLLISION ─────────────────────────────────│
   │  Precise check: "Did they actually collide?"          │
   │  Calculate contact points, penetration depth          │
   ▼                                                       │
5. SOLVE CONSTRAINTS ──────────────────────────────────────│
   │  Resolve overlaps, apply friction                     │
   │  Process joints, limits                               │
   ▼                                                       │
6. CALLBACKS ──────────────────────────────────────────────│
   │  OnCollisionEnter/Exit, OnTriggerEnter/Exit           │
   └──────────────────────────────────────────────────────┘

Collision Layers and Masks

Layer Collision Matrix:

              Player  Enemy  Bullet  Terrain  Trigger
            ┌───────┬───────┬───────┬───────┬───────┐
Player      │   X   │   ✓   │   ✓   │   ✓   │   ✓   │
            ├───────┼───────┼───────┼───────┼───────┤
Enemy       │   ✓   │   X   │   ✓   │   ✓   │   X   │
            ├───────┼───────┼───────┼───────┼───────┤
Bullet      │   ✓   │   ✓   │   X   │   ✓   │   X   │
            ├───────┼───────┼───────┼───────┼───────┤
Terrain     │   ✓   │   ✓   │   ✓   │   X   │   X   │
            └───────┴───────┴───────┴───────┴───────┘

X = No collision    ✓ = Collide

Benefits:
• Player bullets don't hit player
• Enemies don't collide with each other (less CPU)
• Triggers only respond to player

// Working with layers in code
public class LayerExample : MonoBehaviour
{
    // Define layers (set up in Project Settings > Tags and Layers)
    public LayerMask groundLayer;
    public LayerMask enemyLayer;
    
    void Start()
    {
        // Get layer number
        int playerLayer = LayerMask.NameToLayer("Player");
        int enemyLayer = LayerMask.NameToLayer("Enemy");
        
        // Disable collision between layers
        Physics.IgnoreLayerCollision(playerLayer, enemyLayer, true);
        
        // Create layer mask from names
        LayerMask mask = LayerMask.GetMask("Ground", "Obstacles");
        
        // Raycast only against specific layers
        if (Physics.Raycast(transform.position, Vector3.down, 2f, groundLayer))
        {
            Debug.Log("Standing on ground!");
        }
    }
}

// Performance tips
public class PhysicsOptimization : MonoBehaviour
{
    void Start()
    {
        // 1. Sleep rigidbodies that aren't moving
        Rigidbody rb = GetComponent<Rigidbody>();
        rb.sleepThreshold = 0.5f;  // Default
        
        // 2. Use simpler colliders for moving objects
        // Mesh Collider → Box/Sphere/Capsule
        
        // 3. Reduce physics iterations if needed
        Physics.defaultSolverIterations = 6;  // Default
        
        // 4. Use fixed timestep wisely
        // Time.fixedDeltaTime = 0.02f;  // 50 physics updates/sec
    }
}