Enemy AI and NPC Basics

# CHAPTER 11

Enemy AI and NPC Basics #

1. Introduction #

2. Learning Objectives #

• Explain what a Navigation Mesh (NavMesh) is.

• "Bake" a NavMesh onto your 3D level geometry.

• Apply and configure a NavMeshAgent component to an enemy.

• Write C# code to command an agent to chase a moving target.

• Implement a basic waypoint patrol system.

3. The NavMesh (The Blue Walkway) #

1. 1. Go to Window -> AI -> Navigation.

1. 2. Select your floor and walls in the Hierarchy. In the Navigation window (Object tab), check Navigation Static.

1. 3. Go to the Bake tab and click Bake.

1. 4. You will see a blue overlay appear on the floor. The AI can now navigate your world!

4. The NavMeshAgent (The AI Brain) #

• This component replaces standard Rigidbodies for movement. It handles its own gravity, collision avoidance (steering around other enemies), and velocity.

• You can tune its Speed, Angular Speed (how fast it turns), and Stopping Distance.

5. Chasing the Player (C# Logic) #

using UnityEngine;
using UnityEngine.AI; // REQUIRED for AI code!

public class EnemyChase : MonoBehaviour
{
    public Transform player;
    private NavMeshAgent agent;

    void Start()
    {
        // Grab the brain
        agent = GetComponent<NavMeshAgent>();
    }

    void Update()
    {
        // Constantly update the destination to wherever the player is right now.
        // The Agent automatically calculates the path around walls!
        agent.SetDestination(player.position);
    }
}

6. Patrolling with Waypoints #

• We create empty GameObjects in the scene to act as invisible "Waypoints".

• The AI script holds an Array (a list) of these waypoints.

• It tells the agent to go to Waypoint 0. When agent.remainingDistance is near 0, it tells the agent to go to Waypoint 1, and so on.

7. Combining States (Basic AI Brain) #

• Use Vector3.Distance(transform.position, player.position) to check how far away the player is.

• If distance is > 10, run the Patrol code.

• If distance is <= 10, run the Chase code!

8. Visual Learning: NavMesh Pathfinding #

[ Enemy ]                      [ Player ]
    \                          /
     \                        /
      \___ [ Solid Wall ] ___/
           [ NavMesh Hole ]
           
*The NavMeshAgent knows it cannot walk through the hole where the wall is, so it automatically generates a path AROUND the wall to reach the player.*

9. Best Practices #

• NavMesh Obstacles: If you have a moving object in your game (like a physics crate the player can push), it wasn't baked into the static NavMesh. Enemies will try to walk through it. Attach a NavMeshObstacle component to the crate and check the Carve box. This tells the AI to dynamically cut a hole in the NavMesh wherever the crate moves, forcing enemies to walk around it in real-time!

10. Common Mistakes #

• Baking Moving Floors: The NavMesh can only be baked on objects marked as "Static" (objects that will never move). If you bake a NavMesh onto an elevator or a moving platform, the blue grid will stay floating in the air when the elevator moves down, and the AI will walk right off the edge! Moving AI platforms requires complex dynamic NavMesh scripts.

11. Mini Project: Create an Enemy Chase System #

1. 1. Build a simple maze using 3D Cubes.

1. 2. Mark the floor and the maze walls as Navigation Static, then hit Bake in the Navigation window. (Ensure you see the blue floor).

1. 3. Create a Capsule named "Player" and place it at one end.

1. 4. Create a red Capsule named "Zombie" at the other end.

1. 5. Add a NavMeshAgent component to the Zombie. Change its speed to 3.5.

1. 6. Create the EnemyChase.cs script (from Section 5) and attach it to the Zombie.

1. 7. Drag the Player into the script's target slot.

1. 8. Press Play and move the Player around. Watch the zombie navigate the maze perfectly to reach you!

12. Practice Exercises #

1. 1. What specific component must be attached to an enemy character so it can utilize the baked pathfinding data?

1. 2. What using namespace must be added to the top of your script to gain access to the NavMesh classes?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the relationship between a baked NavMesh and a NavMeshAgent. How does this system save the CPU from performing heavy pathfinding calculations every frame?

• Q: Describe how you would build a basic AI State Machine in C# that transitions an enemy from a "Patrol" state to a "Chase" state when the player gets within 15 meters.

• Q: What is a NavMeshObstacle? Contrast its usage with a static baked obstacle, and provide a gameplay example where a NavMeshObstacle with "Carving" enabled is required.

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #