Player Movement and Input Systems

# CHAPTER 6

Player Movement and Input Systems #

1. Introduction #

2. Learning Objectives #

• Read keyboard and mouse input using Unity's Input class.

• Move an object manually using transform.Translate.

• Understand the critical importance of Time.deltaTime.

• Move a physics-based object using Rigidbody.AddForce.

• Understand the basics of a 3D camera follow script.

3. Reading Player Input #

• Button Presses: Input.GetKeyDown(KeyCode.Space) returns true the exact frame you press the spacebar.

• Axes (Smooth Input): Instead of checking specifically for the 'W' or 'A' keys, Unity uses "Axes".

• Input.GetAxis("Horizontal") returns -1 if you press A (left) and 1 if you press D (right).

• Input.GetAxis("Vertical") returns -1 for S (backward) and 1 for W (forward).

4. Method 1: Transform Translation (Kinematic) #

public float speed = 5f;

void Update()
{
    float h = Input.GetAxis("Horizontal"); // A/D keys
    float v = Input.GetAxis("Vertical");   // W/S keys

    // Move along the X and Z axes
    Vector3 moveDirection = new Vector3(h, 0, v);
    
    transform.Translate(moveDirection * speed * Time.deltaTime);
}

What is Time.deltaTime? If you run Translate in Update, it runs every frame. If someone has a fast PC (144 FPS), the player will run twice as fast as someone on a slow PC (60 FPS). Multiplying by Time.deltaTime normalizes the speed to "meters per second," ensuring the game plays exactly the same on all computers!

5. Method 2: Rigidbody Movement (Physics) #

public Rigidbody rb;
public float pushPower = 10f;

void FixedUpdate() // ALWAYS use FixedUpdate for Rigidbody physics!
{
    float h = Input.GetAxis("Horizontal");
    float v = Input.GetAxis("Vertical");
    
    Vector3 pushDirection = new Vector3(h, 0, v);
    rb.AddForce(pushDirection * pushPower);
}

6. Method 3: CharacterController (The Industry Standard) #

7. Camera Follow Basics #

• The Bad Way: Making the Camera a "child" of the Player. If the player turns around or rolls, the camera will violently spin and make the player nauseous!

• The Good Way: A C# script on the camera that copies the player's position, but adds an "Offset" so it stays slightly behind and above.

public Transform player;
public Vector3 offset = new Vector3(0, 5, -10);

void LateUpdate() // Runs right after Update, ensuring the player has finished moving first!
{
    transform.position = player.position + offset;
}

8. Visual Learning: The Input Pipeline #

[ Player Presses 'W' ]
         |
[ Input.GetAxis("Vertical") returns 1.0 ]
         |
[ Calculate: Vector3(0, 0, 1.0) * Speed * DeltaTime ]
         |
[ transform.Translate() applies movement ]
         |
[ Camera LateUpdate() follows the new position ]

9. Best Practices #

• Use SerializeField for References: When writing the Camera script, don't use GameObject.Find("Player") in the Update loop—it causes massive lag. Instead, use public Transform player; and drag the Player object into the slot in the Inspector before the game starts.

10. Common Mistakes #

• Applying DeltaTime to AddForce: You must multiply Translate by Time.deltaTime to fix framerate issues. However, if you are using Rigidbody.AddForce() inside FixedUpdate(), do NOT multiply by Time.deltaTime! FixedUpdate already runs on a perfectly locked timer, so adding DeltaTime to it will cause your math to break.

11. Mini Project: Create a Controllable 3D Player #

1. 1. Create a 3D Object -> Plane (The Floor).

1. 2. Create a 3D Object -> Cube (The Player).

1. 3. Create a C# script named PlayerMovement.

1. 4. Open the script and paste the Transform Translation code from Section 4.

1. 5. Save the script and drag it onto the Cube.

1. 6. Create a C# script named CameraFollow and paste the code from Section 7.

1. 7. Attach CameraFollow to the Main Camera. Click the Camera, look at the Inspector, and drag the Cube into the empty "Player" slot.

1. 8. Press Play and drive your cube around!

12. Practice Exercises #

1. 1. What built-in Unity variable must you multiply movement by to ensure your game speed is independent of the computer's framerate?

1. 2. If you are applying physics forces to a Rigidbody, which specific update loop must you place the code in?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the necessity of Time.deltaTime. What specific gameplay bug occurs if a developer uses transform.Translate(Vector3.forward) inside Update() without it?

• Q: Contrast transform.Translate with Rigidbody.AddForce. In what specific game genre or scenario would you choose one over the other?

• Q: Why is a camera follow script traditionally placed in the LateUpdate() method rather than the standard Update() method?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #