Character Controllers and Platformer Physics

# CHAPTER 7

Character Controllers and Platformer Physics #

1. Introduction #

2. Learning Objectives #

• Explain why standard Rigidbodies make poor platformer characters.

• Implement robust Ground Detection using SphereCasts/Raycasts.

• Calculate and apply movement perpendicular to sloped surfaces.

• Program "Coyote Time" and "Jump Buffering" for enhanced game feel.

• Decide between Kinematic and Dynamic character controllers.

3. The Rigidbody Friction Problem #

• The Fix: You must create a Physics Material with 0 Friction and apply it to the player's collider. This makes the player perfectly slippery, allowing them to slide down walls naturally.

4. Advanced Ground Detection #

• The Industry Standard: Use a mathematical laser pointing down from the feet.

bool isGrounded;
public Transform feetPosition;
public float checkRadius = 0.2f;
public LayerMask groundLayer;

void Update()
{
    // Draw an invisible mathematical sphere at the feet.
    // If it overlaps with anything on the "Ground" layer, we are grounded!
    isGrounded = Physics.CheckSphere(feetPosition.position, checkRadius, groundLayer);
}

5. Handling Slopes #

1. 1. Shoot a Raycast down to hit the slope.

1. 2. Get the "Normal" of the slope (the mathematical arrow pointing straight out from the ramp's surface).

1. 3. Use Vector Cross Products (or engine projection methods) to angle the forward movement vector parallel to the ramp.

6. The Magic of "Coyote Time" #

float coyoteTime = 0.1f;
float coyoteTimeCounter;

void Update()
{
    if (isGrounded)
    {
        coyoteTimeCounter = coyoteTime; // Reset timer
    }
    else
    {
        coyoteTimeCounter -= Time.deltaTime; // Tick down in the air
    }

    // You can jump even if you are slightly off the ledge!
    if (Input.GetKeyDown("space") && coyoteTimeCounter > 0f)
    {
        Jump();
        coyoteTimeCounter = 0f; // Consume the jump
    }
}

7. Visual Learning: Slope Movement #

[ Standard Forward Vector (Smashes into ramp) ]
     -->
        /
       / (Ramp)

[ Projected Slope Vector (Slides up ramp) ]
      /^
     / |
    /

8. Best Practices #

• Jump Buffering: Along with Coyote Time, implement a Jump Buffer. If the player is falling and presses "Jump" 0.1 seconds *before* they hit the ground, save that input. The moment they touch the ground, execute the jump automatically. This makes the controls feel incredibly responsive and fluid.

9. Common Mistakes #

• Applying Velocity Directly: Many tutorials teach rb.velocity = new Vector3(x, rb.velocity.y, z);. This manually overrides the physics engine every frame. It works for simple games, but if a moving platform or an explosion tries to push the player, this line of code will instantly delete that external force! It is safer to use rb.AddForce or dedicated Kinematic Character Controllers.

10. Mini Project: Build a Responsive Platformer Controller #

class PlatformerPlayer
{
    public Rigidbody2D rb;
    public float speed = 10f;
    public float jumpForce = 15f;
    
    private float coyoteTimer;
    private bool isGrounded;

    void Update()
    {
        isGrounded = Physics2D.OverlapCircle(transform.position, 0.2f, LayerMask.GetMask("Ground"));

        if (isGrounded) coyoteTimer = 0.1f;
        else coyoteTimer -= Time.deltaTime;

        // X Movement
        float x = Input.GetAxisRaw("Horizontal");
        rb.velocity = new Vector2(x * speed, rb.velocity.y);

        // Jump
        if (Input.GetKeyDown(KeyCode.Space) && coyoteTimer > 0f)
        {
            rb.velocity = new Vector2(rb.velocity.x, jumpForce);
            coyoteTimer = 0f;
        }
    }
}

11. Practice Exercises #

1. 1. Why must you apply a zero-friction Physics Material to a Rigidbody player character in a platformer?

1. 2. Explain the concept of "Coyote Time" and why it improves player experience.

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the difference between a Dynamic Rigidbody controller and a Kinematic Character Controller (KCC). Why do most AAA first-person shooters use KCCs instead of standard Rigidbodies?

• Q: Walk me through the implementation of "Jump Buffering." How does it solve the issue of a player pressing the jump button slightly too early while falling?

• Q: A player is pushing against a vertical wall in mid-air. They let go of the keyboard, but they do not fall; they remain stuck to the wall. Identify the physics engine interaction causing this and provide the solution.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #