2D Platformer Game Basics

Learn the essential scripts for creating a 2D platformer game in Unity. We'll implement basic game mechanics including player movement, jumping, camera tracking, and scene transitions.

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Game Structure

A 2D platformer game consists of three core scripts:

Script	Role
PlayerController	Player movement, jumping, animation, collision handling
CameraController	Camera that follows the player
ClearManager	Game clear scene management

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PlayerController - Player Controller

The core script that controls player movement, jumping, and animation.

Complete Code

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

public class PlayerController : MonoBehaviour
{
    Rigidbody2D rb;
    Animator animator;
    float jumpForce = 680.0f;
    float walkForce = 30.0f;
    float maxWalkSpeed = 2.0f;

    void Start()
    {
        rb = GetComponent<Rigidbody2D>();
        animator = GetComponent<Animator>();
    }

    void Update()
    {
        // Jump
        if (Input.GetKeyDown(KeyCode.Space) && rb.velocity.y == 0)
        {
            rb.AddForce(transform.up * jumpForce);
            animator.SetTrigger("Jump");
        }

        // Left/Right movement input
        int key = 0;
        if (Input.GetKey(KeyCode.RightArrow)) key = 1;
        if (Input.GetKey(KeyCode.LeftArrow)) key = -1;

        // Player speed
        float speedx = Mathf.Abs(rb.velocity.x);

        // Max speed limit
        if (speedx < maxWalkSpeed)
        {
            rb.AddForce(transform.right * key * walkForce);
        }

        // Direction change
        if (key != 0)
        {
            transform.localScale = new Vector3(key, 1, 1);
        }

        // Animation speed adjustment
        animator.speed = speedx / 2.0f;

        // Restart game if out of screen
        if (transform.position.x < -3.5f || transform.position.x > 3.5f || transform.position.y < -10f)
        {
            SceneManager.LoadScene("GameScene");
        }
    }

    private void OnTriggerEnter2D(Collider2D collision)
    {
        Debug.Log("OnTriggerEnter2D: " + collision.gameObject.name);
        SceneManager.LoadScene("ClearScene");
    }
}

Key Concepts Explained

1. Component References

Rigidbody2D rb;
Animator animator;

void Start()
{
    rb = GetComponent<Rigidbody2D>();
    animator = GetComponent<Animator>();
}

• Rigidbody2D: Movement and collision handling through physics engine
• Animator: Sprite animation control
• Get component references in Start()

2. Jump Implementation

if (Input.GetKeyDown(KeyCode.Space) && rb.velocity.y == 0)
{
    rb.AddForce(transform.up * jumpForce);
    animator.SetTrigger("Jump");
}

• Input.GetKeyDown(): Executes only once when key is pressed
• rb.velocity.y == 0: Only jump when on ground (prevents double jump)
• AddForce(): Apply physical force to jump
• SetTrigger(): Trigger jump animation

3. Horizontal Movement

int key = 0;
if (Input.GetKey(KeyCode.RightArrow)) key = 1;
if (Input.GetKey(KeyCode.LeftArrow)) key = -1;

float speedx = Mathf.Abs(rb.velocity.x);

if (speedx < maxWalkSpeed)
{
    rb.AddForce(transform.right * key * walkForce);
}

• Input.GetKey(): Executes continuously while key is held
• maxWalkSpeed: Speed limit for natural movement
• AddForce(): Physics-based movement for smooth acceleration/deceleration

4. Direction Change (Using LocalScale)

if (key != 0)
{
    transform.localScale = new Vector3(key, 1, 1);
}

• Set localScale.x to 1 or -1 to flip sprite horizontally
• Using localScale instead of flipX also flips child objects

Tip: SpriteRenderer.flipX only flips the sprite, while localScale flips colliders and child objects as well.

5. Animation Speed Control

animator.speed = speedx / 2.0f;

• Adjust animation speed proportional to player movement speed
• Animation stops when stationary, speeds up when moving fast

6. Screen Boundary Detection

if (transform.position.x < -3.5f || transform.position.x > 3.5f || transform.position.y < -10f)
{
    SceneManager.LoadScene("GameScene");
}

• Restart game when player goes off screen
• Check left, right, and bottom boundaries

7. Trigger Collision Detection

private void OnTriggerEnter2D(Collider2D collision)
{
    SceneManager.LoadScene("ClearScene");
}

• Place a collider with Is Trigger checked at the goal point
• Transition to clear scene when player reaches the goal

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CameraController - Camera Tracking

A camera script that follows the player's Y-axis position.

Complete Code

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

public class CameraController : MonoBehaviour
{
    public GameObject player;

    void Update()
    {
        Vector3 playerPos = player.transform.position;
        transform.position = new Vector3(transform.position.x, playerPos.y, transform.position.z);
    }
}

Key Concepts Explained

Public Variable for Player Reference

public GameObject player;

• Drag and connect player object in Inspector
• public declaration makes it configurable in Inspector

Y-Axis Only Tracking

Vector3 playerPos = player.transform.position;
transform.position = new Vector3(transform.position.x, playerPos.y, transform.position.z);

• Maintain camera's original X and Z position
• Only Y-axis follows player position
• Suitable for vertical scrolling platformers

Extension Tip: Use playerPos.x to also track X-axis, and use Vector3.Lerp() for smooth tracking.

Smooth Camera Tracking (Improved Version)

public class CameraController : MonoBehaviour
{
    public GameObject player;
    public float smoothSpeed = 5.0f;

    void LateUpdate()
    {
        Vector3 targetPos = new Vector3(transform.position.x, player.transform.position.y, transform.position.z);
        transform.position = Vector3.Lerp(transform.position, targetPos, smoothSpeed * Time.deltaTime);
    }
}

• LateUpdate(): Move camera after player movement (executes after Update)
• Vector3.Lerp(): Smooth interpolation for natural camera movement

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ClearManager - Clear Scene Management

A script that handles game restart after clearing.

Complete Code

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

public class ClearManager : MonoBehaviour
{
    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            SceneManager.LoadScene("GameScene");
        }
    }
}

Key Concepts Explained

Mouse Click Detection

if (Input.GetMouseButtonDown(0))
{
    SceneManager.LoadScene("GameScene");
}

• Input.GetMouseButtonDown(0): Left mouse button click
• Return to game scene when clicking on clear screen

Build Settings Configuration

To use scene transitions, you must add scenes in File > Build Settings:

1. GameScene - Main game scene
2. ClearScene - Clear screen scene

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Collision Detection Callbacks Summary

Callback functions used for Unity 2D collision detection:

Callback Function	Description
OnCollisionEnter2D	Physics collision started
OnCollisionStay2D	Physics collision ongoing
OnCollisionExit2D	Physics collision ended
OnTriggerEnter2D	Trigger collision started
OnTriggerStay2D	Trigger collision ongoing
OnTriggerExit2D	Trigger collision ended

Collision vs Trigger: Collision involves physical interaction (pushing), Trigger only detects passing through.

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Unity Setup Checklist

Player Object

• Add Rigidbody2D component
• Add Collider2D component (BoxCollider2D or CapsuleCollider2D)
• Add Animator component (if using animation)
• Add PlayerController script

Camera Object

• Add CameraController script
• Connect Player object to player variable in Inspector

Goal Object (Goal Point)

• Add Collider2D component
• Check Is Trigger

Build Settings

• Add GameScene
• Add ClearScene

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Summary

Item	Content
Player Movement	Physics-based movement with Rigidbody2D.AddForce()
Jump	AddForce(transform.up * jumpForce), only when grounded
Direction Change	Set localScale.x to 1 or -1
Camera Tracking	Follow player's Y-axis position only
Scene Transition	SceneManager.LoadScene("SceneName")
Trigger Detection	Use OnTriggerEnter2D() callback

By combining these three scripts, you can implement the core mechanics of a basic 2D platformer game.

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