Unity 2d

Unity 2D Development

2D Project Setup

Unity supports dedicated 2D project creation. When starting a 2D project:

• Select the 2D template (or 2D (URP) for 2D lighting support) when creating a new project
• The 2D template auto-installs required packages: 2D Sprite, 2D Tilemap Editor, 2D Animation
• The Scene view defaults to orthographic (top-down XY plane)
• The camera is set to Orthographic projection by default
• Imported images default to Sprite (2D and UI) texture type

Core 2D subsystems from the docs:

• Sprites: "2D graphic objects" -- the foundation for 2D games
• Tilemaps: "A GameObject that allows you to quickly create 2D levels using tiles and a grid overlay"
• 2D Physics: Dedicated physics system with 2D-optimized components
• 2D Rendering in URP: 2D lights, lighting effects, and pixelated visual styles

Source: https://docs.unity3d.com/6000.3/Documentation/Manual/Unity2D.html

Sprites and SpriteRenderer

Sprite Assets

Sprites are "a type of 2D asset you can use in your Unity project." They function as 2D graphic objects with specialized import and management features. The 2D Sprite package must be installed (included with 2D templates).

Key sprite capabilities:

• Import sprites or spritesheets from textures
• Use the Sprite Editor to cut sprites from textures (slicing)
• Apply cropping to remove transparent areas
• Create collision geometry for sprite physics
• 9-slicing for reusing sprites at various sizes
• Masking to hide or reveal parts of a sprite or group of sprites

Source: https://docs.unity3d.com/6000.3/Documentation/Manual/sprite/sprite-landing.html

SpriteRenderer Component

The SpriteRenderer component controls how sprites display in scenes. Key properties:

Draw Modes:

• Simple (default): Uniformly scales the entire sprite
• Sliced: For 9-sliced sprites; scales according to 9-slice regions using Width/Height
• Tiled: For 9-sliced sprites; tiles the middle section. Tile Mode options: Continuous (even tiling) or Adaptive (stretches until threshold, then tiles)

Default material: Sprite-Lit-Default (customizable via Material picker)

// SpriteRenderer basic usage
SpriteRenderer sr = GetComponent<SpriteRenderer>();
sr.sprite = newSprite;
sr.color = Color.red;
sr.flipX = true;
sr.flipY = true;
sr.sortingLayerName = "Foreground";
sr.sortingOrder = 5;

See references/sprites-and-atlas.md for draw mode examples, sprite masking, and change callbacks.

Source: https://docs.unity3d.com/6000.3/Documentation/ScriptReference/SpriteRenderer.html

Sprite Atlas

A Sprite Atlas is "a utility that packs several sprite textures tightly together within a single texture known as an atlas." This reduces draw calls -- Unity uses one draw call for the atlas instead of separate calls per sprite.

Create: Assets > Create > 2D > Sprite Atlas (generates .spriteatlas file)

Sprite Atlas Properties

Runtime API: "You can use the Sprite Atlas API to control loading the Sprite Atlases at your project's runtime."

// Load a sprite from a SpriteAtlas at runtime
using UnityEngine;
using UnityEngine.U2D;

public class AtlasLoader : MonoBehaviour
{
    [SerializeField] private SpriteAtlas atlas;
    private SpriteRenderer spriteRenderer;

    void Start()
    {
        spriteRenderer = GetComponent<SpriteRenderer>();
        Sprite sprite = atlas.GetSprite("player_idle");
        spriteRenderer.sprite = sprite;
    }
}

Late binding: Subscribe to SpriteAtlasManager.atlasRequested to load atlases on demand (for Addressables/AssetBundle workflows). See references/sprites-and-atlas.md for full late binding example.

Source: https://docs.unity3d.com/6000.3/Documentation/Manual/sprite/atlas/atlas-landing.html

Tilemap System

Tilemaps allow rapid 2D level creation using tiles and a grid overlay. The Tilemap component "stores and manages Tile Assets" and "transfers the required information from the tiles placed on it to other related components such as the Tilemap Renderer and the Tilemap Collider 2D."

Tilemap Component Properties

TilemapRenderer Modes

Additional TilemapRenderer properties:

• Sort Order: Direction tiles are sorted during rendering
• Detect Chunk Culling Bounds: Auto or Manual (prevents sprite clipping)
• Material: Material for rendering sprite textures
• Mask Interaction: None, Visible Inside Mask, or Visible Outside Mask
• Sorting Layer / Order in Layer: Render priority controls

Tilemap Scripting API

using UnityEngine;
using UnityEngine.Tilemaps;

Tilemap tilemap = GetComponent<Tilemap>();

// Single tile operations
tilemap.SetTile(new Vector3Int(x, y, 0), tile);      // Place tile
tilemap.SetTile(new Vector3Int(x, y, 0), null);       // Remove tile
TileBase t = tilemap.GetTile(new Vector3Int(x, y, 0)); // Read tile

// Batch operations (faster than individual SetTile in loops)
tilemap.SetTiles(positionsArray, tilesArray);
tilemap.BoxFill(pos, tile, startX, endX, startY, endY);
tilemap.FloodFill(pos, tile);
tilemap.SwapTile(oldTile, newTile);

// Coordinate conversion
Vector3 worldPos = tilemap.CellToWorld(cellPos);
Vector3Int cellPos = tilemap.WorldToCell(worldPosition);

// Bounds management -- ALWAYS call after procedural generation
tilemap.CompressBounds();
BoundsInt bounds = tilemap.cellBounds;

// Clear
tilemap.ClearAllTiles();
tilemap.RefreshAllTiles();

Key properties: cellBounds, localBounds, size, origin, animationFrameRate Key events: tilemapTileChanged, tilemapPositionsChanged, loopEndedForTileAnimation

See references/tilemaps.md for procedural generation examples, Rule Tiles, and TilemapCollider2D setup.

Source: https://docs.unity3d.com/6000.3/Documentation/ScriptReference/Tilemaps.Tilemap.html

2D Physics Overview

Unity's 2D physics system provides optimized components for 2D interactions. "Unity's physics system lets you handle 2D physics to make use of optimizations available with 2D."

Core Components

Rigidbody2D Key Properties

Rigidbody2D Key Methods

Rigidbody2D rb = GetComponent<Rigidbody2D>();

// Apply forces
rb.AddForce(Vector2.up * 10f);
rb.AddForce(Vector2.right * 5f, ForceMode2D.Impulse);
rb.AddForceAtPosition(force, position);
rb.AddTorque(15f);

// Move (use in FixedUpdate for physics-safe movement)
rb.MovePosition(rb.position + velocity * Time.fixedDeltaTime);
rb.MoveRotation(rb.rotation + rotationSpeed * Time.fixedDeltaTime);

// Query
rb.Cast(direction, results, distance);
rb.ClosestPoint(worldPoint);
rb.Distance(otherCollider);
rb.GetContacts(contacts);
rb.IsAwake();
rb.IsSleeping();

Physics2D Static Methods (Queries)

// Raycast
RaycastHit2D hit = Physics2D.Raycast(origin, direction, distance, layerMask);
RaycastHit2D[] hits = Physics2D.RaycastAll(origin, direction);

// Shape casts
RaycastHit2D boxHit = Physics2D.BoxCast(origin, size, angle, direction, distance);
RaycastHit2D circleHit = Physics2D.CircleCast(origin, radius, direction, distance);

// Overlap detection
Collider2D col = Physics2D.OverlapCircle(point, radius, layerMask);
Collider2D[] cols = Physics2D.OverlapCircleAll(point, radius);
Collider2D boxCol = Physics2D.OverlapBox(point, size, angle, layerMask);
Collider2D pointCol = Physics2D.OverlapPoint(point, layerMask);

// Utilities
float dist = Physics2D.Distance(colliderA, colliderB).distance;
bool touching = Physics2D.IsTouching(colliderA, colliderB);

// 2D platformer: ground check + jump + horizontal movement
Rigidbody2D rb = GetComponent<Rigidbody2D>();

// In Update: ground check and jump
bool isGrounded = Physics2D.OverlapCircle(groundCheck.position, 0.2f, groundLayer);
// Note: Uses legacy Input Manager for simplicity. See unity-input for the new Input System.
if (Input.GetButtonDown("Jump") && isGrounded)
    rb.AddForce(Vector2.up * jumpForce, ForceMode2D.Impulse);

// In FixedUpdate: horizontal movement (preserves vertical velocity)
float move = Input.GetAxisRaw("Horizontal");
rb.linearVelocity = new Vector2(move * moveSpeed, rb.linearVelocity.y);

Source: https://docs.unity3d.com/6000.3/Documentation/ScriptReference/Rigidbody2D.html Source: https://docs.unity3d.com/6000.3/Documentation/ScriptReference/Physics2D.html

2D Lighting (URP)

2D Lighting requires the Universal Render Pipeline (URP) with a 2D Renderer. Use the 2D (URP) template or configure URP manually.

Light 2D Types

Light 2D Properties

using UnityEngine;
using UnityEngine.Rendering.Universal;

public class DayNightCycle : MonoBehaviour
{
    [SerializeField] private Light2D globalLight;
    [SerializeField] private Color dayColor = Color.white;
    [SerializeField] private Color nightColor = new Color(0.1f, 0.1f, 0.3f);
    [SerializeField] private float cycleDuration = 60f;

    void Update()
    {
        float t = Mathf.PingPong(Time.time / cycleDuration, 1f);
        globalLight.color = Color.Lerp(dayColor, nightColor, t);
        globalLight.intensity = Mathf.Lerp(1f, 0.3f, t);
    }
}

Source: https://docs.unity3d.com/6000.3/Documentation/Manual/urp/2d-light-properties-explained.html

Sorting Layers and Order

"Sorting Groups allow you to group GameObjects with Sprite Renderers together, and control the order in which they render their sprites." Unity treats all Sprite Renderers within the same Sorting Group as a unified rendering unit.

Sorting Layer System

Sorting layers define the broad rendering order. Within each layer, Order in Layer (integer) sets the specific priority. Lower numbers render first (appear behind higher numbers).

Sorting Group Component

The Sorting Group component organizes child renderers into a single sortable unit. Use cases:

• Complex multi-sprite characters (body parts as separate sprites)
• Groups of related objects that should sort together against the environment

Properties:

• Sorting Layer: Assigns the group to a sorting layer
• Order in Layer: Priority within the sorting layer

Nested Sorting Groups are supported: inner groups sort among themselves first, then the outer group sorts as a unit against other renderers.

// Y-sort for top-down games (attach to entities)
void LateUpdate()
{
    int order = Mathf.RoundToInt(-transform.position.y * 100);
    GetComponent<SpriteRenderer>().sortingOrder = order;
}

// Set sorting layer and order via script
SpriteRenderer sr = GetComponent<SpriteRenderer>();
sr.sortingLayerName = "Characters";
sr.sortingOrder = 10;
sr.sortingLayerID = SortingLayer.NameToID("Characters"); // faster

Source: https://docs.unity3d.com/6000.3/Documentation/Manual/sprite/sorting-group/sorting-group-landing.html

Common Patterns (C#)

Top-Down Movement (Rigidbody2D, no gravity)

public class TopDownMovement : MonoBehaviour
{
    [SerializeField] private float moveSpeed = 5f;
    private Rigidbody2D rb;
    private Vector2 moveInput;

    void Awake() { rb = GetComponent<Rigidbody2D>(); rb.gravityScale = 0f; }
    void Update() // legacy Input; see unity-input
    {
        moveInput = new Vector2(Input.GetAxisRaw("Horizontal"),
            Input.GetAxisRaw("Vertical")).normalized;
    }
    void FixedUpdate()
    {
        rb.MovePosition(rb.position + moveInput * moveSpeed * Time.fixedDeltaTime);
    }
}

Sprite Direction Flip / Camera Follow / Tilemap Click

// Flip sprite based on movement direction (legacy Input; see unity-input)
float h = Input.GetAxisRaw("Horizontal");
if (h != 0) spriteRenderer.flipX = h < 0;

// Simple 2D camera follow (attach to Camera, set offset.z = -10)
void LateUpdate()
{
    Vector3 desired = target.position + offset;
    transform.position = Vector3.Lerp(transform.position, desired, smoothSpeed * Time.deltaTime);
}

// Place tile at mouse click
Vector3 worldPos = cam.ScreenToWorldPoint(Input.mousePosition);
Vector3Int cellPos = tilemap.WorldToCell(worldPos);
tilemap.SetTile(cellPos, tileToPlace);

Anti-Patterns

1. Using 3D physics for 2D games: Always use Rigidbody2D and Collider2D, never Rigidbody/BoxCollider. The 2D physics engine is separate and optimized for 2D.

2. Moving Rigidbody2D with Transform: Never use transform.position to move physics objects. Use Rigidbody2D.MovePosition() in FixedUpdate or AddForce(). Direct Transform manipulation bypasses physics simulation and breaks collision detection.

3. Scaling sprites with Transform.localScale at runtime: Use SpriteRenderer.size with Sliced/Tiled draw modes instead. Transform scale causes issues with colliders and is less performant.

4. Forgetting to call CompressBounds() on procedural Tilemaps: After generating tiles procedurally, call tilemap.CompressBounds() to recalculate bounds. Without it, iteration over cellBounds includes empty space.

5. One draw call per sprite (no atlasing): Pack related sprites into a Sprite Atlas. Without atlasing, each unique texture triggers a separate draw call.

6. Enabling Read/Write on Sprite Atlas unnecessarily: This doubles memory usage. Only enable when you need script access via Texture2D.SetPixels.

7. Using Allow Rotation on UI sprites: Disable Allow Rotation in Sprite Atlas settings when packing sprites used in Canvas UI, as rotation breaks UI layout.

8. Not setting TilemapRenderer mode appropriately: Use Chunk mode for static tilemaps (best performance). Use Individual mode only when tiles need to interact with other renderers or require custom sorting.

9. Using continuous collision detection everywhere: Only set collisionDetectionMode to Continuous on fast-moving objects. Discrete is cheaper and sufficient for most 2D objects.

10. Ignoring Sorting Layers: Relying solely on Order in Layer leads to magic numbers. Define meaningful Sorting Layers (Background, Midground, Characters, Foreground, UI) and use Order in Layer for fine-tuning within each layer.

Key API Quick Reference

Related Skills

• unity-foundations -- GameObjects, Components, Transforms, Prefabs, ScriptableObjects
• unity-physics -- In-depth 3D/2D physics, joints, raycasting, physics materials
• unity-graphics -- Rendering pipelines, materials, shaders, post-processing
• unity-lighting-vfx -- URP lighting setup, visual effects, particle systems

Additional Resources

• Unity 2D Overview
• Sprites
• SpriteRenderer Reference
• Sprite Atlas
• Tilemap Reference
• Tilemap Scripting API
• Rigidbody2D API
• Physics2D API
• 2D Light Properties
• Sorting Groups