Phaser Gamedev

Phaser Game Development

Build 2D browser games using Phaser 3's scene-based architecture and physics systems.

STOP: Before Loading Any Spritesheet

Read spritesheets-nineslice.md FIRST.

Spritesheet loading is fragile—a few pixels off causes silent corruption that compounds into broken visuals. The reference file contains the mandatory inspection protocol.

Quick rules (details in reference):

1. Measure the asset before writing loader code—never guess frame dimensions
2. Character sprites use SQUARE frames: If you calculate frameWidth=56, try 56 for height first
3. Different animations have different frame sizes: A run cycle needs wider frames than idle; an attack needs extra width for weapon swing. Measure EACH spritesheet independently
4. Check for spacing: Gaps between frames require spacing: N in loader config
5. Verify the math: imageWidth = (frameWidth × cols) + (spacing × (cols - 1))

Reference Files

Read these BEFORE working on the relevant feature:

Architecture Decisions (Make Early)

Physics System Choice

Scene Structure

scenes/
├── BootScene.ts      # Asset loading, progress bar
├── MenuScene.ts      # Title screen, options
├── GameScene.ts      # Main gameplay
├── UIScene.ts        # HUD overlay (launched parallel)
└── GameOverScene.ts  # End screen, restart

Scene Transitions

this.scene.start('GameScene', { level: 1 });     // Stop current, start new
this.scene.launch('UIScene');                     // Run in parallel
this.scene.pause('GameScene');                    // Pause
this.scene.stop('UIScene');                       // Stop

Core Patterns

Game Configuration

const config: Phaser.Types.Core.GameConfig = {
  type: Phaser.AUTO,
  width: 800,
  height: 600,
  scale: {
    mode: Phaser.Scale.FIT,
    autoCenter: Phaser.Scale.CENTER_BOTH
  },
  physics: {
    default: 'arcade',
    arcade: { gravity: { y: 300 }, debug: false }
  },
  scene: [BootScene, MenuScene, GameScene]
};

Scene Lifecycle

class GameScene extends Phaser.Scene {
  init(data) { }      // Receive data from previous scene
  preload() { }       // Load assets (runs before create)
  create() { }        // Set up game objects, physics, input
  update(time, delta) { }  // Game loop, use delta for frame-rate independence
}

Frame-Rate Independent Movement

// CORRECT: scales with frame rate
this.player.x += this.speed * (delta / 1000);

// WRONG: varies with frame rate
this.player.x += this.speed;

Viewport Scaling

CRITICAL: Use Phaser's Scale Manager for ALL viewport scaling. This is the PRIMARY and ONLY correct approach.

Why Phaser Scale Manager?

Phaser's Scale Manager handles:

• Responsive viewport sizing
• Aspect ratio preservation
• Device pixel ratio (DPR) handling
• Window resize events
• Canvas scaling and centering
• Input coordinate transformation

Manual JavaScript/CSS scaling is an anti-pattern that breaks Phaser's coordinate system, input handling, and physics calculations.

Scale Manager Configuration

The Scale Manager is configured in your game config's scale property:

const config: Phaser.Types.Core.GameConfig = {
  type: Phaser.AUTO,
  width: 800,   // Base game width
  height: 600,  // Base game height
  scale: {
    mode: Phaser.Scale.FIT,              // Primary scaling mode
    autoCenter: Phaser.Scale.CENTER_BOTH, // Center the game canvas
    parent: 'game-container',            // Optional: parent DOM element
    width: 800,                          // Base width (can differ from top-level)
    height: 600,                         // Base height (can differ from top-level)
    min: { width: 400, height: 300 },    // Optional: minimum size
    max: { width: 1920, height: 1080 },  // Optional: maximum size
  },
  // ... rest of config
};

Common Scaling Modes

Scaling for 70-80% Viewport Usage

To make the game fill 70-80% of the viewport while maintaining aspect ratio:

const config: Phaser.Types.Core.GameConfig = {
  type: Phaser.AUTO,
  width: 800,
  height: 600,
  scale: {
    mode: Phaser.Scale.FIT,
    autoCenter: Phaser.Scale.CENTER_BOTH,
    // Calculate target size as percentage of viewport
    // Phaser will automatically scale to fit while maintaining aspect ratio
  },
  // ... rest of config
};

// In your scene, you can also adjust dynamically:
this.scale.setGameSize(
  Math.floor(window.innerWidth * 0.75),  // 75% of viewport width
  Math.floor(window.innerHeight * 0.75)  // 75% of viewport height
);

Dynamic Resize Handling

Phaser automatically handles window resize events when using Scale Manager. To respond to resize:

this.scale.on('resize', (gameSize: Phaser.Structs.Size) => {
  // Game has been resized
  // gameSize.width and gameSize.height are the new dimensions
  // Adjust camera, UI, or game elements as needed
});

Accessing Scale Information

// In any scene:
const scaleManager = this.scale;

// Get current game size (after scaling)
const gameWidth = this.scale.gameSize.width;
const gameHeight = this.scale.gameSize.height;

// Get display size (actual canvas size)
const displayWidth = this.scale.displaySize.width;
const displayHeight = this.scale.displaySize.height;

// Get scale factor
const scaleX = this.scale.displaySize.width / this.scale.gameSize.width;
const scaleY = this.scale.displaySize.height / this.scale.gameSize.height;

Anti-Pattern: Manual JavaScript/CSS Scaling

❌ NEVER do this:

// WRONG: Manual DOM/CSS scaling
const gameCanvas = document.querySelector('canvas');
gameCanvas.style.width = '75%';
gameCanvas.style.height = '75%';
gameCanvas.style.transform = 'scale(0.75)';

// WRONG: Manual JavaScript resize handlers
window.addEventListener('resize', () => {
  const container = document.getElementById('game-container');
  container.style.width = `${window.innerWidth * 0.75}px`;
  container.style.height = `${window.innerHeight * 0.75}px`;
});

// WRONG: MutationObserver for scaling
const observer = new MutationObserver(() => {
  // Manual scaling logic
});

Problems with manual scaling:

• Breaks Phaser's coordinate system
• Input coordinates don't match visual positions
• Physics calculations become incorrect
• Camera and viewport calculations fail
• No automatic DPR handling
• Resize events not properly coordinated

✅ ALWAYS use Phaser Scale Manager:

// CORRECT: Use Phaser Scale Manager
const config: Phaser.Types.Core.GameConfig = {
  scale: {
    mode: Phaser.Scale.FIT,
    autoCenter: Phaser.Scale.CENTER_BOTH,
  },
};

When Skills Don't Provide Enough Detail

If scaling requirements are complex or the skill guidance is insufficient:

1. Search official Phaser documentation:

   • Use mcp_web_fetch to fetch: https://photonstorm.github.io/phaser3-docs/Phaser.Scale.ScaleManager.html
   • Search for: "Phaser Scale Manager", "Phaser.Scale.FIT", "Phaser viewport scaling"

2. Search Stack Overflow:

   • Search for: "Phaser 3 scaling", "Phaser responsive game", "Phaser Scale Manager"

3. Check GitHub discussions:

   • Phaser 3 GitHub issues: "Phaser.Scale" or "scaling" issues

Never fall back to manual JavaScript scaling - always use Phaser's built-in Scale Manager.

Anti-Patterns

Remember

Phaser provides powerful primitives—scenes, sprites, physics, input—but architecture is your responsibility.

Before coding: What scenes? What entities? How do they interact? What physics model?

Claude can build complete, polished Phaser games. These guidelines illuminate the path—they don't fence it.