Spatial Calculation and UI Layout

Spatial calculation helpers and UI layout patterns to reduce UI positioning iterations from 3-5 to 1-2. Provides coordinate system documentation, text width measurement utilities, and layout calculation patterns.

Overview

Problem: UI positioning tasks require 3-5 iterations due to spatial calculation challenges. Agents struggle with coordinate systems, text width calculations, and layout positioning.

Solution: Coordinate system documentation, text width measurement utilities, layout calculation patterns, and common gotchas.

Impact: Reduces UI positioning iterations from 3-5 to 1-2, saves 2-3 minutes per UI layout task, improves first-attempt success rate

Coordinate System Documentation

World Coordinates vs Screen Coordinates

Understanding coordinate systems is critical for UI positioning tasks.

World Coordinates:

•Game world space (e.g., 800x600 game world)
•Camera-independent (objects exist in world space)
•Used for game objects, sprites, physics bodies
•Example: sprite.x = 400 (400 pixels from world origin)

Screen Coordinates:

•Viewport/camera space (what player sees)
•Camera-dependent (changes with camera scroll)
•Used for UI elements, HUD, overlays
•Example: ui.x = 400 (400 pixels from screen edge, camera-independent)

Key Difference:

typescript

// World coordinates (game object)
sprite.x = 400;  // 400 pixels in world space

// Screen coordinates (UI element)
ui.x = 400;  // 400 pixels from screen edge (camera-independent)

Framework-Specific Coordinate Systems

Phaser 3:

•World coordinates: sprite.x, sprite.y (in world space)
•Screen coordinates: ui.x, ui.y (camera-independent)
•Camera scroll: this.cameras.main.scrollX, this.cameras.main.scrollY

React:

•Screen coordinates: style.left, style.top (relative to viewport)
•Container coordinates: Relative to parent container

Canvas/WebGL:

•World coordinates: Canvas drawing coordinates
•Screen coordinates: Viewport-relative coordinates

Camera Scroll Offset Handling

When calculating positions, account for camera scroll:

typescript

// ❌ WRONG: Not accounting for camera scroll
const worldX = 400;
sprite.x = worldX;  // May be off-screen if camera scrolled

// ✅ CORRECT: Account for camera scroll
const cameraX = this.cameras.main.scrollX;
const worldX = 400;
sprite.x = cameraX + worldX;  // Correct position relative to camera

For UI Elements (Screen Coordinates):

typescript

// UI elements use screen coordinates (camera-independent)
ui.x = 400;  // Always 400 pixels from screen edge, regardless of camera

Text Width Measurement Utilities

Measure Text Width Before Positioning

Always measure text width before calculating positions:

typescript

// ❌ WRONG: Assuming fixed width
text.x = 400;  // May not be centered if text width varies

// ✅ CORRECT: Calculate based on actual width
const textWidth = text.width;
const screenWidth = this.cameras.main.width;
const centerX = (screenWidth - textWidth) / 2;
text.x = centerX;

Text Width Measurement Patterns

Pattern 1: Measure Existing Text

typescript

// Measure text that's already created
const textWidth = text.width;
const textHeight = text.height;

Pattern 2: Measure Text Before Creation

typescript

// Create temporary text to measure
const tempText = this.add.text(0, 0, "Score: 100", style);
const textWidth = tempText.width;
const textHeight = tempText.height;
tempText.destroy();  // Clean up temporary text

// Use measurements for positioning
const centerX = (screenWidth - textWidth) / 2;
text.x = centerX;

Pattern 3: Measure Text with Different Content

typescript

// Measure text with longest expected content
const longestText = "Score: 999999";
const tempText = this.add.text(0, 0, longestText, style);
const maxWidth = tempText.width;
tempText.destroy();

// Use max width for layout
const centerX = (screenWidth - maxWidth) / 2;

Framework-Specific Text Measurement

Phaser 3:

typescript

const text = this.add.text(0, 0, "Text", style);
const width = text.width;
const height = text.height;

Canvas:

typescript

const ctx = canvas.getContext('2d');
ctx.font = '16px Arial';
const width = ctx.measureText("Text").width;

React:

typescript

const textRef = useRef<HTMLDivElement>(null);
const width = textRef.current?.offsetWidth || 0;

Layout Calculation Patterns

Pattern 1: Center Text on Screen

typescript

// Calculate text width first
const textWidth = text.width;
const screenWidth = this.cameras.main.width;
const centerX = (screenWidth - textWidth) / 2;

text.x = centerX;  // Center text horizontally

Common Mistake: Not accounting for text width

typescript

// ❌ WRONG: Assuming text is centered at screenWidth / 2
text.x = screenWidth / 2;  // Text starts at center, not centered

// ✅ CORRECT: Account for text width
text.x = (screenWidth - text.width) / 2;  // Text is centered

Pattern 2: Position Relative to Another Object

typescript

// Position button below text
const textBottom = text.y + text.height;
const spacing = 20;
button.y = textBottom + spacing;

Common Mistake: Not accounting for object height

typescript

// ❌ WRONG: Assuming objects align at same y
button.y = text.y;  // Button overlaps text

// ✅ CORRECT: Account for text height
button.y = text.y + text.height + spacing;

Pattern 3: Account for Origin

typescript

// Sprite origin affects position calculation
sprite.setOrigin(0.5, 0.5);  // Center origin
sprite.x = 400;  // Center of sprite at x=400

// If origin is (0, 0), sprite.x is top-left corner
sprite.setOrigin(0, 0);
sprite.x = 400;  // Top-left corner at x=400

Common Mistake: Not accounting for origin

typescript

// ❌ WRONG: Assuming origin is (0, 0)
sprite.x = 400;  // May not be where expected if origin is (0.5, 0.5)

// ✅ CORRECT: Account for origin
sprite.setOrigin(0.5, 0.5);
sprite.x = 400;  // Center of sprite at x=400

Pattern 4: Layout with Spacing

typescript

// Layout multiple elements with consistent spacing
const elements = [text1, text2, text3];
const spacing = 20;
let currentY = 100;

elements.forEach(element => {
  element.y = currentY;
  currentY += element.height + spacing;
});

Pattern 5: Responsive Layout

typescript

// Responsive layout based on screen size
const screenWidth = this.cameras.main.width;
const screenHeight = this.cameras.main.height;

// Center horizontally
text.x = (screenWidth - text.width) / 2;

// Position from bottom
const bottomMargin = 50;
text.y = screenHeight - text.height - bottomMargin;

Common Positioning Gotchas

Gotcha 1: Not Accounting for Text Width

typescript

// ❌ WRONG: Assuming fixed width
text.x = 400;  // May not be centered if text width varies

// ✅ CORRECT: Calculate based on actual width
const textWidth = text.width;
const centerX = (screenWidth - textWidth) / 2;
text.x = centerX;

Gotcha 2: Confusing World vs Screen Coordinates

typescript

// ❌ WRONG: Using world coordinates for UI
ui.x = sprite.x;  // UI will move with camera scroll

// ✅ CORRECT: Use screen coordinates for UI
ui.x = 400;  // UI stays fixed on screen

Gotcha 3: Not Considering Sprite Origin Offsets

typescript

// ❌ WRONG: Assuming origin is (0, 0)
sprite.x = 100;  // May not be where expected if origin is (0.5, 0.5)

// ✅ CORRECT: Account for origin
sprite.setOrigin(0.5, 0.5);
sprite.x = 100;  // Center of sprite at x=100

Gotcha 4: Not Accounting for Object Height

typescript

// ❌ WRONG: Assuming objects align at same y
button.y = text.y;  // Button overlaps text

// ✅ CORRECT: Account for text height
button.y = text.y + text.height + spacing;

Gotcha 5: Assuming HMR Has Applied Changes

typescript

// ❌ WRONG: Assuming changes are applied immediately
makeCodeChange();
captureScreenshot();  // May capture stale state

// ✅ CORRECT: Verify HMR has applied changes
makeCodeChange();
waitForHMR();  // Wait for HMR to apply
captureScreenshot();  // Now captures updated state

Visual Debugging Techniques

Technique 1: Programmatic Verification

Verify calculations programmatically before visual verification:

typescript

// Calculate position
const centerX = (screenWidth - text.width) / 2;
text.x = centerX;

// Verify calculation
const expectedX = (screenWidth - text.width) / 2;
console.log(`Text x: ${text.x}, Expected: ${expectedX}`);
if (Math.abs(text.x - expectedX) < 1) {
  console.log('Position correct');
} else {
  console.log('Position incorrect');
}

Technique 2: Test Seam Verification

Use test seam commands to verify positions:

bash

# Verify text position via test seam
agent-browser eval "window.__TEST__?.getCurrentScene()?.texts?.score?.x"
agent-browser eval "window.__TEST__?.getCurrentScene()?.texts?.score?.y"

Technique 3: Visual Markers

Add visual markers for debugging:

typescript

// Add visual marker at expected position
const marker = this.add.rectangle(centerX, 300, 5, 5, 0xff0000);
// Text should be centered at marker

Examples of Successful Layout Calculations

Example 1: Centered Score Display

typescript

// Calculate text width first
const scoreText = "Score: 100";
const text = this.add.text(0, 0, scoreText, style);
const textWidth = text.width;
const screenWidth = this.cameras.main.width;

// Center text horizontally
const centerX = (screenWidth - textWidth) / 2;
text.x = centerX;

// Position from top with margin
const topMargin = 50;
text.y = topMargin;

Key Points:

•Calculate text width before positioning
•Account for screen width (not world width)
•Use spacing constants for consistency

Example 2: Button Below Text

typescript

// Position text first
const text = this.add.text(100, 100, "Click Me", textStyle);
const textBottom = text.y + text.height;

// Position button below text with spacing
const spacing = 20;
const button = this.add.rectangle(100, textBottom + spacing, 200, 50, 0x00ff00);

Key Points:

•Calculate text bottom position
•Add spacing between elements
•Align horizontally (same x for text and button)

Example 3: Responsive Bottom Bar

typescript

// Get screen dimensions
const screenWidth = this.cameras.main.width;
const screenHeight = this.cameras.main.height;

// Create bottom bar
const barHeight = 60;
const bar = this.add.rectangle(
  screenWidth / 2,  // Center horizontally
  screenHeight - barHeight / 2,  // Position from bottom
  screenWidth,  // Full width
  barHeight,
  0x000000
);

// Add text on bar
const text = this.add.text(0, 0, "Game Over", textStyle);
text.x = (screenWidth - text.width) / 2;  // Center text
text.y = screenHeight - barHeight / 2 - text.height / 2;  // Center vertically on bar

Key Points:

•Use screen dimensions for responsive layout
•Calculate center positions
•Account for object dimensions

Programmatic Verification Patterns

Pattern 1: Verify Before Visual Check

Verify calculations programmatically before capturing screenshots:

typescript

// Calculate position
const centerX = (screenWidth - text.width) / 2;
text.x = centerX;

// Verify calculation
const isCentered = Math.abs(text.x - centerX) < 1;
if (!isCentered) {
  console.error('Text not centered');
  // Fix calculation before proceeding
}

// Only capture screenshot after verification
agent-browser screenshot screenshots/verification.png;

Pattern 2: Test Seam Position Verification

Use test seam to verify positions:

bash

# Verify text position
agent-browser eval "
  const scene = window.__TEST__?.getCurrentScene();
  const text = scene?.texts?.score;
  if (text) {
    const screenWidth = scene.cameras.main.width;
    const expectedX = (screenWidth - text.width) / 2;
    Math.abs(text.x - expectedX) < 1 ? 'Centered' : 'Not centered'
  } else {
    'Text not found'
  }
"

Pattern 3: Calculation Validation

Validate calculations before applying:

typescript

// Validate calculation
function validateCenterPosition(text: Phaser.GameObjects.Text, screenWidth: number): boolean {
  const expectedX = (screenWidth - text.width) / 2;
  const actualX = text.x;
  const tolerance = 1;  // 1 pixel tolerance
  
  return Math.abs(actualX - expectedX) < tolerance;
}

// Use validation
if (!validateCenterPosition(text, screenWidth)) {
  console.error('Position calculation incorrect');
  // Recalculate
}

Best Practices

•Calculate text width first before positioning
•Account for screen width (not world width) for UI elements
•Use spacing constants for consistency
•Account for origin when positioning sprites
•Verify calculations programmatically before visual verification
•Use test seam commands for position verification
•Document coordinate system used (world vs screen)
•Test at different screen sizes for responsive layouts

Integration with Other Skills

•phaser-game-testing: Uses coordinate system patterns
•agent-browser: Uses test seam for position verification
•screenshot-handling: Captures screenshots after programmatic verification

Related Skills

•phaser-game-testing - Phaser testing patterns
•agent-browser - Browser automation
•screenshot-handling - Screenshot capture

Remember

•Measure text width before positioning
•Use screen coordinates for UI elements
•Account for origin when positioning sprites
•Verify programmatically before visual verification
•Use test seam for position verification
•Document coordinate system used
•Test responsive layouts at different screen sizes