# Agent Instructions

## Overview

Scry is a Magic: The Gathering card scanner app built with .NET MAUI. It uses perceptual hashing to match photographed cards against a database of known card images from Scryfall.

**Key components:**
- Mobile scanning app (MAUI/Android)
- Card recognition via perceptual hashing (not OCR)
- SQLite database with pre-computed hashes
- Scryfall API integration for card data

## Build Commands

Use `just` commands (defined in `.justfile`):

| Task | Command | Notes |
|------|---------|-------|
| Build project | `just build` | Builds Android debug |
| Run tests | `just test` | Runs xUnit tests |
| Generate card database | `just gen-db` | Downloads from Scryfall, computes hashes |
| Publish app | `just publish` | Creates release APK |
| Hot reload dev | `just dev` | Uses `dotnet watch` |
| Start emulator | `just emu` | Virtual camera with Serra Angel |
| Install to device | `just install` | Installs release APK |

### Database Generator Options

```bash
just gen-db                          # Default: 500 cards with test images
dotnet run --project tools/DbGenerator -- -c 1000  # More cards
dotnet run --project tools/DbGenerator -- --force  # Rebuild from scratch
dotnet run --project tools/DbGenerator -- --no-test-cards  # Skip priority test cards
```

## Project Structure

```
src/
├── Scry.App/           # MAUI mobile app (Android target)
│   ├── Views/          # XAML pages (ScanPage, CollectionPage, etc.)
│   ├── ViewModels/     # MVVM ViewModels using CommunityToolkit.Mvvm
│   ├── Services/       # App-layer services (ICardRecognitionService, ICardRepository)
│   ├── Converters/     # XAML value converters
│   ├── Models/         # App-specific models (CollectionEntry)
│   └── Resources/Raw/  # Bundled card_hashes.db
│
└── Scry.Core/          # Platform-independent core library
    ├── Recognition/    # CardRecognitionService, RecognitionOptions
    ├── Imaging/        # PerceptualHash, ImagePreprocessor, CardDetector
    ├── Data/           # CardDatabase (SQLite)
    ├── Models/         # Card, Oracle, Set, ScanResult
    └── Scryfall/       # ScryfallClient for API/bulk data

test/
└── Scry.Tests/         # xUnit tests

tools/
└── DbGenerator/        # CLI tool to generate card_hashes.db

TestImages/             # Test images organized by category
├── reference_alpha/    # Alpha/Beta cards for testing
├── single_cards/       # Individual card photos
├── varying_quality/    # Different lighting/quality
├── hands/              # Cards held in hand
├── foil/               # Foil cards with glare
└── ...                 # More categories
```

## Architecture

### Recognition Pipeline

```
Camera Image
     │
     ▼
┌─────────────────────┐
│ CardDetector        │  ← Edge detection, find card quad
│ (optional)          │
└─────────────────────┘
     │
     ▼
┌─────────────────────┐
│ PerspectiveCorrection│  ← Warp to rectangle
└─────────────────────┘
     │
     ▼
┌─────────────────────┐
│ ImagePreprocessor   │  ← CLAHE for lighting normalization
│ (ApplyClahe)        │
└─────────────────────┘
     │
     ▼
┌─────────────────────┐
│ PerceptualHash      │  ← Compute 192-bit color hash (24 bytes)
│ (ComputeColorHash)  │
└─────────────────────┘
     │
     ▼
┌─────────────────────┐
│ CardRecognitionService│ ← Hamming distance match against DB
└─────────────────────┘
```

### Data Model

Three-table schema mirroring Scryfall:

- **oracles** - Abstract game cards (one per unique card name)
- **sets** - MTG sets with metadata
- **cards** - Printings with perceptual hashes (one per unique artwork)

The `Card` model includes denormalized Oracle fields for convenience.

### Key Classes

| Class | Purpose |
|-------|---------|
| `CardRecognitionService` | Main recognition logic, caches DB, handles rotation matching |
| `PerceptualHash` | DCT-based color hash (192-bit = 8 bytes × 3 RGB channels) |
| `ImagePreprocessor` | CLAHE, resize, grayscale conversions |
| `CardDetector` | Edge detection + contour analysis to find card boundaries |
| `PerspectiveCorrection` | Warp detected quad to rectangle |
| `CardDatabase` | SQLite wrapper with batch insert, queries |
| `ScryfallClient` | Bulk data streaming, image downloads |

## Code Conventions

### General

- **Target**: .NET 10.0 (net10.0-android for app, net10.0 for Core/tools)
- **Nullable**: Enabled everywhere (`<Nullable>enable</Nullable>`)
- **Warnings as errors**: `<TreatWarningsAsErrors>true</TreatWarningsAsErrors>`
- **Central package management**: Versions in `Directory.Packages.props`

### C# Style

- Records for data models (`record Card`, `record ScanResult`)
- `required` properties for non-nullable required fields
- Extension methods for conversions (`ScryfallCard.ToCard()`)
- Static classes for pure functions (`PerceptualHash`, `ImagePreprocessor`)
- `using` declarations (not `using` blocks) for disposables
- File-scoped namespaces
- Primary constructors where appropriate
- `CancellationToken` parameter on all async methods

### MVVM (App layer)

- `CommunityToolkit.Mvvm` for source generators
- `[ObservableProperty]` attributes for bindable properties
- `[RelayCommand]` for commands
- ViewModels in `Scry.ViewModels` namespace

### Naming

- Services: `ICardRecognitionService`, `CardRecognitionService`
- Database methods: `GetCardsWithHashAsync`, `InsertCardBatchAsync`
- Hash methods: `ComputeColorHash`, `HammingDistance`
- Test methods: `RecognizeAsync_ExactMatch_ReturnsSuccess`

## Testing

Tests are in `test/Scry.Tests` using xUnit.

```bash
just test              # Run all tests
dotnet test --filter "FullyQualifiedName~PerceptualHash"  # Filter by name
```

### Test Categories

| Test Class | Tests |
|------------|-------|
| `PerceptualHashTests` | Hash computation, Hamming distance |
| `CardRecognitionTests` | End-to-end recognition |
| `CardDatabaseTests` | SQLite CRUD operations |
| `ImagePreprocessorTests` | CLAHE, resize |
| `RobustnessAnalysisTests` | Multiple image variations |

### Test Images

TestImages directory contains categorized reference images:
- `reference_alpha/` - Alpha/Beta cards (matching DbGenerator priority cards)
- `single_cards/` - Clean single card photos
- `varying_quality/` - Different lighting/blur conditions

## Key Algorithms

### Perceptual Hash (pHash)

Color-aware 192-bit hash:
1. Resize to 32×32
2. For each RGB channel:
   - Compute 2D DCT
   - Extract 8×8 low-frequency coefficients (skip DC)
   - Compare each to median → 63 bits per channel
3. Concatenate R, G, B hashes → 24 bytes (192 bits)

Matching uses Hamming distance with threshold ≤25 bits and minimum confidence 85%.

### CLAHE (Contrast Limited Adaptive Histogram Equalization)

Applied in LAB color space to L channel only:
- Tile-based histogram equalization (8×8 tiles)
- Clip limit prevents over-amplification of noise
- Bilinear interpolation between tiles for smooth output

### Card Detection

Pure SkiaSharp implementation:
1. Grayscale → Gaussian blur → Canny edge detection
2. Contour tracing via flood fill
3. Douglas-Peucker simplification → Convex hull
4. Find best quadrilateral matching MTG aspect ratio (88/63 ≈ 1.397)
5. Order corners: top-left, top-right, bottom-right, bottom-left

## Debug Mode

Set `RecognitionOptions.DebugOutputDirectory` to save pipeline stages:
- `01_input.png` - Original image
- `02_detection.png` - Card detection visualization
- `03_perspective_corrected.png` - Warped card
- `05_clahe_*.png` - After CLAHE preprocessing

On Android: `/sdcard/Download/scry-debug` (pull with `adb pull`)

## Dependencies

### Core Library (Scry.Core)

- **SkiaSharp** - Image processing, DCT, edge detection
- **Microsoft.Data.Sqlite** - SQLite database
- **Microsoft.Extensions.Options** - Options pattern

### App (Scry.App)

- **CommunityToolkit.Maui** - MAUI extensions
- **CommunityToolkit.Maui.Camera** - Camera integration
- **CommunityToolkit.Mvvm** - MVVM source generators

### DbGenerator Tool

- **Spectre.Console** / **Spectre.Console.Cli** - Rich terminal UI

## Common Tasks

### Adding a New Card to Priority Test Set

1. Add image to `TestImages/reference_alpha/` or appropriate folder
2. Add entry to `GenerateCommand.PriorityCardsWithSets` dictionary
3. Run `just gen-db` to regenerate database

### Debugging Recognition Failures

1. Enable debug output in `MauiProgram.cs`:
   ```csharp
   options.DebugOutputDirectory = "/sdcard/Download/scry-debug";
   ```
2. Run recognition
3. Pull debug images: `adb pull /sdcard/Download/scry-debug`
4. Compare `05_clahe_*.png` with reference images in database

### Modifying Hash Algorithm

1. Update `PerceptualHash.ComputeColorHash()`
2. Update `CardRecognitionService.ColorHashBits` constant
3. Regenerate database: `just gen-db --force`
4. Run tests: `just test`

## Gotchas

1. **Hash size is 24 bytes (192 bits)** - 3 RGB channels × 8 bytes each
2. **Confidence threshold is 85%** - Configurable in `CardRecognitionService.MinConfidence`
3. **Card detection is optional** - Controlled by `RecognitionOptions.EnableCardDetection`
4. **Rotation matching tries 4 orientations** - Controlled by `RecognitionOptions.EnableRotationMatching`
5. **Database is bundled in APK** - Copied on first run to app data directory
6. **Multi-face cards** - Only front face image is used for hashing
7. **Rate limiting** - DbGenerator uses 50ms delay between Scryfall image downloads

## CI/CD

Forgejo Actions workflow (`.forgejo/workflows/release.yml`):
- Builds for win-x64, linux-x64, osx-x64
- Creates "standard" and "embedded" (with APK) variants
- Publishes to Forgejo releases

## External Resources

- [Scryfall API](https://scryfall.com/docs/api) - Card data source
- [CARD_RECOGNITION.md](docs/CARD_RECOGNITION.md) - Detailed architecture doc