Modularize photon mapping system for better separation of concerns and testability

[Copilot]

Problem

The original photon mapping implementation suffered from several architectural issues:

• Tight coupling: Photon mapping code was tightly integrated into the Scene class, violating single responsibility principle
• Mixed responsibilities: Scene class handled both scene management AND photon mapping operations
• Poor testability: Photon mapping functionality was embedded in Scene, making it difficult to test independently
• No clear abstraction: Photon generation, storage, and querying were all mixed together without clear interfaces

Solution

This PR introduces a modular photon mapping system that separates concerns into three distinct components:

1. PhotonMap (include/photon/photon_map.hpp)

• Responsibility: Photon storage and spatial queries
• Stores photons using KD-tree for efficient spatial queries
• Provides nearest neighbor queries with radius constraints
• Thread-safe and memory efficient

2. PhotonMapper (include/photon/photon_mapper.hpp)

• Responsibility: Photon generation and ray tracing
• Generates photons from light sources in a scene
• Traces photons through the scene using Monte Carlo methods
• Separates regular photons from caustic photons

3. PhotonMappingRenderer (include/photon/photon_mapping_renderer.hpp)

• Responsibility: Radiance estimation using photon maps
• Performs photon density estimation for diffuse surfaces
• Handles specular and transmission bounces
• Integrates with existing kernel system

Key Changes

• New directory structure: Created include/photon/ and src/photon/ for modular components
• Backward compatibility: Existing PhotonMappingStrategy automatically uses the new system
• Improved API: Clean, focused interfaces for each component
• Better testing: Added comprehensive tests for each module
• Enhanced documentation: Detailed documentation with usage examples

Usage Example

// Create photon mapper
auto photonMapper = std::make_shared<photon::PhotonMapper>();

// Generate photon maps
photonMapper->generatePhotonMaps(scene, 10000, 5);

// Create renderer
auto renderer = std::make_shared<photon::PhotonMappingRenderer>();
renderer->setPhotonMapper(photonMapper);

// Use in rendering
RGB color = renderer->renderPixel(viewDirection, intersection, scene, config, kernel, maxBounces);

Benefits

1. Separation of concerns: Each class has a single, well-defined responsibility
2. Testability: Each component can be tested independently
3. Reusability: PhotonMapper can be used with different rendering strategies
4. Maintainability: Easier to understand and modify photon mapping code
5. Extensibility: Easy to add new photon mapping algorithms or optimizations

Testing

• Added test/test_photon_mapping.cpp for unit testing photon mapping components
• Added test/test_photon_render.cpp for integration testing with actual rendering
• All existing tests continue to pass, ensuring no regression

Files Changed

• New modules: include/photon/, src/photon/ with modular photon mapping implementation
• Updated Scene: Added interface methods for the new photon mapper
• Updated PhotonMappingStrategy: Now uses the new modular system
• Updated Makefile: Added support for photon module compilation
• Documentation: Added comprehensive documentation in docs/photon_mapping_modularization.md

The modularization makes the photon mapping system much more maintainable and testable while preserving all existing functionality and performance characteristics.

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