Story 3.5: Cook-Torrance Scene Material Integration

[nurusanwe]

Summary

Complete implementation of Story 3.5: Cook-Torrance Scene Material Integration, enabling Cook-Torrance materials in scene files alongside Lambert materials with comprehensive polymorphic architecture.

• ✅ Polymorphic Material System - Clean Material base class with virtual evaluate_brdf() method
• ✅ Extended Scene File Format - Support for both material_lambert and material_cook_torrance syntax
• ✅ Cook-Torrance Showcase Scene - Comprehensive educational demonstration of microfacet theory
• ✅ Robust Error Handling - Graceful handling of malformed material definitions with educational feedback
• ✅ Memory Optimization - Enhanced polymorphic material container memory calculations

Key Architectural Changes

Material Polymorphism

• NEW: src/materials/material_base.hpp - Abstract Material base class with virtual methods
• Enhanced: Lambert and Cook-Torrance materials now inherit from Material base
• Enhanced: Scene class uses std::vector<std::unique_ptr<Material>> for polymorphic storage

Scene File Format Extension

# Lambert materials
material_lambert matte_red 0.8 0.3 0.3

# Cook-Torrance materials  
material_cook_torrance gold_mirror 1.0 0.8 0.3 0.02 1.0 0.04
material_cook_torrance plastic_green 0.2 0.8 0.4 0.5 0.0 0.04

Educational Showcase

• NEW: assets/cook_torrance_showcase.scene - Comprehensive material demonstration
• Demonstrates metallic vs dielectric behavior
• Shows roughness progression effects
• Includes Lambert reference materials for comparison
• Extensive educational comments explaining microfacet theory

Quality Assurance

Testing Coverage

• ✅ Multi-material scene loading with mixed Lambert and Cook-Torrance materials
• ✅ Energy conservation validation across both material types
• ✅ Educational console output preserved for both material types
• ✅ Scene file error handling with malformed material definitions
• ✅ Existing single-sphere Cook-Torrance mode remains functional

Performance & Memory

• Optimized polymorphic material memory calculations
• Linear O(n) scene intersection performance maintained
• Minimal virtual method call overhead for BRDF evaluation
• Enhanced memory usage analysis and reporting

Test Plan

Prerequisites: All existing tests must continue to pass

New Functionality Testing:

1. Load assets/cook_torrance_showcase.scene - should render successfully with mixed materials
2. Test malformed scene files - should provide clear error messages
3. Verify --cook-torrance single-sphere mode still works
4. Run ./build/test_math_correctness - all tests should pass
5. Memory usage analysis should show accurate polymorphic material accounting

Integration Testing:

# Test multi-material scene rendering
./build/raytracer --scene assets/cook_torrance_showcase.scene --resolution 200x200

# Test error handling
echo "material_lambert bad" | ./build/raytracer --scene /dev/stdin

# Test single-sphere mode
./build/raytracer --cook-torrance --resolution 100x100

Code Quality

• Maintainability: Clean polymorphic architecture with proper inheritance
• Educational Value: Comprehensive mathematical explanations and console output
• Error Handling: Robust validation with educational feedback
• Performance: Optimized memory usage and minimal overhead
• Testing: Extensive integration test coverage

This implementation provides a solid foundation for future material system extensions (OpenPBR, MaterialX) while maintaining excellent educational value and code quality.