# Simple Ray Tracer - A physical-based path tracing in C

A high-performance Monte Carlo path tracer written from scratch in pure C, featuring BVH acceleration, multiple importance sampling (MIS), Sobol sequences, and energy-conserving BSDFs.

## ✨ Features
- **Pure C implementation** — zero external dependencies beyond Assimp for mesh loading and cglm for math
- **BVH Acceleration** — SAH-optimized binary tree with per-ray traversal stats
- **Multiple Importance Sampling** — combines BSDF and light sampling using the balance heuristic
- **Next-Event Estimation** — direct lighting with directional lights and sky
- **HDR Environment Lighting** — MIS-enabled sampling of HDR sky using precomputed CDFs.
- **Sobol Quasi-Random Sampling** — fast convergence and low-discrepancy stratification
- **Recursive Path Integration** — with Russian Roulette termination for energy conservation
- **OBJ Support** — via Assimp for mesh import and material parsing (OpenPBR-compatible)

## 📸 Example Render

Sponza rendered with 64 spp:
![preview](readme-assets/preview.png)
Sponza rendered with 64 spp and hdri sky only:
![preview-hdri](readme-assets/preview-hdri.png)

## 🔧 Build Instructions

### Requirements

- C compiler (GCC, Clang, MSVC)
- OpenMP
- CMake

### Build

You can use the provided CMakeLists.txt to build the project. Just run the following commands in the root directory of the project:
```bash
cmake -S . -B build
```
If you have powershell installed, you can run the build.ps1 script in project root directory.

## 🧠 Technical Breakdown

### BVH Acceleration
- Built with Surface Area Heuristic (SAH) to reduce average traversal cost.
- Debug counters log how many BVH nodes each ray touches, for profiling scene structure.

### Multiple Importance Sampling (MIS)
- Balance heuristic weights BSDF and light sampling PDFs.
- Supports both directional lights and sky light via next-event estimation. Produce noise-free renders with low sample counts.

### HDR Sky Lighting
- Supports importance sampling of environment maps using a hierarchical CDF over solid angles.
- Combined with MIS and BSDF sampling for noise-free global illumination, even with strong dynamic ranges.

### Sobol Sampling
- 32D low-discrepancy sequences for camera rays, light sampling, and Russian roulette.
- Faster convergence and lower variance than pure RNG at 64 spp.

### Materials (BSDFs)
- Lambertian, and specular supported.
- Designed to align with the OpenPBR material model.

### Path Tracing
- Recursive, physically-based integration with energy conservation checks.
- Russian Roulette applied adaptively after second bounce.

## 🚧 Roadmap

- [x] AOV support (e.g. normals, albedo, depth)
- [ ] Support for more complex materials (e.g. glass)
- [ ] Standardize light unit
- [ ] General speed improvements
- [ ] Add GPU backend with CUDA
- [ ] Support for glossy microfacet models (GGX)
- [ ] Support for better diffuse models (Oren-Nayar)
- [ ] Support for volumetric scattering (homogeneous media)
- [ ] Light hierarchy for large emitter sets
- [ ] Better multithreaded rendering
- [ ] GUI frontend with real-time control