Real-Time Ray Tracing and Path Tracing in Three.js: A Guide to Photorealistic Rendering

Photorealistic rendering has long been a benchmark for high-quality graphics in 3D applications. Ray tracing and path tracing are at the forefront of this revolution, allowing developers to simulate realistic lighting, reflections, and refractions. With advances in WebGPU and GPU acceleration, Three.js now has the potential to achieve real-time ray tracing and path tracing in web-based applications. This article explores the steps to implement these techniques and optimize them for real-time performance.

1. Setting Up Ray Tracing with GPU Acceleration

Modern web technologies like WebGPU and GPU-based libraries (e.g., RTX-compatible GPUs) make it feasible to achieve ray tracing in real-time within a browser.

• WebGPU Integration: WebGPU is a cutting-edge API that provides lower-level access to the GPU compared to WebGL. It allows for advanced computations, enabling efficient ray tracing.
• Use libraries like three-gpu-pathtracer or integrate custom shaders for ray-tracing calculations.
• Ensure that WebGPU is enabled in your browser. (Currently supported in Chrome Canary and specific builds of Firefox.)
• Example setup:

const renderer = new WebGPURenderer();
renderer.setSize(window.innerWidth, window.innerHeight);
document.body.appendChild(renderer.domElement);

• Hardware Acceleration: Leverage RTX GPUs for ray tracing. Use Three.js extensions or third-party libraries like NVIDIA’s OptiX for better GPU acceleration.

2. Integrating Progressive Path Tracing Algorithms

Path tracing simulates the physical behavior of light, offering a more realistic approach to rendering than traditional ray tracing.

• Path Tracing Basics:
• Unlike ray tracing, which traces individual rays, path tracing sends multiple rays per pixel and averages the results. This accounts for indirect lighting and global illumination.
• Use libraries such as three-mesh-bvh (bounding volume hierarchy) to speed up path tracing by optimizing ray-scene intersection tests.
• Implementing Path Tracing in Three.js:
• Set up a fragment shader for path tracing logic:

// Example GLSL fragment shader
void main() {
  vec3 color = vec3(0.0);
  for (int i = 0; i < MAX_RAYS; i++) {
    color += traceRay(rayOrigin, rayDirection);
  }
  gl_FragColor = vec4(color / float(MAX_RAYS), 1.0);
}

• Iterate over rays and compute lighting contributions for each bounce.

3. Handling Complex Materials

For photorealistic results, materials must mimic real-world properties, including light scattering and spectral effects.

• Subsurface Scattering:
• Simulate effects like translucent skin or marble by scattering light inside materials. Use volumetric shaders or integrate a subsurface scattering BRDF (Bidirectional Reflectance Distribution Function).
• Example:

material.onBeforeCompile = (shader) => {
  shader.fragmentShader = shader.fragmentShader.replace(
    '/* insert subsurface logic here */',
    'color = subsurfaceScattering(color, normal, light);'
  );
};

• Spectral Rendering:
• Handle the dispersion of light into its component wavelengths for effects like rainbows or diamond-like materials.
• Integrate wavelength-based calculations in shaders to produce spectral highlights.

4. Performance Optimization for Real-Time Visuals

Real-time ray tracing and path tracing are computationally expensive. Optimizing your Three.js implementation is crucial for achieving smooth performance.

• Dynamic Sampling:
• Adjust the number of rays per pixel based on scene complexity and distance from the camera.
• Use fewer rays for distant objects and more for areas of high importance.
• Temporal Denoising:
• Accumulate samples over time and apply denoising filters to smooth out noise caused by insufficient ray samples.
• Libraries like Open Image Denoise can be integrated for post-processing.
• Adaptive Resolution:
• Dynamically lower the rendering resolution for complex scenes and upscale using techniques like DLSS (Deep Learning Super Sampling).
• Bounding Volume Hierarchies (BVH):
• Use BVH to accelerate ray-scene intersection tests by organizing scene geometry into hierarchical structures.
• Example with three-mesh-bvh:

const bvh = new MeshBVH(geometry);
scene.traverse((object) => {
  if (object.isMesh) object.geometry.boundsTree = bvh;
});

Conclusion

Real-time ray tracing and path tracing represent the pinnacle of rendering technology and integrating them into Three.js brings a whole new level of realism to web-based graphics. By leveraging GPU acceleration, implementing advanced algorithms, and optimizing performance, developers can push the boundaries of what’s possible in the browser.

This exploration into ray tracing and path tracing in Three.js is just the beginning. With the continuous evolution of WebGPU and graphics hardware, the future of photorealistic rendering on the web is incredibly bright.