Introduction
Real-time ray tracing and path tracing are rendering techniques that produce highly realistic graphics by simulating the physical behavior of light. Ray tracing tracks the path of light rays as they interact with objects, capturing effects like reflections, refractions, and shadows. Path tracing is a form of ray tracing that samples many light paths per pixel to approximate global illumination effects like diffuse interreflection and caustics.
In recent years, dedicated ray tracing hardware has enabled real-time ray tracing and path tracing in consumer gaming GPUs. This article provides an in-depth guide to understanding these cutting-edge rendering techniques and leveraging them in real-time graphics applications.
Ray Tracing Basics
How Ray Tracing Works
Ray tracing simulates the physics of light by tracing rays from the camera into the 3D scene. Here’s a high-level overview of the ray tracing process:
- Camera rays are traced from the camera position through each pixel.
- When a ray intersects a surface, a shading point is generated.
- The surface properties and shading normals at the intersection point are evaluated.
- Shadow rays may be traced towards light sources to check for blockers.
- Reflection rays and refraction rays may be recursively traced for reflective and transparent surfaces.
- The color contribution of each ray is accumulated to determine the final pixel color.
By modeling the interactions of light in this way, ray tracing can capture photorealistic optical effects like reflections, refractions, shadows, ambient occlusion and global illumination.
Ray Tracing vs. Rasterization
Rasterization is the standard rendering technique used in real-time graphics. It projects and rasterizes 3D geometric primitives to a 2D image plane. Lighting is approximated using techniques like shadow maps, screen space reflections, and light probes.
Ray tracing complements rasterization by providing physically accurate light transport and optical effects. However, rasterization is generally faster for primary visibility and simple shading. Hybrid rendering uses rasterization for the scene geometry and ray tracing for the lighting simulation.
Benefits of Ray Tracing
Key benefits of ray tracing include:
- Photorealistic rendering quality with accurate optical effects.
- A unified simulation framework for modeling light propagation.
- Scales well with additional computing power and scene complexity.
- Natively handles complex effects like reflections, refractions, and shadows.
Limitations of Ray Tracing
Limitations and challenges of ray tracing include:
- Computational complexity makes real-time application challenging.
- Diffuse interreflections require sampling many rays per pixel.
- Acceleration structures are needed to efficiently trace rays.
- Advanced effects like subsurface scattering remain difficult to simulate.
Path Tracing
Overview of Path Tracing
Path tracing is a specific ray tracing technique that focuses on accurate simulation of global illumination effects like diffuse interreflection. It works by randomly sampling many ray paths per pixel and accumulating their contributions:
- Rays are recursively traced through the scene as they reflect, refract, and scatter.
- Each ray path contributes direct and indirect illumination.
- Hundreds of paths per pixel are sampled to converge to a noise-free image.
By modeling diffuse interreflection, path tracing can create realistic soft lighting and indirect shadows. Effects like caustics from light focusing through transparent objects also emerge from the lighting simulation.
Strengths of Path Tracing
Key strengths of path tracing include:
- Realistic global illumination with diffuse interreflection.
- Unified simulation framework handles a variety of light transport.
- Minimal parameter tweaking required for different scenes.
- Naturally extends to advanced effects like volumetrics.
Limitations of Path Tracing
Limitations and downsides of path tracing:
- High sampling requirement results in noise and long render times.
- Materials must be made “raytrace friendly” to avoid noise.
- Acceleration structures are critical to make it feasible.
- Still cannot easily handle some effects like subsurface scattering.
Real-Time Ray Tracing
Dedicated Ray Tracing Hardware
While ray tracing has existed for decades, real-time performance was long constrained by the computational demands.
Dedicated ray tracing hardware like NVIDIA’s RTX GPUs now enable real-time ray tracing by accelerating specific ray tracing workloads:
- Ray tracing cores handle ray-triangle intersection tests.
- RT Cores accelerate bounding volume hierarchy (BVH) traversal.
- Rapid packed math and SFUs handle ray tracing shader work.
This hardware brings the benefits of ray tracing to real-time rendering while keeping rasterization for base performance.
Hybrid Rendering with Rasterization
The key to performance is hybrid rendering combining rasterization with ray tracing:
- Rasterization handles primary visibility and basic shading.
- Ray tracing adds photorealistic optical effects where needed.
- Rasterization first creates a G-Buffer for the scene geometry.
- Rays are traced against the G-Buffer for selective effects.
This maintains good performance while bringing in ray tracing where it has the most impact.
Real-Time Ray Tracing Effects
Some examples of real-time ray tracing effects include:
- Ray traced shadows – accurate hard & soft shadows from area lights.
- Ray traced reflections – glossy planar, spherical and curved reflections.
- Ray traced ambient occlusion – subtle shadowing in corners and crevices.
- Path traced global illumination – diffuse interreflection with ray traced GI.
These selective ray traced effects can be blended with rasterized buffers to balance quality and performance.
Challenges of Real-Time Ray Tracing
Real-time ray tracing introduces optimization challenges including:
- Building efficient acceleration structures for dynamic scenes.
- Reducing ray tracing bandwidth and memory usage.
- Mitigating noise from low path sampling counts.
- Load balancing GPU workloads between ray tracing and rasterization.
Getting Started with Ray Tracing
Prerequisite Knowledge
To begin working with real-time ray tracing, you should have:
- Strong C++ skills and GPU programming experience.
- Understanding of computer graphics concepts like lighting, shading.
- Familiarity with DirectX 12 or Vulkan graphics APIs.
- Experience with HLSL, GLSL, or other shader languages.
Tutorials and Samples
Good resources to learn hands-on are:
- NVIDIA’s RTX ray tracing tutorials – Range from basics to advanced effects.
- Microsoft DXR raytracing samples – From basic to hybrid rendering.
- Khronos ray tracing tutorials – Integrating ray tracing with Vulkan.
Start with simpler tutorials on the foundations before moving to advanced topics like path tracing.
Development Tools
You’ll need:
- DXR for DirectX 12 or VK_KHR_ray_tracing for Vulkan.
- A DXR-capable GPU like NVIDIA’s RTX cards.
- The latest drivers, SDKs, and shader compilers.
- Rendering middleware like NVIDIA’s RTXGI for global illumination.
- Debugging tools like NVIDIA NSIGHT for graphics debugging.
Be prepared to optimize scene representations, shaders, and algorithms to achieve real-time performance.
Porting Rasterized Content
To extend existing rasterized content:
- Identify effects to selectively ray trace like shadows, AO, reflections.
- Audit shaders and content for ray tracing compatibility.
- Build acceleration structures for static and dynamic geometry.
- Implement hybrid rasterized/ray traced rendering.
- Profile, optimize and iterate the ray tracing code.
Balancing rasterization with ray tracing is key for good real-time performance.
Conclusion
Real-time ray tracing opens up new possibilities for interactive photorealism. With the right skills and optimization, ray tracing can take real-time graphics to the next level of immersion and realism. This guide provides foundational knowledge to get started with harnessing real-time ray tracing in your own projects. The future looks bright for this exciting advancing rendering technology!
