Have you ever considered how computer graphics achieve such realistic lighting effects? This article demystifies ray tracing techniques for optical simulation.
Ray tracing is a fundamental technique for reproducing the propagation and interaction of light in a scene. While the concept itself is well established, the details and related notions can be quite complex. As a starting point for further exploration, we provide below a brief overview of the main ray tracing techniques.
Ray Tracing
In ray tracing, we simulate light by following rays as they travel through a scene. These rays shoot out from a virtual camera, mimicking how light behaves in the real world. When a ray hits an object, the program determines how the light interacts with the surface material. Light can bounce (reflect), bend (refract), or be absorbed depending on the material properties. Reflected rays continue their journey, potentially creating realistic shadows and reflections. Refracted rays bend as they pass through transparent materials.
By tracking these light paths and considering their intensity after each interaction, ray tracing creates a highly realistic image with accurate lighting effects.
Ray Casting
Ray casting is a simpler technique used to understand object visibility in a scene. It works by sending rays outward from a viewpoint, typically the camera. The program then calculates the closest object each ray hits. This essentially creates a depth map, indicating which objects are in front of others. Ray casting is useful for basic visibility checks but doesn’t account for complex light interactions like reflections or refractions. This means it cannot simulate realistic lighting effects.
While not ideal for photorealistic images, ray casting can be computationally efficient for determining object visibility in simpler scenarios.
Path Tracing
Path tracing builds upon ray tracing to achieve even more realistic images. It simulates the complete journey of light particles, not just single rays. A light source is emitting countless photons. Path tracing tracks these simulated photons as they bounce around the scene, reflecting, refracting, and potentially getting absorbed by objects. The final image is built by accumulating the contribution of these simulated light paths at each pixel.
This approach allows path tracing to capture complex lighting phenomena like caustics, and indirect illumination (light bouncing off multiple surfaces). However, path tracing can be computationally expensive due to the high number of light paths simulated.
Bidirectional Path Tracing (BDPT)
Bidirectional Path Tracing (BDPT) improves upon path tracing by tracing light paths in two directions. In traditional path tracing, light paths start from the light source and travel towards the camera. BDPT adds light paths that originate from the camera and travel outwards. This allows for more efficient intersection calculations, especially in scenes with complex lighting.
By considering both directions, BDPT can achieve better results with fewer samples compared to traditional path tracing. However its implementation is more complex.
Read our article “Pathtracing vs Raytracing: what is the difference for accurate rendering” to learn more about how Ocean™ is using BDPT to generate highly accurate virtual prototypes.
Metropolis Bidirectional Path Tracing (MBPT)
Metropolis Path Tracing is a process that is used in Ocean™ for the optimization of complex environments.
Metropolis Bidirectional Path Tracing (MBPT) is a further refinement of BDPT. It utilizes a specific type of random sampling technique to dynamically adjust how it samples light paths. It focuses on statistically important paths that contribute more significantly to the final image. MBPT prioritizes paths that are more likely to lead to the “exit” (the camera) and avoids getting stuck in dead ends (paths with low light contribution).
This approach improves the efficiency of BDPT even further, particularly in challenging lighting scenarios with complex paths. In comparison to the standard BDPT, the MBPT requires even more computing capacity for implementation.
Path Tracing Techniques for Leveraging Innovation with Ocean™
Bidirectional Path Tracing and Metropolis Bidirectional Path Tracing are two techniques implemented in Ocean™ that enable physically accurate rendering.
This advanced ability to accurately reproduce the way light interacts within a scene, combined with optical measurements of materials, enables Ocean™ to generate predictive images and data that can be used for a wide range of applications where accurate appearance is decisive.
Explore the differences between path and ray tracing and how BDPT and MBPT are used in Ocean™ to create physically true simulations, in our dedicated article.
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