Sphere Tracing is a rendering method used for visualizing 3D scenes without explicitly storing geometry as polygons. It is based on Signed Distance Functions (SDF) and is a variant of Ray Marching, but with an optimized stepping approach that allows for more precise and efficient intersection detection with objects.
Core Principle
Instead of advancing the ray uniformly, as in classical Ray Marching, Sphere Tracing uses distance information to the nearest surface to determine the optimal step size. This significantly reduces the number of iterations and minimizes the likelihood of rendering artifacts.
The algorithm works as follows:
- Ray Generation: A ray is cast from the camera, with its direction computed based on pixel coordinates.
- Tracing Steps: The ray moves forward, not with fixed steps, but with steps equal to the value of the SDF. This ensures that the ray never overshoots an object.
- Hit Detection: If the SDF value becomes smaller than a given threshold (e.g., 0.001), the ray is considered to have reached the surface.
- Iteration Limit: If the number of steps exceeds a set limit, the pixel is treated as background.
- Lighting and Shadows Calculation: After detecting an intersection, normals, shadows, and other effects are computed.
Advantages of Sphere Tracing
- More efficient rendering – Compared to traditional Ray Marching, it requires fewer iterations since it adapts step sizes based on distance information.
- Guaranteed no surface overshooting – The ray cannot pass through an object, eliminating rendering errors.
- Good compatibility with procedural graphics – Enables the visualization of complex shapes without storing polygonal meshes.
Limitations
- Not suitable for highly detailed scenes – In areas where SDF gradients change sharply, accuracy issues may arise.
- Requires predefined SDF descriptions – Unlike classical Ray Tracing, which can work with any surface, Sphere Tracing relies on predefined Signed Distance Functions.
Conclusion
Sphere Tracing is a powerful method for rendering SDF-based scenes, significantly improving the efficiency of Ray Marching. It is widely used in procedural graphics, the demoscene, and generative art, providing fast and accurate visualization of complex forms.
