Ray tracing is a method to produce realistic images; it determines visible surfaces in an image at the pixel level (Appel, 1968; Kay & Greenberg, 1979; Whitted, 1980). Unlike the z-buffer and BSP tree, ray tracing operates pixel-by-pixel rather than primitive-by-primitive. This tends to make ray tracing relatively slow for scenes with large objects in screen space. However, it has a variety of nice features which often make it the right choice for batch rendering and even for some interactive applications.