one farther down the tree. The time spent constructing the hierarchy should be much less than the time saved by using it. Interactive ray tracing The first implementation of an interactive ray tracer was the LINKS-1 Computer Graphics System built in 1982 at Osaka University's School of Engineering, by professors Ohmura Kouichi, Shirakawa Isao and Kawata Toru with 50 students. It was a massively parallel processing computer system with 514 microprocessors (257 Zilog Z8001s and 257 iAPX 86s), used for rendering realistic 3D computer graphics with high-speed ray tracing. According to the Information Processing Society of Japan: "The core of 3D image rendering is calculating the luminance of each pixel making up a rendered surface from the given viewpoint, light source, and object position. The LINKS-1 system was developed to realize an image rendering methodology in which each pixel could be parallel processed independently using ray tracing. By developing a new software methodology specifically for high-speed image rendering, LINKS-1 was able to rapidly render highly realistic images." It was used to create an early 3D planetarium-like video of the heavens made completely with computer graphics. The video was presented at the Fujitsu pavilion at the 1985 International Exposition in Tsukuba." It was the second system to do so after the Evans & Sutherland Digistar in 1982. The LINKS-1 was reported to be the world's most powerful computer in 1984. The earliest public record of "real-time" ray tracing with interactive rendering (i.e., updates greater than a frame per second) was credited at the 2005 SIGGRAPH computer graphics conference as being the REMRT/RT tools developed in 1986 by Mike Muuss for the BRL-CAD solid modeling system. Initially published in 1987 at USENIX, the BRL-CAD ray tracer was an early implementation of a parallel network distributed ray tracing system that achieved several frames per second in rendering performance. This performance was attained by means of the highly optimized yet platform independent LIBRT ray tracing engine in BRL-CAD and by using solid implicit CSG geometry on several shared memory parallel machines over a commodity network. BRL-CAD's ray tracer, including the REMRT/RT tools, continue to be available and developed today as open source software. Since then, there have been considerable efforts and research towards implementing ray tracing at real-time speeds for a variety of purposes on stand-alone desktop configurations. These purposes include interactive 3D graphics applications such as demoscene productions, computer and video games, and