Using a computer for ray tracing to generate shaded pictures was first attempted by Goldstein and Nagel of MAGI (Mathematics Applications Group, Inc.). Their work cites earlier work for “visual simulation of three-dimensional objects” by Arthur Appel that “employs ‘quantitative invisibility’ to eliminate hidden lines, and further program to produce grayscale output”, so it is not truly “ray tracing”. In Goldsein’s and Nagel’s paper, “3-D Visual Simulation”, ray tracing is used to make shaded pictures of solids by simulating the photographic process in reverse. For each picture element (pixel) in the screen, they cast a light ray through it into the scene to identify the visible surface. The surface intersected by the ray, found by “tracing” along it, was the visible one. At the ray-surface intersection point found, they computed the surface normal and, knowing the position of the light source, computed the brightness of the pixel in the screen. Their publication describes a short (30 second) film “made using the University of Maryland’s display hardware outfitted with a 16mm camera. The film showed the helicopter and a simple ground level gun emplacement. The helicopter was programmed to undergo a series of maneuvers including turns, take-offs, and landings, etc., until it eventually is shot down and crashed.” A CDC 6600 computer was used.