bp@pixar.UUCP (Bruce Perens) (08/29/89)
About the "expensive magic", the holographic images American Bank Note produced for us are holograms of a two-dimensional movie, not of three-dimensional objects. In other words, they're cheating. Here's how it's done: A movie is prepared of an object, with the point of view rotating around the vertical axis of the object at a constant rate per frame. The hologram plate is exposed with a frame of the movie, in such a way that the frame will be visible only when the eye is in in a horizontally narrow region in front of the plate. The plate is rotated around its veritcal axis as subsequent frames of the movie are exposed, so that multiple images are stored on the plate, each with their own particular viewing angle. You can "play" all of the frames of the movie in sequence by rotating the plate around its vertical axis, or moving your head from side to side. The stereoscopic effect is that of a simple two-frame stereogram, since each eye sees a different frame of the movie. One can take advantage of this "movie" effect to put live action in the hologram, as in "The Kiss", an old hologram in which a woman is seen to blow a kiss and wink at the viewer as the viewer's eyes move around the image. You can use this for animation as well, as was the case in the Tin-Toy hologram. In one of the Tin-Toy holograms, you can see the baby's eyes blink seperately - first in your right eye, then in your left - this scheme doesn't work well for rapid action. You won't see any vertical perspective in these holograms. If you use brute-force ray tracing methods to calculate where the fringes belong on a holographic plate, rather than using the analog method detailed above, you can show both vertical and horizontal perspective as in a normal hologram, though many fringe-calculated holograms cheat by throwing away the vertical perspective to save a few CPU-days.