dave@onfcanim.UUCP (Dave Martindale) (02/02/89)
In article <18070@glacier.STANFORD.EDU> jbn@glacier.UUCP (John B. Nagle) writes: > > Showscan is a straightforward movie system using 70mm film at about 60 >frames per second. At this speed, the illusion of motion achieved at >24 frames per second is much improved. The screen is also made sufficiently >large to cover the entire human field of view. The overall effect is said >to approximate reality. It can be considered a benchmark as to how good >a display system has to be before it disappears and becomes a virtual reality. Time to mention the *other* high-definition film system - IMAX/OMNIMAX: IMAX is a system that uses 15-perforation frames running horizontally - each frame uses 3 times the film area as Showscan. IMAX runs at a conventional 24 frames per second. OMNIMAX use the same film, camera, and projectors as IMAX, except that the camera uses a sort of fisheye lens to obtain a 180 degree field of view, and the image is projected onto the inner surface of a dome. Showscan and IMAX consume just about the same amount of film per second _ Showscan runs 2.5 times as fast as normal 70mm but the frame is the same size, while IMAX uses 3 times the film area at the normal frame rate. So the "data rate" is the same, but the two systems use it differently. The Showscan screen may be quite *wide*, but it's not very high. The frame size is the same as normal theatrical 70mm, 2.072 x 0.906, 5 perforations per frame. IMAX frames are 2.032 x 2.772 inches. (Both figures are camera apertures; I'll ignore projector apertures for the sake of simplicity). If we pick an arbitrary screen width of 60 feet, a Showscan screen will be only 26 feet high while an IMAX screen is 44 feet high. Thus, IMAX does a much better job of "covering the human field of view" vertically. Showscan does have the advantage of a brighter image on screen, mostly because it is covering less screen. When covering the same screen width, a Showscan frame is being magnified 34% more than an IMAX one, so each IMAX image is sharper. On the other hand, projecting more images per second increases apparent sharpness, so Showscan probably doesn't lose anything here. Showscan uses standard 70mm equipment, modified a bit. IMAX uses custom-built cameras with 4 registration pins and a vacuum pressure plate to hold the film flat. The projectors use fixed registration pins and hold the film flat against a glass block using compressed air. As a result, IMAX has very little image jitter on screen compared with any other film format. Note that this is not a fundamental difference between the two formats, just a practical one. So what's the net result? For rapid motion, either of the camera or the subject, Showscan looks great while IMAX images flicker and strobe. But when the camera and scene are stationary or moving slowly, IMAX gives a "window into the world" quality that Showscan can't, because of the narrower field and more jitter. (If you get a chance, see the footage of the earth shot from space in "Hail Columbia" - you'll see what I mean.) Which system is better depends on the subject matter. By the way, to compare this to computer displays, a good IMAX projection print will have 40 lp/mm resolution. That's 80 pixels/mm, 2032 pixels/inch, for an equivalent resolution of 5600 x 4130 pixels. What sort of computer display hardware would be needed to generate that at 24 fps? As someone else pointed out, flight simulator people are working on generating high resolution only where the pilot is currently looking, with quite low resolution elsewhere, to minimize computing. However, you can't do that with an audience of more than one. About 3D: IMAX and Omnimax are particularly good systems for 3-D. Vertical misalignment between the two images of a stereo pair gives people headaches, and vertical jitter produces this sort of misalignment. IMAX has very low jitter, and any jitter that remains is mostly horizontal. Omnimax has the potential of really surrounding you with a 3-D environment. 3D films have been made in both IMAX and Omnimax. If there is interest, I can re-post an article on IMAX 3D that I originally posted about 3 years ago. Dave Martindale
twgee@rose.waterloo.edu (Thomas Gee) (02/03/89)
When I was down in Texas this summer I saw an IMAX movie concerning the early exploration of the Grand Canyon. Although the show itself was fascinating, and the material gave the producers a great excuse for many breathtaking helicopter and ultralight based tours, I found that the pictures were badly distorted at the edges of the screen. The focal width and length of the IMAX method did not seem very large. Outside of the most central part of the picture, the images were blurry, as they were at the edges of the screen. Worse, the images at the very edges of the view were badly curved, somewhat similar to the effect of a fisheye lens. The scenes filmed from a helicopter flying over Dallas (part of the IMAX introduction) emphasized this effect by showing warped and curved buildings and streets. All in all, however, it was thoroughly enjoyable. 'Twould be nice to have such things here in the Great White North. Just think of what the hockey games would look like! :-) -------------------------------------------------------------------------------- The Proctor was uttered by the alleyway like an | Thomas Gee untranslateable word. | CS Student, UW '90. -- James Blish, "Cities in Flight" | watmath!rose!twgee