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Chapter 4 : A HISTORY OF COMPUTER ANIMATION 3/20/92 2 4 computer," the Whitney films, like John's Catalogue (1961) and James's, Lapis (1963) represent a center position between the older abstract animators like Fishinger (sp?), and the still-to-emerge computerization (fig . 52) . Another pioneer from this analog yet automated era is Doug Trumbull, largely responsible for introducing, in the movie 2001 (1968), a time lapse exposure technique called slit-scan in which light is smeared onto the individual frames of film (fig . 53) . 52 . Analog computer animation . This image is made by successive rotation and multiple exposure of a single template, successive images are formed by local rotations and translations to the template itself . Color is via table lookup . From a descriptive point of view it is immaterial if the imaging aparatus is analogue with physical templates or digital with virtual templates . 53 . Slit scan technique makes use of narrow line (the slit) which travels across the artwork or the recording medium (the scan) while the exposure is made . Physically the slit is actually a narrow opening cut into a piece of very thin metal . The regular camera shutter remains open while the slit is in motion, although it is closed while the film is advanced . In effect the slit functions like a focal plane shutter . If the camera and artwork (or prop) is held steady during the exposure scan then the newly recorded image looks normal and undistorted . But if the camera and/or the artwork is moving during the scan, something magical occures . For example, if the slit is a horizontal line progressing from the bottom of the screen to the top (A1), and the camera moves closer to the image while this is happening, then the resultanting image appears in tilted perspective, closer to the viewer at the top of the screen (A2) . The direction arrows superimposed throughout this figure indicate the direction and travel of the slit (in A1 this is the up arrow) and the effect of the camera zooming on the artwork (the two arrows from the larger rectangle to the smaller one) during the exposure of a single frame . The generation of successive frames requires changing a parameter on a frame by basis . For example, if the start position of the artwork is translated vertically (B1), while repeating the identical slit scan and zoom camera action employed for the first frame, the resultant image will seem to animate along the plane of the tilted perspective (B2) . A program for this might look like : Set initial position of artwork Repeat frames = 1 to 100
Chapter 4 : A HISTORY OF COMPUTER ANIMATION 3/20/92 2 5 Translate artwork up some small constant amount Position slit at bottom of frame Position camera to zoomed out position Open the real camera shutter Repeat slitincrements = 1 to 1000 Translate slit up 1/1000 of frameheight Zoom camera in 1/1000 of zoomdistance End repeat Close the real camera shutter Frame advance End repeat Variation in the slit scan process include changing the thickness of the line (a wider line produces a softer, less sharply focused image) ; the geometry of the line (C) ; the movement of the line (direction and speed) ; even animating the line's geometry on a frame by frame basis . And obviously the camera can pan and rotate as well as zoom . More of these parameters can change on a frame by frame basis too, so the range of effects is quite large . For example, given a live action scene (D1), if the slit starting position and camera zoom distance are held constant (D2), then the result will be alive action scene with (consistant) forced perspective (D3) . Or, given an unchanging grid (E1) and consistant zoom (E2), animation of the slit from a straight line to a circle (E2) will caluse the resultant image to progress from a flat plane in perspective to a tunnel (E3) . In (F1) the image and slit are constant but the amount of camera zoom changes on a frame by frame basis . The result is an image changing from deep perspective to one parallel to the image plane . Do recognize that with most physical systems both the slit and the camera zoom are moving continuously, and that the program example above is a discrete approximation . Assuming the resolution is fine enough such a strategy does work, although it is prone to aliasing . To simulate slit scan techniques entirely in a digital domain the input image must be digitized and the slit though of as a window projected onto a narrow strip of the resultant image . Most of the time a better approach would be to simply determine the mapping from the input image to the result and calculate the result parametrically . Slit scan techniques are slow because the time to move the camera and artwork during each frame is measured in minutes . The computer's role in this automation include not only reducto tedium,
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Chapter 4: A HISTORY OF COMPUTER ANIMATION ... - Vasulka.org

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