Definitions

Motion control

Motion control photography

Motion control photography is a special effects technique used in film that enables precise repetition of camera movements, usually to facilitate special effects photography. The process is generally utilized by filming several elements using the same camera motion, and then compositing the elements into a single image. Other effects are often used along with motion control, such as chroma key to aid the compositing. Today's computer technology allows the programmed camera movement to processed, such as having the move scaled up or down for different sized elements. Common applications of this process include shooting with miniatures, either to composite several miniatures or to composite miniatures with full-scale elements.

The process is also commonly used when duplication of an element which cannot be physically duplicated is required; motion control is the primary method of featuring multiple instances of the same actor in a shot that involves camera movement. For this technique, the camera typically films the exact same motion in the exact same location while the actor performs different parts. A blank take (with no actor in the shot) is sometimes also taken to give compositors a reference of what parts of the shot is different in each take. In today's film, the reference take is also useful for digital manipulation of the shots, or for adding digital elements. A simple duplication shot confines each "copy" of an element to one part of the screen. It is far more difficult to composite the shots when the duplicate elements cross paths, though digital technology has made this easier to achieve. Several basic camera tricks are sometimes utilized with this technique, such as having the hand of a body double enter a shot to interact with the actor while the duplicate's arm is to be off-screen. For the sake of compositing, the background elements of the scene must remain identical between takes, requiring anything movable to be locked down; the blank reference take can aid in resolving any discrepancies between the other shots.

Similar technology in modern film allows for a camera to record its exact motion during a shot so that the motion can be duplicated by a computer in the creation of computer generated elements for the same shot.

History

Modelmaking for scenery has long been used in the film industry, but when a model is too small it often loses its illusion and becomes "obviously a model". Solving this by building a larger model introduces a catch-22: larger models are more difficult to build and often too fragile to move smoothly. The solution is to move the camera, rather than the model, and the advent of compact lightweight 35mm cameras has made machine-controlled motion control feasible. Motion-control also requires control over other photographic elements, such as frame rates, focus, and shutter speeds. By changing the frame rates and the depth of field, models can seem to be much larger than they actually are, and the speed of the camera motion can be increased or decreased accordingly.

Early attempts at motion control came about when John Whitney pioneered several motion techniques using old anti-aircraft analog computers (Kerrison Predictor) connected to servos to control the motion of lights and lit targets. The 1968 film 2001: A Space Odyssey pioneered motion control in two respects. The film's model photography was conducted with large mechanical rigs that enabled precise and repeatable camera and model motion. The film's finale was created with mechanically-controlled slit-scan photography, which required precise camera motion control during the exposure of single frames. The first large-scale application of motion control was in Star Wars Episode IV: A New Hope (1977), where a digitally-controlled camera known as the Dykstraflex performed complex and repeatable motions around stationary spaceship models. This enabled a greater complexity in the spaceship-battle sequences, as separately filmed elements (spaceships, backgrounds, etc.) could be better coordinated with one another with greatly reduced error.

The simultaneous increase in power and affordability of computer-generated imagery in the 21st century, and the ability for CGI specialists to duplicate even hand-held camera motion (see Match moving), has made the use of motion control photography less common.

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