The atomic unit of computer graphics is the pixel (a contraction of picture element). Low-resolution displays, often found in personal computers, have resolutions of 640 X 480 pixels. This resolution is sufficient for most NTSC video work. However, motion picture work requires higher resolution displays with resolutions of 1280 pixels X 1024 lines or greater. Upcoming high-definition television systems will have displays approaching 2,000 horizontal pixels by 1,000 vertical lines.
The computer calculates the color for each pixel and displays it by varying the intensity of the Red, Green and Blue (RGB) signal. To represent color as perceived by the human eye, each pixel must span a range of 16 million to 68 billion colors (256 to 4,096 intensity values per R, G, B component). Internally the computer stores the RGB values in memory, with between 8 and 12 bits representing each R, G and B value. Each pixel, therefore, requires 24 to 36 bits of storage. Even for the low resolution of NTSC video, the computer must calculate and then store over 1 megabyte of data for each frame. A single Academy-aperture 35mm color negative frame, at the theatrical screening resolution of 4,096 pixels x 3,072 lines, requires aroimd 56 megabytes of storage. A 65mm 5-perf motion-picture image requires a screen resolution of 6,000 X 2,500 pixels or higher. With 12 bits per R, G and B value, a frame would require 67.5 megabytes of memory, i.e., 6,000 pixels X 2,500 lines X 3 colors (RGB) X 1.5 bytes (1 byte = 8 bits). The computer must calculate this data then move it from its internal memory to the display memory of the film recorder.
The film recorder displays the data on a cathode-ray tube (CRT) or writes directly to the raw camera stock with a scanning RGB laser. This means that in order to make computer graphics economical, you must not only have an extremely fast computer, but you must also have high bandwidth pathways (called channels) between storage devices, the computer and the film recorder. For comparison, personal computers with 2,400 baud modems transfer data at 240 bytes per second. A high-performance CRT-based film recorder, in order to record a single 35mm frame in approximately six seconds, needs the channels to transfer 56 megabytes of data at 10,000,000 bytes (10 megabytes) per second.
Currently, no computer can create computer graphic frames at film resolution in real time. Often a frame may take from several seconds to many hours to compute and record. Whole scenes often take days to weeks of computer time. Because of these factors, computer graphics can be expensive, but the virtues of computer imagery often outweigh the costs.
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