Introduction - Computer Animation: Algorithms and Techniques - page 21

the consumer. Second, in the film industry it has meant that compositing is no longer optical. Optically
compositing each element in a film meant another pass of the negative through an optical film printer,
which meant additional degradation of image quality. With the advent of digital compositing (see
Appendix A.2 ), the limit on the number of composited elements is removed. Third, once films are rou-
tinely stored digitally, digital techniques can be used for wire removal and to apply special effects.
These digital techniques have become the bread and butter of computer graphics in the film industry.
When one works with digital video, there are several issues that need to be addressed to determine
the cost, speed, storage requirements, and overall quality of the resulting system. Compression tech-
niques can be used to conserve space or reduce transmission time, but some compression compromises
the quality of the image and the speed of compression/decompression may restrict a particular tech-
nique's suitability for a given application. During video capture, any image compression must operate
in real time. Formats used for storage and playback can be encoded off-line, but the decoding must
support real-time playback. Video resolution, video frame rates, and full-color imagery require that
27 MB/sec be supported for video playback. 10 An hour of uncompressed video requires just under
100 GB of storage. 11 While lossless compression (the original data can be exactly reconstructed) is
possible, most video compression is lossy (not all of the original signal is recoverable) because of
the favorable quality/space trade-off. There are several digital video formats used by different manu-
facturers of video equipment for various applications as well as video formats for streaming video and
storage; these formats include D1, D2, D3, D5, miniDV, DVC, Digital8, MPEG-4, digital Betacam,
H.261, and H.263. Better signal quality can be attained with the use of component instead of composite
signals. Discussion of these and other issues related to digital video is beyond the scope of this topic.
Information on some of the more popular formats can be found in Appendix B.10 .
1.4.4 Digital audio
Audio is just as important to computer animation as it is to traditional animation. Over the years, audio
technology, like image technology, has gone digital. Early audio recordings used an electromechanical
stylus to etch representations of the signal into wax drums or plastic platters. Later, the signal was used
to modulate the magnetization of some type of ferromagnetic material on plastic tape. Digital audio has
since taken over. Digital audio has the same advantages as digital imagery when it comes to duplicating
and editing. Digital audio can be copied, cut and pasted, transitioned, and looped over without any
degradation in signal quality—a distinct advantage over its analog counterpart. The sound capability
in personal computers has dramatically improved over the years so that now high-quality sound capa-
bility is standard. As with digital imagery, there are file formats and compression standards to consider
when dealing with digital audio. In addition, there is a standard for digitally controlling musical
devices.
Digital musical device control
Musical instrument digital interface (MIDI) is a standard developed in 1983 to control musical instru-
ments without being tied to any one instrument in particular. MIDI commands are keynote commands
to musical devices and are intended to represent a musical performance. Mostly, the commands take the
10 640 pixels/scanline 480 scanlines/frame 3 bytes/pixel 30 fps ¼ 27,630,000 bytes/sec.
11 27,630,000 bytes/sec 3600 sec/hour ¼ 99,468,000,000 bytes/hour.