Time
Image compression is difficult but worthwhile--images, not to mention long sequences of images, take a lot of memory. Time management in QuickTime is equally essential. You must understand time management to understand the QuickTime functions and data structures.Seemingly simple issues prove interesting--for example, determining the proper length (duration) of a movie. For many movies, the proper duration is the time required to play them in "real" time--that is, a rate in which human actions appear natural, and objects fall to earth accelerating at 32 feet per second per second. But what is the length of a movie that shows spreadsheet data charted over time, or a map of the earth that recapitulates continental drift? Add to this the differing clock speeds of different platforms, and the need to decompress in real time, and time proves, as ever, complex.
To manage these situations, QuickTime defines time coordinate systems, which anchor movies and their media data structures to a common temporal reality, the second. A time coordinate system contains a time scale that provides the translation between real time and the time in a movie. Time scales are marked in time units. The number of units that pass per second quantifies the scale--that is, a time scale of 26 means that 26 units pass per second and each time unit is 1/26 of a second. A time coordinate system also contains a duration, which is the length of a movie or a media in the number of time units it contains. Particular points in a movie can be identified by a time value, the number of time units elapsed to that point.
Each media has its own time coordinate system, which starts at time 0. The Movie Toolbox maps each type of media data from the movie's time coordinate system to the media's time coordinate system.
Time bases and time coordinate systems are described in the chapter "Movie Toolbox" later in this book.