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Movie Structures

QuickTime movies have a time dimension defined by a time scale and a duration, which are specified by a time coordinate system. Figure 1-1 illustrates a movie’s time coordinate system. A movie always starts at time 0. The time scale defines the unit of measure for the movie’s time values. The duration specifies how long the movie lasts.

A movie can contain one or more tracks. Each track refers to media data that can be interpreted within the movie’s time coordinate system. Each track begins at the beginning of the movie; however, a track can end at any time. In addition, the actual data in the track may be offset from the beginning of the movie. Tracks with data that does not commence at the beginning of a movie contain empty space that precedes the track data.

At any given point in time, one or more tracks may or may not be enabled.

However, no single track needs to be enabled during the entire movie. As you move through a movie, you gain access to the data that is described by each of the enabled tracks. Figure 1-2 shows a movie that contains five tracks. The lighter shading in each track represents the time offset between the beginning of the movie and the start of the track’s data (this lighter shading corresponds to empty space at the beginning of these tracks). When the movie’s time value is 6, there are three enabled tracks: Video 1 and Audio 1, and Video 2, which is just being enabled. The Other 1 track does not become enabled until the time value reaches 8. The Audio 2 track becomes enabled at time value 10.

A movie can contain one or more layers. Each layer contains one or more tracks that may be related to one another. The Movie Toolbox builds up a movie’s visual representation layer by layer. In Figure 1-2, for example, if the images contained in the Video 1 and Video 2 tracks overlap spatially, the user sees the image that is stored in the front layer. You can assign individual tracks to movie layers using Movie Toolbox functions that are described in QuickTime Movie Internals Guide.

The Movie Toolbox allows you to define both a movie preview and a movie poster for a QuickTime movie. A movie preview is a short dynamic representation of a movie. Movie previews typically last no more than 3 to 5 seconds, and they should give the user some idea of what the movie contains. (An example of a movie preview is a narrative track.) You define a movie preview by specifying its start time, its duration, and its tracks. A movie may contain tracks that are used only in its preview.

A movie poster is a single visual image representing the movie. You specify a poster as a point in time in the movie. As with the movie itself and the movie preview, you define which tracks are enabled in the movie poster.

Figure 1-3 shows an example of a movie’s tracks. The video track is used for the movie, the preview, and the poster. The movie audio track is used only for the movie. The preview audio track is used only for the preview. The poster graphic track is used only for the poster.

In this section:

Tracks
Media Structures

Tracks

A movie can contain one or more tracks. Each track represents a single stream of data in a movie and is associated with a single media. The media has control information that refers to the actual movie data.

All of the tracks in a movie use the movie’s time coordinate system. That is, the movie’s time scale defines the basic time unit for each of the movie’s tracks. Each track begins at the beginning of the movie, but the track’s data might not begin until some time value other than 0. This intervening time is represented by blank space. In an audio track the blank space translates to silence; in a video track the blank space generates no visual image. Each track has its own duration. This duration need not correspond to the duration of the movie. Movie duration always equals the maximum duration of all the tracks. An example of this is shown in Figure 1-4.

A track is always associated with one media. The media contains control information that refers to the data that constitutes the track. The track contains a list of references that identify portions of the media that are used in the track. In essence, these references are an edit list of the media. Consequently, a track can play the data in its media in any order and any number of times. Figure 1-5 shows how a track maps data from a media into a movie.

Media Structures

A media describes the data for a track. The data is not actually stored in the media. Rather, the media contains references to its media data, which may be stored in disk files, on CD-ROM discs, or other appropriate storage devices. Note that the data referred to by one media may be used by more than one movie, though the media itself is not reused.

Each media has its own time coordinate system, which defines the media’s time scale and duration. A media’s time coordinate system always starts at time 0, and it is independent of the time coordinate system of the movie that uses its data. Tracks map data from the movie’s time coordinate system to the media’s time coordinate system. Figure 1-6 shows how tracks perform this mapping.

Each supported data type has its own media handler. The media handler interprets the media’s data. The media handler must be able to randomly access the data and play segments at rates specified by the movie. The track determines the order in which the media is played in the movie and maps movie time values to media time values.

Figure 1-6 shows the final link to the data. The media in the figure references digital video frames on a CD-ROM disc.

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