Quicktime File Format (2012-08-14).pdf

Media Data Atom TypesModifier TracksModifier TracksThe addition of modifier tracks in QuickTime 2.1 introduced the capability for creating dynamic movies. (Amodifier track sends data to another track; by comparison, a track reference is an association.) For example,instead of playing video in a normal way, a video track could send its image data to a sprite track. The spritetrack then could use that video data to replace the image of one of its sprites. When the movie is played, thevideo track appears as a sprite.Modifier tracks are not a new type of track. Instead, they are a new way of using the data in existing tracks. Amodifier track does not present its data, but sends it to another track that uses the data to modify how itpresents its own data. Any track can be either a sender or a presenter, but not both. Previously, all tracks werepresenters.Another use of modifier tracks is to store a series of sound volume levels, which is what occurs when you workwith a tween track. These sound levels can be sent to a sound track as it plays to dynamically adjust the volume.A similar use of modifier tracks is to store location and size information. This data can be sent to a video trackto cause it to move and resize as it plays.Because a modifier track can send its data to more than one track, you can easily synchronize actions betweenmultiple tracks. For example, a single modifier track containing matrices as its samples can make two separatevideo tracks follow the same path.See “Creating Movies with Modifier Tracks” (page 343) for more information about using modifier tracks.Limitations of Spatial Modifier TracksA modifier track may cause a track to move outside of its original boundary regions. This may present problems,since applications do not expect the dimensions or location of a QuickTime movie to change over time.To ensure that a movie maintains a constant location and size, the Movie Toolbox limits the area in which aspatially modified track can be displayed. A movie’s “natural” shape is defined by the region returned by theGetMovieBoundsRgn function. The toolbox clips all spatially modified tracks against the region returned byGetMovieBoundsRgn. This means that a track can move outside of its initial boundary regions, but it cannotmove beyond the combined initial boundary regions of all tracks in the movie. Areas uncovered by a movingtrack are handled by the toolbox in the same way as areas uncovered by tracks with empty edits.If a track has to move through a larger area than that defined by the movie’s boundary region, the movie’sboundary region can be enlarged to any desired size by creating a spatial track (such as a video track) of thedesired size but with no data. As long as the track is enabled, it contributes to the boundary regions of themovie.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.247