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

Some Useful Examples and ScenariosCreating an Effect DescriptionAn input map works in concert with track reference atoms in the source tracks. A track reference atom of typekTrackModifierReference is added to each source track, which causes that source track’s output to beredirected to the effects track. An input map is added to the effects track to identify the source tracks and givea name to each source, such as 'srcA' and 'srcB'. The effect can then refer to the sources by name, specifyingthat 'srcB' should slide in over 'srcA', for example.Structure of an Input MapThe input map contains a set of atoms that refer to the tracks used as sources for the effect. Each source trackis represented by one track reference atom of type kTrackModifierInput.Each modifier input atom contains two children, one of type kEffectDataSourceType, and one of typekTrackModifierType, which hold the name and type of the source.The name of the source is a unique identifier that you create, which is used in the effect description to referencethe track. Any four-character name is valid, as long as it is unique in the set of source names.Important Apple recommends you adopt the standard naming convention 'srcX', where X is a letterof the alphabet. Thus, your first source would be named 'srcA', the second 'srcB', and so forth. Thisconvention is used here in this chapter.The child atom of type kTrackModifierType indicates the type of the track being referenced. For a videotrack the type is VideoMediaType, for a sprite track it is SpriteMediaType, and so forth. Video tracks arethe most common track type used as sources for effects. Only tracks that have a visible output, such as videoand sprite tracks, can be used as sources for an effect. This means, for example, that sound tracks cannot besources for an effect.Figure 6-6 (page 342) shows a completed input map that references two sources. The first source is a videotrack and is called 'srcA'. The second source, also a video track, is called 'srcB'.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.341