Track References

Track references are a feature of QuickTime that allows you to relate a movie’s tracks to one another. The QuickTime track-reference mechanismsupports many-to-many relationships. That is, any movie track may contain one or more track references, and any track may be related to one or more other tracks in the movie.

Track references can be useful in a variety of ways. For example, track references can be used to relate timecode tracks to other movie tracks. Another use of track references is to associate one or more text tracks that contain subtitles with the appropriate audio track or tracks.

Track references are also used to create alternate tracks, as discussed in Working With Alternate Tracks.

Every movie track contains a list of its track references. Each track reference identifies another, related track. That related track is identified by its track identifier. The track reference itself contains information that allows you to classify the references by type. This type information is stored in an OSType data type. You are free to specify any type value you want. Note, however, that Apple has reserved all lowercase type values.

You may create as many track references as you want, and you may create more than one reference of a given type. Each track reference of a given type is assigned an index value. The index values start at 1 for each different reference type. QuickTime maintains these index values so that they always start at 1 and count by 1.

Creating Modifier Tracks

Modifier tracks are used to modify other tracks. Redirecting the output of a modifier track to the input of another track lets you create a wide variety of effects, such as panning audio or presenting video data as a sprite.

Any QuickTime track can be made into a modifier track. For example, you can use the output of a video track as a sprite’s image source, or the output of two video tracks as inputs to a transition effect in an effect track.

To create a modifier track, you need to use track references and input maps. A track reference lets you specify which track receives a modifier track’s output. An input map specifies the part or parts of the receiving track to be modified by the output of the modifier track. For example, the output of a tween track might modify a sound track’s volume, balance, or both.

Creating Movies With Modifier Tracks

To create a movie with modifier tracks, first you create a movie with all the desired tracks, then you create the modifier track. To link the modifier track to the track that it modifies, use the AddTrackReference function shown in Listing 5-1.

Listing 5-1  Linking a modifier track to the track it modifies

long addedIndex;

AddTrackReference   (aVideoTrack,

                     aModifierTrack,

                     kTrackModifierReference,

                     &addedIndex);

The reference doesn’t completely describe the modifier track’s relationship to the track it modifies. Instead, the reference simply tells the modifier track to send its data to the specified track. The receiving track doesn’t know what it should do with that data. A single track may also be receiving data from more than one modifier track. You have to update the media input map of the receiving track to specify how the data is to be interpreted.

Manipulating Media Input Maps

Each track has particular attributes such as size, position, and volume associated with it. The media input map of that track describes where the variable parameters are stored so that modifier tracks know where to send their data. When a track is copied, its input map is also copied. CopyTrackSettings also transfers the media input map.

QuickTime provides two functions you can use to maintain media input maps:

• GetMediaInputMap

• SetMediaInputMap

To describe how each modifier input should be used, each track’s media has an input map. The media’s input map describes how the data being sent to each input of a track should be interpreted by the receiving track. After creating the reference, it is necessary to update the receiving track’s media input map. When AddTrackReference is called, it returns the index of the reference added. That index is the index of the input that needs to be described in the media input map. If the modifier track created above contains regions to change the shape of the video track, the code shown in Listing 5-2 updates the input map appropriately.

Listing 5-2  Updating the input map

QTAtomContainer inputMap;

QTAtom inputAtom;

OSType inputType;

Media aVideoMedia = GetTrackMedia(aVideoTrack);

GetMediaInputMap (aVideoMedia, &inputMap);

QTInsertChild(

         inputMap, kParentAtomIsContainer,

         kTrackModifierInput,

         addedIndex,

         0,

         0,

         nil,

         &inputAtom);

inputType = kTrackModifierTypeClip;

QTInsertChild (

         inputMap,

         inputAtom,

         kTrackModifierType,

         1,

         0,

         sizeof(inputType),

         &inputType,

         nil);

SetMediaInputMap(aVideoMedia, inputMap);

QTDisposeAtomContainer(inputMap);

The media input map allows you to store additional information for each input. In the preceding example, only the type of the input is specified. In other types of references, you may need to specify additional data.

When a modifier track is sending an empty track edit, or is disabled or deleted, all receiving tracks are notified that the track input is inactive. When an input becomes inactive, it is reset to its default value. For example, if a track is receiving data from a clip modifier track and that input becomes inactive, the shape of the track reverts to the shape it would have if there were no clip modifier track.

Working With Alternate Tracks

QuickTime lets you define alternate tracks in a movie. You can use alternate tracks to support multiple languages or to present different levels of visual quality in the movie. You collect alternate tracks into groups. Alternate track groups are collections of tracks that represent some conceptual data but are appropriate for use in different play environments. For example, you might have some 4-bit data in one track and some 8-bit data in another. Working with alternate tracks allows you to set up alternatives from which QuickTime can choose.

QuickTime selects one track from each alternate group when it plays the movie. For example, you could create a movie that has three separate audio tracks: one in English, one in French, and one in Spanish. You would collect these audio tracks into an alternate group. When the user plays the movie, QuickTime selects the track from this group that corresponds to the current language setting for the movie.

Similarly, you can use alternate tracks to store data of different quality. When the user plays the movie, QuickTime selects the track that best suits the capabilities of the user’s computer on which the movie is being played. In this manner, you can create a single movie that can accommodate the playback characteristics of a number of different computer configurations.

QuickTime allows you to store quality information for media structures that are assigned to either sound or video tracks. For all tracks, QuickTime uses bits 6 and 7 of the quality setting. These bits encode a relative quality value. These values range from 0 to 3. You can use higher quality values to indicate larger sample sizes. For example, consider a movie that has two sound tracks that are alternates for each other: one contains 8-bit sound and the other contains 16-bit sound. You could assign a quality value of mediaQualityNormal to the 8-bit media and a value of mediaQualityBetter to the 16-bit media. QuickTime would play the 16-bit media only if the user’s configuration could handle 16-bit sound. Otherwise, QuickTime would use the 8-bit media. The sound media handler determines the sample size for each sound media for QuickTime by examining the media’s sound description structure.

In addition, QuickTime also uses bits 0 through 5 (the low-order bits) of the quality setting. You use these bits to indicate the pixel depths at which the media should be played. Each bit corresponds to a single depth value, ranging from 1-bit pixels to 32-bit pixels. You may use these bits to control the playback of both video and sound tracks.

As an example, consider a movie that contains three video tracks with the following characteristics:

You could assign the following quality values to these track’s media structures:

QuickTime would always use Track A when playing the movie on 1-bit and 2-bit displays. At the other pixel depths, the video media handler determines which track to use by examining the availability and performance of the specified decompressors. If the JPEG decompressor can play back at full frame rate, QuickTime would use Track C. Otherwise, QuickTime uses Track B. The video media handler determines the compressor that is appropriate for each media by examining the media’s image description structure.

You set a movie’s language by calling the SetMovieLanguage function.

To establish alternate groups of tracks, you can use the SetTrackAlternate and GetTrackAlternate functions.

You can work with the language and quality characteristics of media by calling the GetMediaLanguage, SetMediaLanguage, GetMediaQuality, and SetMediaQuality functions.

By default, QuickTime automatically selects the appropriate tracks to play according to a movie’s quality and language settings, as well as the capabilities of the user’s computer. Whenever your application calls theSetMovieGWorld, SetMovieBox, UpdateMovie, or SetMovieMatrix function, QuickTime checks each alternate group for an appropriate track. However, you can control this selection process. Use the SetAutoTrackAlternatesEnabled function to enable or disable automatic track selection. The SelectMovieAlternates function instructs QuickTime to select appropriate tracks immediately. If no tracks in an alternate track group are enabled, then QuickTime does not activate any track from that group during automatic track selection.