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

Movie AtomsTrack AtomsAlternate groupA 16-bit integer that identifies a collection of movie tracks that contain alternate data for one another.This same identifier appears in each 'tkhd' atom of the other tracks in the group. QuickTime choosesone track from the group to be used when the movie is played. The choice may be based on suchconsiderations as playback quality, language, or the capabilities of the computer.A value of zero indicates that the track is not in an alternate track group.The most common reason for having alternate tracks is to provide versions of the same track in differentlanguages. Figure 2-8 (page 60) shows an example of several tracks. The video track’s Alternate GroupID is 0, which means that it is not in an alternate group (and its language codes are empty; normally,video tracks should have the appropriate language tags). The three sound tracks have the same GroupID, so they form one alternate group, and the subtitle tracks have a different Group ID, so they formanother alternate group. The tracks would not be adjacent in an actual QuickTime file; this is just a listof example track field values.Figure 2-8Example of alternate tracks in two alternate groupsAlternate Extended LanguageTrack Type Group ID Language Tag Codevideo (vide) 0sound (soun) 1 en-US engsound 1 fr-FR frasound 1 jp-JP jpnsubtitle (subt) 2 en-US engsubtitle 2 fr-FR fraVolumeA 16-bit fixed-point value that indicates how loudly this track’s sound is to be played. A value of 1.0indicates normal volume.ReservedA 16-bit integer that is reserved for use by Apple. Set this field to 0.Matrix structureThe matrix structure associated with this track. See Figure 2-3 (page 44) for an illustration of a matrixstructure.Track widthA 32-bit fixed-point number that specifies the width of this track in pixels.Track heightA 32-bit fixed-point number that indicates the height of this track in pixels.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.60