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

Media Data Atom TypesMusic Media●A subtitle’s placement is determined by the top coordinate of one of two rectangles. If the override textbox sample data extension ('tbox') is present, it is used. Otherwise, the default text box in the sampledescription is used. Some players will use the top coordinate to determine whether the subtitle is in thetop half of the track dimensions and place the subtitle at the top of the video, otherwise placing it at thebottom of the video. Other players might use the top coordinate precisely, placing the subtitle at thespecified vertical coordinate. As both playback environments are possible for a piece of content, it isrecommend that a top coordinate of 0 be used for placing at the top and a top coordinate equal to thetrack height minus the subtitle height be used. In this way, if the content is played in either kind of player,its placement is predictable.Referencing a Related Forced Subtitle TrackA subtitle track can contain a track reference of type 'forc' to a paired subtitle track that contains only forcedsubtitles.Pairing two subtitle tracks might be necessary if the timing of forced subtitle samples (see 'frcd') differsfrom the regular subtitle text, such as when a forced subtitle display would overlap in time with the display ofthe regular subtitle. If timings are the same, a single subtitle track should be used.To pair two tracks, one subtitle track can contain any combination of forced and non-forced (regular) subtitlesamples and the other track must contain only forced subtitles. The tracks must be in the same alternate groupand be tagged with the same extended language tag and language code. The first, regular track then uses atrack reference of type 'forc' to reference the second, forced-only track. (Mixing extended language tags orcodes for the same language in the same alternate group is undefined.)Note The regular track in a pair provides a complete transcription of the audio as subtitle text. Thisallows a user to listen in one language but to read subtitled dialogue in another language.See “Alternate Subtitle Tracks” (page 335) and “Track Reference Atoms” (page 73) for more information.Music MediaMusic media is used to store note-based audio data, such as MIDI data, in QuickTime movies. It has a mediatype of 'musi'.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.206