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

Media Data Atom TypesSubtitle Mediatop is used to place the text; the height is determined by the bottom minus the top. Neither the top northe bottom should be outside the subtitle track dimensions. See “Subtitle Track Header Size andPlacement” (page 205).Subtitle Track Header Size and PlacementIndividual subtitles can be placed only within the subtitle track’s dimensions, adjusted by the subtitle track’smatrix. This is expressed relative to the main video track, allowing subtitles to overlay the video. Typically, allsubtitles are placed at the bottom of the video. Alternatively, subtitles can be placed at a different verticallocation, which allows individual subtitles at the bottom or the top of the associated video. This section describeshow this is controlled and how track and subtitle geometry is established.The value of the track dimensions and track matrix differ depending upon the absence or presence of theVertical Placement (0x20000000) flag in the subtitle sample description’s display flags. When Vertical Placementis not set, subtitles are always placed at the bottom of the video. When Vertical Placement is set, the verticalposition of subtitles can vary based upon the Text Box atom ('tbox') in each sample.In both cases, the subtitle track width must be the same as that of its associated main video ('vide') track.If the the Vertical Placement flag (0x20000000) display flag of the sample description is not set, the followingshould be true:●●●The subtitle track’s height should be 0.15 * the 'vide' track header height. This allows room for two linesof subtitle text. For example, if the 'vide' track header height is 720 pixels, then the 'sbtl' track headerheight should be 108 (pixels).The subtitle track’s vertical placement is determined by the track matrix, which should be a simple verticaltranslation matrix that shifts the subtitle down by 0.85 * the 'vide' track header height. For a subtitlemedia handler that obeys the tx3g rules, this positions the subtitles atop the bottom 15 percent of thevideo. Media handlers may choose to shift the subtitles further down in some modes; for example, in aplayback mode that displays black bars above and below content, the video could be shifted up and thesubtitles moved down into the black area.Subtitle samples must not contain a text box sample data extension ('tbox') because no control oververtical placement is allowed.Alternatively, if the the Vertical Placement flag (0x20000000) display flag of the sample description is set, thefollowing should be true:●●The height of the subtitle track should be the height of the video track header instead of 0.15 * the videotrack height. Because the subtitle track dimensions match the video track dimensions, subtitle text canbe positioned at the bottom or top of the video, unlike when the Vertical Placement flag is not set.The track matrix should be the identity matrix.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.205