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

Media Data Atom TypesSubtitle MediaSubtitle Sample DescriptionThe subtitle sample description contains information that defines how to interpret subtitle media data. Thissample description is based on the standard sample description header, as described in “Sample DescriptionAtoms” (page 99).Note Only one sample description is permitted in a 'sbtl' track.The data format field in the sample description is currently always set to 'tx3g'. Unrecognized data formatsshould be ignored. The text media described here is based on the text box defined in the 3GPP Timed Textspecification but provides a different track type and media handler designed specifically for subtitles.The subtitle media handler adds some of its own fields to the sample description.Display flagsA 32-bit integer containing flags that describe how the subtitle text should be drawn. The following flagsare defined.Vertical placementControls vertical placement of the subtitle text. If this flag is set, the subtitle media handler usesthe top coordinate of the display bounds of the override 'tbox' text box to determine thesubtitle’s vertical placement as described in “Subtitle Track Header Size and Placement” (page205). Otherwise, the subtitle displays at the bottom of the video. This flag’s value is 0x20000000.Some samples are forcedIndicates whether any subtitle samples contain forced atoms. If this flag is set, at least one samplecontains a forced ('frcd') atom as described in “Subtitle Sample Data” (page 202). This flag’svalue is 0x40000000.All samples are forcedIf this flag is set, the subtitle media handler treats all samples as forced subtitles, regardless ofthe presence or absence of a 'frcd' atom. This flag’s value is 0x80000000. If this flag is set, theSome Samples Are Forced flag must also be set (making 0xC0000000).ReservedAn 8-bit integer that must be set to 1.ReservedAn 8-bit integer that must be set to -1 (negative one).ReservedA 32-bit integer that must be set to 0.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.200