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

Movie AtomsOverview of Movie AtomsAny track of a QuickTime file can be associated with one or more tags that indicate the media’s characteristics.Tags indicate something of interest about a track. For example, a tag could indicate the purpose of the track(it is commentary), an abstract characteristic of the track (it requires hardware decoding), or an indication thatthe track includes legible text ( a chapter track and subtitle track both can be read by the user).Comparison of tags is case sensitive; two tags match if the bytes of the strings match exactly. Two tag stringsdiffering only by case should not be used to avoid possible confusion for developers or content creators.Duplicate tags in a single track are allowed but are discouraged. Duplication has no special meaning.Tag strings are not localized and are meant to be machine interpreted; however, mnemonic strings areencouraged.A tag is either public or private:●●Public tags allow shared semantics to be deployed widely. Public tags are currently defined by Apple.Private tags can be defined for private use.Tag strings have the following structure:●A public tag starts with the prefix “public.”, which is followed by one or more segments separated byperiods. Examples (not defined) might be public.subtitle or public.commentary.director.Note Public tags are public because they are documented in this specification or are availablein Apple APIs. Other definitions of tags with the "public." prefix are prohibited; use private tagsinstead.●●A private tag starts with the private entity’s domain using a reverse DNS naming convention. For example,apple.com becomes com.apple. This is followed by one or more segments separated by periods. Examples(not defined) might be com.apple.this-is-a-tag, com.apple.video.includes-sign-language, andorg.w3c.html5.referenced-video.The only allowed prefixes are “public.” and reversed domains. All other prefixes are reserved for futureuse.Note Generic top-level domains other than “public" (if it were to be assigned) are supported. Thestring “public" is reserved to signal public media characteristic tags.This specification defines the following public media characteristic tags. Other public and private tags couldbe defined outside the specification; unrecognized tags should be ignored.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.52