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

Overview of QTFFQuickTime movies are stored on disk, using two basic structures for storing information: atoms (also knownas simple atoms or classic atoms) and QT atoms. To understand how QuickTime movies are stored, you needto understand the basic atom structures described in this chapter. Most atoms that you encounter in theQuickTime File Format are simple or classic atoms. Both simple atoms and QT atoms, however, allow you toconstruct arbitrarily complex hierarchical data structures. Both also allow your application to ignore data thatthey don’t understand.Media DescriptionA QuickTime file stores the description of its media separately from the media data.The description is called the movie resource, movie atom, or simply the movie, and contains informationsuch as the number of tracks, the video compression format, and timing information. The movie resource alsocontains an index describing where all the media data is stored.The media data is the actual sample data, such as video frames and audio samples, used in the movie. Themedia data may be stored in the same file as the QuickTime movie, in a separate file, in multiple files, in alternatesources such as databases or real-time streams, or in some combination of these.AtomsThe basic data unit in a QuickTime file is the atom. Each atom contains size and type fields that precede anyother data. The size field indicates the total number of bytes in the atom, including the size and type fields.The type field specifies the type of data stored in the atom and, by implication, the format of that data. In somecases, the size and type fields are followed by a version field and a flags field. An atom with these version andflags fields is sometimes called a full atom.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.20