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

Media Data Atom TypesHint Media●●●Media data represented as a set of network-independent standard QuickTime tracks, which may be playedor edited, as normal.A common declaration and base structure for server hint tracks; this common format is protocolindependent, but contains the declarations of which protocols are described in the server tracks.A specific design of the server hint tracks for each protocol which may be transmitted; all these designsuse the same basic structure.The resulting streams, sent by the servers under the direction of hint tracks, do not need to contain any traceof QuickTime information. This approach does not require that QuickTime, or its structures or declaration style,be used either in the data on the wire or in the decoding station. For example, a QuickTime file using H.261video and DVI audio, streamed under Real-Time Protocol (RTP), results in a packet stream which is fully compliantwith the IETF specifications for packing those codings into RTP.Hint tracks are built and flagged, so that when the movie is viewed directly (not streamed), they are ignored.The next section describes a generic format for streaming hints to be stored in a QuickTime movie.Adding Hint Tracks to a MovieTo store packetization hints, one or more hint tracks are added to a movie. Each hint track contains hints forat least one actual media track to be streamed. A streamed media track may have more than one hint track.For example, it might have a separate hint track for the different packet sizes the server supports, or it mighthave different hint tracks for different protocols. It is not required that all media tracks have correspondinghint tracks in a movie.The sample time of a hint sample corresponds to the sample time of the media contained in the packetsgenerated by that hint sample. The hint sample may also contain a transmission time for each packet. (Theformat for the hint sample is specific to the hint track type.)The hint track may have a different time scale than its referenced media tracks.The flags field in the track header atom ('tkhd') must be set to 0x000000, indicating that the track is inactiveand is not part of the movie, preview, or poster.The subType field of the handler description atom ('hdlr') contains 'hint', indicating that the media typeis packetization hints.Note that if a QuickTime media track is edited, any previously stored packetization hints may become invalid.Comparing the modification dates of the media track and the hint track is one way to determine this scenario,but it is far from being foolproof. Since the hint track keeps track of which original track media samples and2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.252