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

Movie AtomsCompressed Movie ResourcesFinding a SampleWhen QuickTime displays a movie or track, it directs the appropriate media handler to access the media datafor a particular time. The media handler must correctly interpret the data stream to retrieve the requesteddata. In the case of video media, the media handler traverses several atoms to find the location and size of asample for a given media time.The media handler performs the following steps:1. Determines the time in the media time coordinate system.2. Examines the time-to-sample atom to determine the sample number that contains the data for the specifiedtime.3. Scans the sample-to-chunk atom to discover which chunk contains the sample in question.4. Extracts the offset to the chunk from the chunk offset atom.5. Finds the offset within the chunk and the sample’s size by using the sample size atom.Finding a Key FrameFinding a key frame for a specified time in a movie is slightly more complicated than finding a sample for aspecified time. The media handler must use the sync sample atom and the time-to-sample atom together inorder to find a key frame.The media handler performs the following steps:1. Examines the time-to-sample atom to determine the sample number that contains the data for the specifiedtime.2. Scans the sync sample atom to find the key frame that precedes the sample number chosen in step 1.3. Scans the sample-to-chunk atom to discover which chunk contains the key frame.4. Extracts the offset to the chunk from the chunk offset atom.5. Finds the offset within the chunk and the sample’s size by using the sample size atom.Compressed Movie ResourcesMost QuickTime movies have metadata in addition to their media data. Media data can be compressed usinga variety of video and sound compression algorithms. Beginning with QuickTime 3, it also became possible tocompress the metadata—more commonly known as the movie resource. However, the movie resource cannotbe compressed by means of a lossy compression algorithm because it contains critical information, such asthe video and audio compression types used, individual frame offsets, and timing information. To compressthe movie resource, therefore, lossless data compression algorithms must be used.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.117