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

Movie AtomsSample AtomsNote If a composition shift least greatest atom is not present, a reader must assumecompositionOffsetToDisplayOffsetShift = 0. The sample tables will need to be scannedto find the least and greatest offsets, as well as the presentation start and end times, to determinethe decode time offset required for presentation.Sync Sample AtomsThe sync sample atom identifies the key frames in the media. In a media that contains compressed data, keyframes define starting points for portions of a temporally compressed sequence. The key frame isself-contained—that is, it is independent of preceding frames. Subsequent frames may depend on the keyframe.The sync sample atom provides a compact marking of the random access points within a stream. The table isarranged in strictly increasing order of sample number. If this table is not present, every sample is implicitly arandom access point.Sync sample atoms have an atom type of 'stss'. The sync sample atom contains a table of sample numbers.Each entry in the table identifies a sample that is a key frame for the media. If no sync sample atom exists, thenall the samples are key frames.Figure 2-41 (page 107) shows the layout of a sync sample atom.Figure 2-41The layout of a sync sample atomBytesSync sample atomAtom sizeType = 'stss'VersionFlagsNumber of entriesSync sample table44134VariableThe sync sample atom contains the following data elements.SizeA 32-bit integer that specifies the number of bytes in this sync sample atom.TypeA 32-bit integer that identifies the atom type; this field must be set to 'stss'.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.107