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

Movie AtomsSample AtomsNote The sample table atom can contain a 64-bit chunk offset atom (STChunkOffset64AID ='co64'). When this atom appears, it is used in place of the original chunk offset atom, which cancontain only 32-bit offsets. When QuickTime writes movie files, it uses the 64-bit chunk offset atomonly if there are chunks that use the high 32-bits of the chunk offset. Otherwise, the original 32-bitchunk offset atom is used to ensure compatibility with previous versions of QuickTime.Figure 2-50 (page 114) shows the layout of a chunk offset atom.Figure 2-50The layout of a chunk offset atomBytesChunk offset atomAtom sizeType = 'stco'Version441Flags 3Number of entriesChunk offset table4VariableThe chunk offset atom contains the following data elements.SizeA 32-bit integer that specifies the number of bytes in this chunk offset atom.TypeA 32-bit integer that identifies the atom type; this field must be set to 'stco'.VersionA 1-byte specification of the version of this chunk offset atom.FlagsA 3-byte space for chunk offset flags. Set this field to 0.Number of entriesA 32-bit integer containing the count of entries in the chunk offset table.Chunk offset tableA chunk offset table consisting of an array of offset values. There is one table entry for each chunk in themedia. The offset contains the byte offset from the beginning of the data stream to the chunk. The tableis indexed by chunk number—the first table entry corresponds to the first chunk, the second table entryis for the second chunk, and so on.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.114