XMP Specification Part 3: Storage in Files - Adobe

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Native metadata Files in many formats can contain metadata in other, previously-defined or native formats. Still image formats, for example, frequently contain IPTC and TIFF/Exif metadata. When a file is imported, it can contain metadata in one or more of these native formats. Similarly, when a file is exported, it may be read by a device or application that is expecting metadata in native formats, and may or may not support the XMP format. The information represented overlaps to a great degree, and values that are present in any format should be reflected in all others. An application that supports XMP must read metadata values from the native formats, and represent it correctly in the XMP metadata. When modifying XMP metadata, an application is responsible for correctly reflecting changes in any other metadata formats that are present. A file that has been manipulated by an application that supports XMP might later be opened in an application that relies on another metadata format, and any changes that have been made to the XMP metadata must be properly represented. Users of the XMP Toolkit, which handles most native metadata reconciliation, should still be aware of the issues in order to recognize potential problems to look for when testing with legacy application or file versions. • For general issues and policy on metadata reconciliation, see 3, “Handling native metadata”. • For an in-depth discussion of complex issues pertaining to still image formats (JPEG, TIFF, and PSD), see 4, “Digital photography native metadata”. 8 ©Adobe Systems Incorporated, 2010
1 XMP packets The XMP packet wrapper can enable the use of embedded XMP by software that does not understand the format of the file. The packet wrapper is not the sole aspect of embedding XMP in a file. The entire XMP packet must still be placed in the file as an appropriate component of the file’s structure. XMP packets: • May be embedded in a wide variety of binary and text formats, including native XML files. • Are delimited by easy-to-scan markers that are XML syntax-compatible (as is the entire packet) to allow transmission to an XML parser without additional filtering. • Deal with arbitrary positioning within a byte stream (so as not to rely on machine word boundaries, and so on). • Enable in-place editing of metadata (if the metadata size does not exceed the packet boundaries). • Allow multiple packets to be embedded in a single data file. 2, “Embedding XMP metadata in application files”, gives information on how XMP packets are embedded in specific file formats. Applications may also scan files for XMP packets without knowledge of the file format itself, although this should be regarded as a last resort (see 1.2, “Scanning files for XMP packets”). 1.1 The XMP packet format The packet wrapper, which consists of a header, padding, and trailer wrapped around serialized XMP, is formally optional; that is, a file that contains only the serialized XMP is legal. The packet wrapper, if used, allows byte-oriented packet scanning and in-place editing of the XMP. Some file formats or use cases might forbid the packet wrapper; for instance, in cases where minimal size is paramount. It can also happen in cases where the XMP might not be contiguous, and thus byte-oriented packet scanning must be prevented. The following figure shows a schematic of an XMP packet. It contains a header, XML data, padding, and a trailer. Header Serialized Padding Trailer Figure 1 — XMP packet schematic Here is an outline of an XMP packet, showing the text of the header and trailer: ... the serialized XMP as described above: ... ... ... XML whitespace as padding ... ©Adobe Systems Incorporated, 2010 9
Page 1 and 2: XMP SPECIFICATION PART 3 STORAGE IN
Page 3 and 4: Contents 1 XMP packets. . . . . . .
Page 5 and 6: Preface This document set provides
Page 7: Introduction This document discusse
Page 11 and 12: 1.1.2 XMP data The bytes of the XMP
Page 13 and 14: The scanner now has enough informat
Page 15 and 16: 2 Embedding XMP metadata in applica
Page 17 and 18: accordingly, eventually arriving so
Page 19 and 20: Byte offset, length 2, 2 bytes 2 +
Page 21 and 22: NOTE TIFF, JPEG, and PSD share comp
Page 23 and 24: 4, 4bytes Offset of the first (0th)
Page 25 and 26: • ASF (WMA, WMV) • AVI • FLV
Page 27 and 28: There appear to be no uses in ASF o
Page 29 and 30: 2.2.4 MOV (QuickTime) The QuickTime
Page 31 and 32: An SWF file consists of a file head
Page 33 and 34: In this example, the files 00001.mp
Page 35 and 36: C0002M01.XMP The top-level folder M
Page 37 and 38: 2.4.2.1 Finding XMP in an InDesign
Page 39 and 40: fSequenceNumber The master page seq
Page 41 and 42: TIFF, JPEG, and PSD share complex i
Page 43 and 44: 2.5.2 XML XMP metadata, because it
Page 45 and 46: The marker must be at the beginning
Page 47 and 48: 2.6.2.1.4Avoiding name conflicts In
Page 49 and 50: into the PDF document information d
Page 51 and 52: 3 Handling native metadata There is
Page 53 and 54: 3.1.3 Use of native metadata digest
Page 55 and 56: The ASF specification says that the
Page 57 and 58: A known genre should be mapped to X
Page 59 and 60:
• In JPEG, the APPn sections also
Page 61 and 62:
4 Digital photography native metada
Page 63 and 64:
The image resources need not be sor
Page 65 and 66:
Table 30 — Photoshop image resour
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2:20 photoshop:SupplementalCategori
Page 69 and 70:
The encoding header is the first 8
Page 71 and 72:
Table 39 — Mapping of outer TIFF
Page 73 and 74:
Table 40 — Mapping of inner Exif
Page 75 and 76:
TIFF Tag 33723 in the 0th IFD; Phot
Page 77 and 78:
As mentioned earlier, the IFDs used
Page 79 and 80:
Each additional item has the struct
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XMP Specification Part 3: Storage in Files - Adobe

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