XMP Specification September 2005 97 Embedding XMP Metadata in Application Files PNG 5 PNG An XMP Packet is embedded in a PNG graphic file by adding a chunk of type iTXt. This chunk is semantically equivalent to the tEXt and zTXt chunks, but the textual data is in the UTF-8 encoding of the Unicode character set, instead of Latin-1. The Chunk Data portion is the XMP Packet. The packet must be marked as read-only. NOTE: XMP software that is not aware of the file format must not be allowed to change the content of the XMP Packet because of the CRC checksum following the chunk data. There should be no more than one chunk containing XMP in each PNG file. Encoders are encouraged to place the chunk at the beginning of the file, but this is not required. PNG Data Format Reference http://www.w3.org/TR/REC-png.html Field Length Comments Length 4 An unsigned integer representing the number of bytes in the chunk’s data field (does not include the chunk type code or the CRC). Chunk Type 4 "iTXt" Chunk Data: Standard iTXt chunk header plus the XMP Packet Keyword 17 "XML:com.adobe.xmp" Null separator 1 value = 0x00 Compression flag 1 value = 0x00, specifies uncompressed data Compression method 1 value = 0x00 Language tag 0 Not used for XMP metadata Null separator 1 value = 0x00 Translated keyword 0 Not used for XMP metadata Null separator 1 value = 0x00 Text length of packet The XMP Packet, must be encoded as UTF-8 CRC 4 The Cyclic Redundancy Check, calculated on the preceding bytes in the chunk, including the chunk type code and chunk data fields, but not including the length field.